interpreter: Holderify arrays and dicts

This is the final refactoring for extracting the bultin object
logic out of Interpreterbase. I decided to do both arrays and
dicts in one go since splitting it would have been a lot more
confusing.

10 files changed, 345 insertions, 424 deletions

diff --git a/mesonbuild/ast/interpreter.py b/mesonbuild/ast/interpreter.py
index 5998e5b..7954b9b 100644
--- a/mesonbuild/ast/interpreter.py
+++ b/mesonbuild/ast/interpreter.py
@@ -30,6 +30,15 @@ from ..interpreterbase import (
     TYPE_nkwargs,
 )
 
+from ..interpreter import (
+    Interpreter,
+    StringHolder,
+    BooleanHolder,
+    IntegerHolder,
+    ArrayHolder,
+    DictHolder,
+)
+
 from ..mparser import (
     AndNode,
     ArgumentNode,
@@ -202,6 +211,9 @@ class AstInterpreter(InterpreterBase):
         assert isinstance(node, mparser.FormatStringNode)
         return node.value
 
+    def evaluate_arraystatement(self, cur: mparser.ArrayNode) -> TYPE_nvar:
+        return self.reduce_arguments(cur.args)[0]
+
     def evaluate_arithmeticstatement(self, cur: ArithmeticNode) -> int:
         self.evaluate_statement(cur.left)
         self.evaluate_statement(cur.right)
@@ -364,18 +376,15 @@ class AstInterpreter(InterpreterBase):
             mkwargs = {} # type: T.Dict[str, TYPE_nvar]
             try:
                 if isinstance(src, str):
-                    from ..interpreter import Interpreter, StringHolder
                     result = StringHolder(src, T.cast(Interpreter, self)).method_call(node.name, margs, mkwargs)
                 elif isinstance(src, bool):
-                    from ..interpreter import Interpreter, BooleanHolder
                     result = BooleanHolder(src, T.cast(Interpreter, self)).method_call(node.name, margs, mkwargs)
                 elif isinstance(src, int):
-                    from ..interpreter import Interpreter, IntegerHolder
                     result = IntegerHolder(src, T.cast(Interpreter, self)).method_call(node.name, margs, mkwargs)
                 elif isinstance(src, list):
-                    result = self.array_method_call(src, node.name, margs, mkwargs)
+                    result = ArrayHolder(src, T.cast(Interpreter, self)).method_call(node.name, margs, mkwargs)
                 elif isinstance(src, dict):
-                    result = self.dict_method_call(src, node.name, margs, mkwargs)
+                    result = DictHolder(src, T.cast(Interpreter, self)).method_call(node.name, margs, mkwargs)
             except mesonlib.MesonException:
                 return None
 
diff --git a/mesonbuild/interpreter/__init__.py b/mesonbuild/interpreter/__init__.py
index c93dbc9..2269837 100644
--- a/mesonbuild/interpreter/__init__.py
+++ b/mesonbuild/interpreter/__init__.py
@@ -35,7 +35,9 @@ __all__ = [
     'ExternalProgramHolder',
     'extract_required_kwarg',
 
+    'ArrayHolder',
     'BooleanHolder',
+    'DictHolder',
     'IntegerHolder',
     'StringHolder',
 ]
@@ -49,7 +51,9 @@ from .interpreterobjects import (ExecutableHolder, BuildTargetHolder, CustomTarg
                                  extract_required_kwarg)
 
 from .primitives import (
+    ArrayHolder,
     BooleanHolder,
+    DictHolder,
     IntegerHolder,
     StringHolder,
 )
diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index b2774f6..f0b650b 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -386,6 +386,8 @@ class Interpreter(InterpreterBase, HoldableObject):
         '''
         self.holder_map.update({
             # Primitives
+            list: P_OBJ.ArrayHolder,
+            dict: P_OBJ.DictHolder,
             int: P_OBJ.IntegerHolder,
             bool: P_OBJ.BooleanHolder,
             str: P_OBJ.StringHolder,
diff --git a/mesonbuild/interpreter/primitives/__init__.py b/mesonbuild/interpreter/primitives/__init__.py
index d6c0795..d876f56 100644
--- a/mesonbuild/interpreter/primitives/__init__.py
+++ b/mesonbuild/interpreter/primitives/__init__.py
@@ -2,13 +2,17 @@
 # SPDX-license-identifier: Apache-2.0
 
 __all__ = [
+    'ArrayHolder',
     'BooleanHolder',
+    'DictHolder',
     'IntegerHolder',
     'StringHolder',
     'MesonVersionString',
     'MesonVersionStringHolder',
 ]
 
+from .array import ArrayHolder
 from .boolean import BooleanHolder
+from .dict import DictHolder
 from .integer import IntegerHolder
 from .string import StringHolder, MesonVersionString, MesonVersionStringHolder
diff --git a/mesonbuild/interpreter/primitives/array.py b/mesonbuild/interpreter/primitives/array.py
new file mode 100644
index 0000000..566dff9
--- /dev/null
+++ b/mesonbuild/interpreter/primitives/array.py
@@ -0,0 +1,103 @@
+# Copyright 2021 The Meson development team
+# SPDX-license-identifier: Apache-2.0
+
+import typing as T
+
+from ...interpreterbase import (
+    ObjectHolder,
+    IterableObject,
+    MesonOperator,
+    typed_operator,
+    noKwargs,
+    noPosargs,
+    noArgsFlattening,
+    typed_pos_args,
+
+    TYPE_var,
+    TYPE_kwargs,
+
+    InvalidArguments,
+)
+
+if T.TYPE_CHECKING:
+    # Object holders need the actual interpreter
+    from ...interpreter import Interpreter
+
+class ArrayHolder(ObjectHolder[T.List[TYPE_var]], IterableObject):
+    def __init__(self, obj: T.List[TYPE_var], interpreter: 'Interpreter') -> None:
+        super().__init__(obj, interpreter)
+        self.methods.update({
+            'contains': self.contains_method,
+            'length': self.length_method,
+            'get': self.get_method,
+        })
+
+
+        self.trivial_operators.update({
+            MesonOperator.EQUALS: (list, lambda x: self.held_object == x),
+            MesonOperator.NOT_EQUALS: (list, lambda x: self.held_object != x),
+            MesonOperator.IN: (object, lambda x: x in self.held_object),
+            MesonOperator.NOT_IN: (object, lambda x: x not in self.held_object),
+        })
+
+        # Use actual methods for functions that require additional checks
+        self.operators.update({
+            MesonOperator.PLUS: self.op_plus,
+            MesonOperator.INDEX: self.op_index,
+        })
+
+    def display_name(self) -> str:
+        return 'array'
+
+    def iter_tuple_size(self) -> None:
+        return None
+
+    def iter_self(self) -> T.Iterator[TYPE_var]:
+        return iter(self.held_object)
+
+    def size(self) -> int:
+        return len(self.held_object)
+
+    @noArgsFlattening
+    @noKwargs
+    @typed_pos_args('array.contains', object)
+    def contains_method(self, args: T.Tuple[object], kwargs: TYPE_kwargs) -> bool:
+        def check_contains(el: T.List[TYPE_var]) -> bool:
+            for element in el:
+                if isinstance(element, list):
+                    found = check_contains(element)
+                    if found:
+                        return True
+                if element == args[0]:
+                    return True
+            return False
+        return check_contains(self.held_object)
+
+    @noKwargs
+    @noPosargs
+    def length_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> int:
+        return len(self.held_object)
+
+    @noArgsFlattening
+    @noKwargs
+    @typed_pos_args('array.get', int, optargs=[object])
+    def get_method(self, args: T.Tuple[int, T.Optional[TYPE_var]], kwargs: TYPE_kwargs) -> TYPE_var:
+        index = args[0]
+        if index < -len(self.held_object) or index >= len(self.held_object):
+            if args[1] is None:
+                raise InvalidArguments(f'Array index {index} is out of bounds for array of size {len(self.held_object)}.')
+            return args[1]
+        return self.held_object[index]
+
+    @typed_operator(MesonOperator.PLUS, object)
+    def op_plus(self, other: TYPE_var) -> T.List[TYPE_var]:
+        if not isinstance(other, list):
+            other = [other]
+        return self.held_object + other
+
+    @typed_operator(MesonOperator.INDEX, int)
+    def op_index(self, other: int) -> TYPE_var:
+        try:
+            return self.held_object[other]
+        except IndexError:
+            raise InvalidArguments(f'Index {other} out of bounds of array of size {len(self.held_object)}.')
diff --git a/mesonbuild/interpreter/primitives/dict.py b/mesonbuild/interpreter/primitives/dict.py
new file mode 100644
index 0000000..24f4d21
--- /dev/null
+++ b/mesonbuild/interpreter/primitives/dict.py
@@ -0,0 +1,87 @@
+# Copyright 2021 The Meson development team
+# SPDX-license-identifier: Apache-2.0
+
+import typing as T
+
+from ...interpreterbase import (
+    ObjectHolder,
+    IterableObject,
+    MesonOperator,
+    typed_operator,
+    noKwargs,
+    noPosargs,
+    noArgsFlattening,
+    typed_pos_args,
+
+    TYPE_var,
+    TYPE_kwargs,
+
+    InvalidArguments,
+)
+
+if T.TYPE_CHECKING:
+    # Object holders need the actual interpreter
+    from ...interpreter import Interpreter
+
+class DictHolder(ObjectHolder[T.Dict[str, TYPE_var]], IterableObject):
+    def __init__(self, obj: T.Dict[str, TYPE_var], interpreter: 'Interpreter') -> None:
+        super().__init__(obj, interpreter)
+        self.methods.update({
+            'has_key': self.has_key_method,
+            'keys': self.keys_method,
+            'get': self.get_method,
+        })
+
+        self.trivial_operators.update({
+            # Arithmetic
+            MesonOperator.PLUS: (dict, lambda x: {**self.held_object, **x}),
+
+            # Comparison
+            MesonOperator.EQUALS: (dict, lambda x: self.held_object == x),
+            MesonOperator.NOT_EQUALS: (dict, lambda x: self.held_object != x),
+            MesonOperator.IN: (str, lambda x: x in self.held_object),
+            MesonOperator.NOT_IN: (str, lambda x: x not in self.held_object),
+        })
+
+        # Use actual methods for functions that require additional checks
+        self.operators.update({
+            MesonOperator.INDEX: self.op_index,
+        })
+
+    def display_name(self) -> str:
+        return 'dict'
+
+    def iter_tuple_size(self) -> int:
+        return 2
+
+    def iter_self(self) -> T.Iterator[T.Tuple[str, TYPE_var]]:
+        return iter(self.held_object.items())
+
+    def size(self) -> int:
+        return len(self.held_object)
+
+    @noKwargs
+    @typed_pos_args('dict.has_key', str)
+    def has_key_method(self, args: T.Tuple[str], kwargs: TYPE_kwargs) -> bool:
+        return args[0] in self.held_object
+
+    @noKwargs
+    @noPosargs
+    def keys_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> T.List[str]:
+        return sorted(self.held_object)
+
+    @noArgsFlattening
+    @noKwargs
+    @typed_pos_args('dict.get', str, optargs=[object])
+    def get_method(self, args: T.Tuple[str, T.Optional[TYPE_var]], kwargs: TYPE_kwargs) -> TYPE_var:
+        if args[0] in self.held_object:
+            return self.held_object[args[0]]
+        if args[1] is not None:
+            return args[1]
+        raise InvalidArguments(f'Key {args[0]!r} is not in the dictionary.')
+
+    @typed_operator(MesonOperator.INDEX, str)
+    def op_index(self, other: str) -> TYPE_var:
+        if other not in self.held_object:
+            raise InvalidArguments(f'Key {other} is not in the dictionary.')
+        return self.held_object[other]
diff --git a/mesonbuild/interpreterbase/__init__.py b/mesonbuild/interpreterbase/__init__.py
index 0375430..14462a2 100644
--- a/mesonbuild/interpreterbase/__init__.py
+++ b/mesonbuild/interpreterbase/__init__.py
@@ -16,6 +16,7 @@ __all__ = [
     'InterpreterObject',
     'MesonInterpreterObject',
     'ObjectHolder',
+    'IterableObject',
     'RangeHolder',
     'MutableInterpreterObject',
 
@@ -76,6 +77,7 @@ from .baseobjects import (
     InterpreterObject,
     MesonInterpreterObject,
     ObjectHolder,
+    IterableObject,
     RangeHolder,
     MutableInterpreterObject,
 
diff --git a/mesonbuild/interpreterbase/_unholder.py b/mesonbuild/interpreterbase/_unholder.py
index 221c52c..7157d17 100644
--- a/mesonbuild/interpreterbase/_unholder.py
+++ b/mesonbuild/interpreterbase/_unholder.py
@@ -16,14 +16,8 @@ from .baseobjects import InterpreterObject, MesonInterpreterObject, ObjectHolder
 from .exceptions import InvalidArguments
 from ..mesonlib import HoldableObject, MesonBugException
 
-import typing as T
-
-def _unholder(obj: T.Union[TYPE_var, InterpreterObject]) -> TYPE_var:
-    if isinstance(obj, list):
-        return [_unholder(x) for x in obj]
-    elif isinstance(obj, dict):
-        return {k: _unholder(v) for k, v in obj.items()}
-    elif isinstance(obj, ObjectHolder):
+def _unholder(obj: InterpreterObject) -> TYPE_var:
+    if isinstance(obj, ObjectHolder):
         assert isinstance(obj.held_object, HoldableTypes)
         return obj.held_object
     elif isinstance(obj, MesonInterpreterObject):
diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index caeb47b..3c7085b 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -20,6 +20,7 @@ from ..mesonlib import HoldableObject, MesonBugException
 import textwrap
 
 import typing as T
+from abc import ABCMeta
 
 if T.TYPE_CHECKING:
     # Object holders need the actual interpreter
@@ -54,7 +55,7 @@ class InterpreterObject:
         self.trivial_operators: T.Dict[
             MesonOperator,
             T.Tuple[
-                T.Type[T.Union[TYPE_var, T.Tuple[TYPE_var, ...]]],
+                T.Union[T.Type, T.Tuple[T.Type, ...]],
                 'OperatorCall'
             ]
         ] = {}
@@ -94,7 +95,7 @@ class InterpreterObject:
             if op[0] is None and other is not None:
                 raise MesonBugException(f'The unary operator `{operator.value}` of {self.display_name()} was passed the object {other} of type {type(other).__name__}')
             if op[0] is not None and not isinstance(other, op[0]):
-                raise InvalidArguments(f'The `{operator.value}` of {self.display_name()} does not accept objects of type {type(other).__name__} ({other})')
+                raise InvalidArguments(f'The `{operator.value}` operator of {self.display_name()} does not accept objects of type {type(other).__name__} ({other})')
             return op[1](other)
         if operator in self.operators:
             return self.operators[operator](other)
@@ -128,8 +129,8 @@ class MesonInterpreterObject(InterpreterObject):
 class MutableInterpreterObject:
     ''' Dummy class to mark the object type as mutable '''
 
-HoldableTypes = (HoldableObject, int, bool, str)
-TYPE_HoldableTypes = T.Union[HoldableObject, int, bool, str]
+HoldableTypes = (HoldableObject, int, bool, str, list, dict)
+TYPE_HoldableTypes = T.Union[TYPE_elementary, HoldableObject]
 InterpreterObjectTypeVar = T.TypeVar('InterpreterObjectTypeVar', bound=TYPE_HoldableTypes)
 
 class ObjectHolder(InterpreterObject, T.Generic[InterpreterObjectTypeVar]):
@@ -163,7 +164,20 @@ class ObjectHolder(InterpreterObject, T.Generic[InterpreterObjectTypeVar]):
     def __repr__(self) -> str:
         return f'<[{type(self).__name__}] holds [{type(self.held_object).__name__}]: {self.held_object!r}>'
 
-class RangeHolder(MesonInterpreterObject):
+class IterableObject(metaclass=ABCMeta):
+    '''Base class for all objects that can be iterated over in a foreach loop'''
+
+    def iter_tuple_size(self) -> T.Optional[int]:
+        '''Return the size of the tuple for each iteration. Returns None if only a single value is returned.'''
+        raise MesonBugException(f'iter_tuple_size not implemented for {self.__class__.__name__}')
+
+    def iter_self(self) -> T.Iterator[T.Union[TYPE_var, T.Tuple[TYPE_var, ...]]]:
+        raise MesonBugException(f'iter not implemented for {self.__class__.__name__}')
+
+    def size(self) -> int:
+        raise MesonBugException(f'size not implemented for {self.__class__.__name__}')
+
+class RangeHolder(MesonInterpreterObject, IterableObject):
     def __init__(self, start: int, stop: int, step: int, *, subproject: str) -> None:
         super().__init__(subproject=subproject)
         self.range = range(start, stop, step)
@@ -177,8 +191,11 @@ class RangeHolder(MesonInterpreterObject):
         except:
             raise InvalidArguments(f'Index {other} out of bounds of range.')
 
-    def __iter__(self) -> T.Iterator[int]:
+    def iter_tuple_size(self) -> None:
+        return None
+
+    def iter_self(self) -> T.Iterator[int]:
         return iter(self.range)
 
-    def __len__(self) -> int:
+    def size(self) -> int:
         return len(self.range)
diff --git a/mesonbuild/interpreterbase/interpreterbase.py b/mesonbuild/interpreterbase/interpreterbase.py
index 4b4b3c0..bc21951 100644
--- a/mesonbuild/interpreterbase/interpreterbase.py
+++ b/mesonbuild/interpreterbase/interpreterbase.py
@@ -24,9 +24,8 @@ from .baseobjects import (
     MutableInterpreterObject,
     InterpreterObjectTypeVar,
     ObjectHolder,
-    RangeHolder,
+    IterableObject,
 
-    TYPE_elementary,
     TYPE_var,
     TYPE_kwargs,
 
@@ -42,7 +41,7 @@ from .exceptions import (
     BreakRequest
 )
 
-from .decorators import FeatureNew, noKwargs
+from .decorators import FeatureNew
 from .disabler import Disabler, is_disabled
 from .helpers import default_resolve_key, flatten, resolve_second_level_holders
 from .operator import MesonOperator
@@ -51,7 +50,6 @@ from ._unholder import _unholder
 import os, copy, re, pathlib
 import typing as T
 import textwrap
-from functools import wraps
 
 if T.TYPE_CHECKING:
     # T.cast is not handled by flake8 to detect quoted annotation use
@@ -64,6 +62,8 @@ HolderMapType = T.Dict[
         T.Type[int],
         T.Type[bool],
         T.Type[str],
+        T.Type[list],
+        T.Type[dict],
     ],
     # For some reason, this has to be a callable and can't just be ObjectHolder[InterpreterObjectTypeVar]
     T.Callable[[InterpreterObjectTypeVar, 'Interpreter'], ObjectHolder[InterpreterObjectTypeVar]]
@@ -74,21 +74,7 @@ FunctionType = T.Dict[
     T.Callable[[mparser.BaseNode, T.List[TYPE_var], T.Dict[str, TYPE_var]], TYPE_var]
 ]
 
-__FN = T.TypeVar('__FN', bound=T.Callable[['InterpreterBase', T.Any], T.Union[TYPE_var, InterpreterObject]])
-def _holderify_result(types: T.Union[None, T.Type, T.Tuple[T.Type, ...]] = None) -> T.Callable[[__FN], __FN]:
-    def inner(f: __FN) -> __FN:
-        @wraps(f)
-        def wrapper(self: 'InterpreterBase', node: mparser.BaseNode) -> T.Union[TYPE_var, InterpreterObject]:
-            res = f(self, node)
-            if types is not None and not isinstance(res, types):
-                raise mesonlib.MesonBugException(f'Expected {types} but got object `{res}` of type {type(res).__name__}')
-            return self._holderify(res)
-        return T.cast(__FN, wrapper)
-    return inner
-
 class InterpreterBase:
-    elementary_types = (list, )
-
     def __init__(self, source_root: str, subdir: str, subproject: str):
         self.source_root = source_root
         self.funcs: FunctionType = {}
@@ -99,8 +85,7 @@ class InterpreterBase:
         self.subdir = subdir
         self.root_subdir = subdir
         self.subproject = subproject
-        # TODO: This should actually be more strict: T.Union[TYPE_elementary, InterpreterObject]
-        self.variables: T.Dict[str, T.Union[TYPE_var, InterpreterObject]] = {}
+        self.variables: T.Dict[str, InterpreterObject] = {}
         self.argument_depth = 0
         self.current_lineno = -1
         # Current node set during a function call. This can be used as location
@@ -190,7 +175,7 @@ class InterpreterBase:
                 raise e
             i += 1 # In THE FUTURE jump over blocks and stuff.
 
-    def evaluate_statement(self, cur: mparser.BaseNode) -> T.Optional[T.Union[TYPE_var, InterpreterObject]]:
+    def evaluate_statement(self, cur: mparser.BaseNode) -> T.Optional[InterpreterObject]:
         self.current_node = cur
         if isinstance(cur, mparser.FunctionNode):
             return self.function_call(cur)
@@ -238,20 +223,18 @@ class InterpreterBase:
             raise ContinueRequest()
         elif isinstance(cur, mparser.BreakNode):
             raise BreakRequest()
-        elif isinstance(cur, self.elementary_types):
-            return cur
         else:
             raise InvalidCode("Unknown statement.")
         return None
 
-    def evaluate_arraystatement(self, cur: mparser.ArrayNode) -> T.List[T.Union[TYPE_var, InterpreterObject]]:
+    def evaluate_arraystatement(self, cur: mparser.ArrayNode) -> InterpreterObject:
         (arguments, kwargs) = self.reduce_arguments(cur.args)
         if len(kwargs) > 0:
             raise InvalidCode('Keyword arguments are invalid in array construction.')
-        return arguments
+        return self._holderify([_unholder(x) for x in arguments])
 
     @FeatureNew('dict', '0.47.0')
-    def evaluate_dictstatement(self, cur: mparser.DictNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_dictstatement(self, cur: mparser.DictNode) -> InterpreterObject:
         def resolve_key(key: mparser.BaseNode) -> str:
             if not isinstance(key, mparser.StringNode):
                 FeatureNew.single_use('Dictionary entry using non literal key', '0.53.0', self.subproject)
@@ -261,17 +244,13 @@ class InterpreterBase:
             return str_key
         arguments, kwargs = self.reduce_arguments(cur.args, key_resolver=resolve_key, duplicate_key_error='Duplicate dictionary key: {}')
         assert not arguments
-        return kwargs
+        return self._holderify({k: _unholder(v) for k, v in kwargs.items()})
 
-    @_holderify_result((bool, Disabler))
-    def evaluate_notstatement(self, cur: mparser.NotNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_notstatement(self, cur: mparser.NotNode) -> InterpreterObject:
         v = self.evaluate_statement(cur.value)
         if isinstance(v, Disabler):
             return v
-        # TYPING TODO: Remove this check once `evaluate_statement` only returns InterpreterObjects
-        if not isinstance(v, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Argument to not ({v}) is not an InterpreterObject but {type(v).__name__}.')
-        return v.operator_call(MesonOperator.NOT, None)
+        return self._holderify(v.operator_call(MesonOperator.NOT, None))
 
     def evaluate_if(self, node: mparser.IfClauseNode) -> T.Optional[Disabler]:
         assert isinstance(node, mparser.IfClauseNode)
@@ -284,10 +263,10 @@ class InterpreterBase:
                 return result
             if not isinstance(result, InterpreterObject):
                 raise mesonlib.MesonBugException(f'Argument to not ({result}) is not an InterpreterObject but {type(result).__name__}.')
-            result = result.operator_call(MesonOperator.BOOL, None)
-            if not isinstance(result, bool):
+            res = result.operator_call(MesonOperator.BOOL, None)
+            if not isinstance(res, bool):
                 raise InvalidCode(f'If clause {result!r} does not evaluate to true or false.')
-            if result:
+            if res:
                 prev_meson_version = mesonlib.project_meson_versions[self.subproject]
                 if self.tmp_meson_version:
                     mesonlib.project_meson_versions[self.subproject] = self.tmp_meson_version
@@ -300,20 +279,7 @@ class InterpreterBase:
             self.evaluate_codeblock(node.elseblock)
         return None
 
-    def validate_comparison_types(self, val1: T.Any, val2: T.Any) -> bool:
-        if type(val1) != type(val2):
-            return False
-        return True
-
-    def _evaluate_in(self, val1: T.Any, val2: T.Any) -> bool:
-        if not isinstance(val1, (str, int, float, mesonlib.HoldableObject)):
-            raise InvalidArguments('lvalue of "in" operator must be a string, integer, float, or object')
-        if not isinstance(val2, (list, dict)):
-            raise InvalidArguments('rvalue of "in" operator must be an array or a dict')
-        return val1 in val2
-
-    @_holderify_result((bool, Disabler))
-    def evaluate_comparison(self, node: mparser.ComparisonNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_comparison(self, node: mparser.ComparisonNode) -> InterpreterObject:
         val1 = self.evaluate_statement(node.left)
         if isinstance(val1, Disabler):
             return val1
@@ -334,105 +300,42 @@ class InterpreterBase:
         }[node.ctype]
 
         # Check if the arguments should be reversed for simplicity (this essentially converts `in` to `contains`)
-        if operator in (MesonOperator.IN, MesonOperator.NOT_IN) and isinstance(val2, InterpreterObject):
-            return val2.operator_call(operator, _unholder(val1))
-
-        # Normal evaluation, with the same semantics
-        elif operator not in (MesonOperator.IN, MesonOperator.NOT_IN) and isinstance(val1, InterpreterObject):
-            return val1.operator_call(operator, _unholder(val2))
-
-        # OLD CODE, based on the builtin types -- remove once we have switched
-        # over to all ObjectHolders.
-
-        # Do not compare the ObjectHolders but the actual held objects
-        val1 = _unholder(val1)
-        val2 = _unholder(val2)
-        if node.ctype == 'in':
-            return self._evaluate_in(val1, val2)
-        elif node.ctype == 'notin':
-            return not self._evaluate_in(val1, val2)
-        valid = self.validate_comparison_types(val1, val2)
-        # Ordering comparisons of different types isn't allowed since PR #1810
-        # (0.41.0).  Since PR #2884 we also warn about equality comparisons of
-        # different types, which is now an error.
-        if not valid and (node.ctype == '==' or node.ctype == '!='):
-            raise InvalidArguments(textwrap.dedent(
-                f'''
-                    Trying to compare values of different types ({type(val1).__name__}, {type(val2).__name__}) using {node.ctype}.
-                    This was deprecated and undefined behavior previously and is as of 0.60.0 a hard error.
-                '''
-            ))
-        if node.ctype == '==':
-            return val1 == val2
-        elif node.ctype == '!=':
-            return val1 != val2
-        elif not valid:
-            raise InterpreterException(
-                'Values of different types ({}, {}) cannot be compared using {}.'.format(type(val1).__name__,
-                                                                                         type(val2).__name__,
-                                                                                         node.ctype))
-        elif not isinstance(val1, self.elementary_types):
-            raise InterpreterException('{} can only be compared for equality.'.format(getattr(node.left, 'value', '<ERROR>')))
-        elif not isinstance(val2, self.elementary_types):
-            raise InterpreterException('{} can only be compared for equality.'.format(getattr(node.right, 'value', '<ERROR>')))
-        # Use type: ignore because mypy will complain that we are comparing two Unions,
-        # but we actually guarantee earlier that both types are the same
-        elif node.ctype == '<':
-            return val1 < val2
-        elif node.ctype == '<=':
-            return val1 <= val2
-        elif node.ctype == '>':
-            return val1 > val2
-        elif node.ctype == '>=':
-            return val1 >= val2
-        else:
-            raise InvalidCode('You broke my compare eval.')
+        if operator in (MesonOperator.IN, MesonOperator.NOT_IN):
+            val1, val2 = val2, val1
 
-    @_holderify_result((bool, Disabler))
-    def evaluate_andstatement(self, cur: mparser.AndNode) -> T.Union[TYPE_var, InterpreterObject]:
+        return self._holderify(val1.operator_call(operator, _unholder(val2)))
+
+    def evaluate_andstatement(self, cur: mparser.AndNode) -> InterpreterObject:
         l = self.evaluate_statement(cur.left)
         if isinstance(l, Disabler):
             return l
-        if not isinstance(l, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Firtst argument to and ({l}) is not an InterpreterObject but {type(l).__name__}.')
         l_bool = l.operator_call(MesonOperator.BOOL, None)
         if not l_bool:
-            return l_bool
+            return self._holderify(l_bool)
         r = self.evaluate_statement(cur.right)
         if isinstance(r, Disabler):
             return r
-        if not isinstance(r, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Second argument to and ({r}) is not an InterpreterObject but {type(r).__name__}.')
-        return r.operator_call(MesonOperator.BOOL, None)
+        return self._holderify(r.operator_call(MesonOperator.BOOL, None))
 
-    @_holderify_result((bool, Disabler))
-    def evaluate_orstatement(self, cur: mparser.OrNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_orstatement(self, cur: mparser.OrNode) -> InterpreterObject:
         l = self.evaluate_statement(cur.left)
         if isinstance(l, Disabler):
             return l
-        if not isinstance(l, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Firtst argument to or ({l}) is not an InterpreterObject but {type(l).__name__}.')
         l_bool = l.operator_call(MesonOperator.BOOL, None)
         if l_bool:
-            return l_bool
+            return self._holderify(l_bool)
         r = self.evaluate_statement(cur.right)
         if isinstance(r, Disabler):
             return r
-        if not isinstance(r, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Second argument to ot ({r}) is not an InterpreterObject but {type(r).__name__}.')
-        return r.operator_call(MesonOperator.BOOL, None)
+        return self._holderify(r.operator_call(MesonOperator.BOOL, None))
 
-    @_holderify_result()
-    def evaluate_uminusstatement(self, cur: mparser.UMinusNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_uminusstatement(self, cur: mparser.UMinusNode) -> InterpreterObject:
         v = self.evaluate_statement(cur.value)
         if isinstance(v, Disabler):
             return v
-        # TYPING TODO: Remove this check once `evaluate_statement` only returns InterpreterObjects
-        if not isinstance(v, InterpreterObject):
-            raise InterpreterException(f'Argument to negation ({v}) is not an InterpreterObject but {type(v).__name__}.')
-        return v.operator_call(MesonOperator.UMINUS, None)
+        return self._holderify(v.operator_call(MesonOperator.UMINUS, None))
 
-    def evaluate_arithmeticstatement(self, cur: mparser.ArithmeticNode) -> T.Union[TYPE_var, InterpreterObject]:
+    def evaluate_arithmeticstatement(self, cur: mparser.ArithmeticNode) -> InterpreterObject:
         l = self.evaluate_statement(cur.left)
         if isinstance(l, Disabler):
             return l
@@ -440,53 +343,21 @@ class InterpreterBase:
         if isinstance(r, Disabler):
             return r
 
-        # New code based on InterpreterObjects
-        if isinstance(l, InterpreterObject):
-            mapping: T.Dict[str, MesonOperator] = {
-                'add': MesonOperator.PLUS,
-                'sub': MesonOperator.MINUS,
-                'mul': MesonOperator.TIMES,
-                'div': MesonOperator.DIV,
-                'mod': MesonOperator.MOD,
-            }
-            res = l.operator_call(mapping[cur.operation], _unholder(r))
-            return self._holderify(res)
-
-        # OLD CODE, based on the builtin types -- remove once we have switched
-        # over to all ObjectHolders.
-
-        if cur.operation == 'add':
-            if isinstance(l, dict) and isinstance(r, dict):
-                return {**l, **r}
-            try:
-                # MyPy error due to handling two Unions (we are catching all exceptions anyway)
-                return l + r  # type: ignore
-            except Exception as e:
-                raise InvalidCode('Invalid use of addition: ' + str(e))
-        elif cur.operation == 'sub':
-            if not isinstance(l, int) or not isinstance(r, int):
-                raise InvalidCode('Subtraction works only with integers.')
-            raise mesonlib.MesonBugException('The integer was not held by an ObjectHolder!')
-        elif cur.operation == 'mul':
-            if not isinstance(l, int) or not isinstance(r, int):
-                raise InvalidCode('Multiplication works only with integers.')
-            raise mesonlib.MesonBugException('The integer was not held by an ObjectHolder!')
-        elif cur.operation == 'div':
-            raise mesonlib.MesonBugException('The integer or string was not held by an ObjectHolder!')
-        elif cur.operation == 'mod':
-            if not isinstance(l, int) or not isinstance(r, int):
-                raise InvalidCode('Modulo works only with integers.')
-            raise mesonlib.MesonBugException('The integer was not held by an ObjectHolder!')
-        else:
-            raise InvalidCode('You broke me.')
-
-    def evaluate_ternary(self, node: mparser.TernaryNode) -> T.Union[TYPE_var, InterpreterObject]:
+        mapping: T.Dict[str, MesonOperator] = {
+            'add': MesonOperator.PLUS,
+            'sub': MesonOperator.MINUS,
+            'mul': MesonOperator.TIMES,
+            'div': MesonOperator.DIV,
+            'mod': MesonOperator.MOD,
+        }
+        res = l.operator_call(mapping[cur.operation], _unholder(r))
+        return self._holderify(res)
+
+    def evaluate_ternary(self, node: mparser.TernaryNode) -> T.Optional[InterpreterObject]:
         assert isinstance(node, mparser.TernaryNode)
         result = self.evaluate_statement(node.condition)
         if isinstance(result, Disabler):
             return result
-        if not isinstance(result, InterpreterObject):
-            raise mesonlib.MesonBugException(f'Ternary condition ({result}) is not an InterpreterObject but {type(result).__name__}.')
         result_bool = result.operator_call(MesonOperator.BOOL, None)
         if result_bool:
             return self.evaluate_statement(node.trueblock)
@@ -494,8 +365,7 @@ class InterpreterBase:
             return self.evaluate_statement(node.falseblock)
 
     @FeatureNew('format strings', '0.58.0')
-    @_holderify_result(str)
-    def evaluate_fstring(self, node: mparser.FormatStringNode) -> str:
+    def evaluate_fstring(self, node: mparser.FormatStringNode) -> InterpreterObject:
         assert isinstance(node, mparser.FormatStringNode)
 
         def replace(match: T.Match[str]) -> str:
@@ -510,38 +380,37 @@ class InterpreterBase:
             except KeyError:
                 raise InvalidCode(f'Identifier "{var}" does not name a variable.')
 
-        return re.sub(r'@([_a-zA-Z][_0-9a-zA-Z]*)@', replace, node.value)
+        res = re.sub(r'@([_a-zA-Z][_0-9a-zA-Z]*)@', replace, node.value)
+        return self._holderify(res)
 
     def evaluate_foreach(self, node: mparser.ForeachClauseNode) -> None:
         assert isinstance(node, mparser.ForeachClauseNode)
         items = self.evaluate_statement(node.items)
-
-        if isinstance(items, (list, RangeHolder)):
-            if len(node.varnames) != 1:
-                raise InvalidArguments('Foreach on array does not unpack')
-            varname = node.varnames[0]
-            for item in items:
-                self.set_variable(varname, self._holderify(item, permissive=True))
-                try:
-                    self.evaluate_codeblock(node.block)
-                except ContinueRequest:
-                    continue
-                except BreakRequest:
-                    break
-        elif isinstance(items, dict):
-            if len(node.varnames) != 2:
-                raise InvalidArguments('Foreach on dict unpacks key and value')
-            for key, value in sorted(items.items()):
-                self.set_variable(node.varnames[0], self._holderify(key))
-                self.set_variable(node.varnames[1], self._holderify(value, permissive=True))
-                try:
-                    self.evaluate_codeblock(node.block)
-                except ContinueRequest:
-                    continue
-                except BreakRequest:
-                    break
-        else:
-            raise InvalidArguments('Items of foreach loop must be an array or a dict')
+        if not isinstance(items, IterableObject):
+            raise InvalidArguments('Items of foreach loop do not support iterating')
+
+        tsize = items.iter_tuple_size()
+        if len(node.varnames) != (tsize or 1):
+            raise InvalidArguments(f'Foreach expects exactly {tsize or 1} variables for iterating over objects of type {items.display_name()}')
+
+        for i in items.iter_self():
+            if tsize is None:
+                if isinstance(i, tuple):
+                    raise mesonlib.MesonBugException(f'Iteration of {items} returned a tuple even though iter_tuple_size() is None')
+                self.set_variable(node.varnames[0], self._holderify(i))
+            else:
+                if not isinstance(i, tuple):
+                    raise mesonlib.MesonBugException(f'Iteration of {items} did not return a tuple even though iter_tuple_size() is {tsize}')
+                if len(i) != tsize:
+                    raise mesonlib.MesonBugException(f'Iteration of {items} did not return a tuple even though iter_tuple_size() is {tsize}')
+                for j in range(tsize):
+                    self.set_variable(node.varnames[j], self._holderify(i[j]))
+            try:
+                self.evaluate_codeblock(node.block)
+            except ContinueRequest:
+                continue
+            except BreakRequest:
+                break
 
     def evaluate_plusassign(self, node: mparser.PlusAssignmentNode) -> None:
         assert isinstance(node, mparser.PlusAssignmentNode)
@@ -551,69 +420,21 @@ class InterpreterBase:
         # Remember that all variables are immutable. We must always create a
         # full new variable and then assign it.
         old_variable = self.get_variable(varname)
-        # TYPING TODO: This should only be InterpreterObject in the future
-        new_value: T.Union[None, TYPE_var, InterpreterObject] = None
-        if isinstance(old_variable, str):
-            if not isinstance(addition, str):
-                raise InvalidArguments('The += operator requires a string on the right hand side if the variable on the left is a string')
-            new_value = old_variable + addition
-        elif isinstance(old_variable, list):
-            if isinstance(addition, list):
-                new_value = old_variable + addition
-            else:
-                new_value = old_variable + [addition]
-        elif isinstance(old_variable, dict):
-            if not isinstance(addition, dict):
-                raise InvalidArguments('The += operator requires a dict on the right hand side if the variable on the left is a dict')
-            new_value = {**old_variable, **addition}
-        elif isinstance(old_variable, InterpreterObject):
-            # TODO: don't make _unholder permissive
-            new_value = self._holderify(old_variable.operator_call(MesonOperator.PLUS, _unholder(addition)))
-        # Add other data types here.
-        else:
-            raise InvalidArguments('The += operator currently only works with arrays, dicts, strings or ints')
+        new_value = self._holderify(old_variable.operator_call(MesonOperator.PLUS, _unholder(addition)))
         self.set_variable(varname, new_value)
 
-    def evaluate_indexing(self, node: mparser.IndexNode) -> T.Union[TYPE_elementary, InterpreterObject]:
+    def evaluate_indexing(self, node: mparser.IndexNode) -> InterpreterObject:
         assert isinstance(node, mparser.IndexNode)
         iobject = self.evaluate_statement(node.iobject)
         if isinstance(iobject, Disabler):
             return iobject
         index = _unholder(self.evaluate_statement(node.index))
 
-        if isinstance(iobject, InterpreterObject):
-            return self._holderify(iobject.operator_call(MesonOperator.INDEX, index))
-        if not hasattr(iobject, '__getitem__'):
-            raise InterpreterException(
-                'Tried to index an object that doesn\'t support indexing.')
-        if isinstance(iobject, dict):
-            if not isinstance(index, str):
-                raise InterpreterException('Key is not a string')
-            try:
-                # The cast is required because we don't have recursive types...
-                return T.cast(T.Union[TYPE_elementary, InterpreterObject], iobject[index])
-            except KeyError:
-                raise InterpreterException('Key %s is not in dict' % index)
-        else:
-            if not isinstance(index, int):
-                raise InterpreterException('Index value is not an integer.')
-            try:
-                # Ignore the MyPy error, since we don't know all indexable types here
-                # and we handle non indexable types with an exception
-                # TODO maybe find a better solution
-                res = iobject[index]  # type: ignore
-                # Only holderify if we are dealing with `InterpreterObject`, since raw
-                # lists already store ObjectHolders
-                if isinstance(iobject, InterpreterObject):
-                    return self._holderify(res)
-                else:
-                    return res
-            except IndexError:
-                # We are already checking for the existence of __getitem__, so this should be save
-                raise InterpreterException('Index %d out of bounds of array of size %d.' % (index, len(iobject)))  # type: ignore
+        if iobject is None:
+            raise InterpreterException('Tried to evaluate indexing on None')
+        return self._holderify(iobject.operator_call(MesonOperator.INDEX, index))
 
-    @_holderify_result()
-    def function_call(self, node: mparser.FunctionNode) -> T.Optional[T.Union[TYPE_var, InterpreterObject]]:
+    def function_call(self, node: mparser.FunctionNode) -> T.Optional[InterpreterObject]:
         func_name = node.func_name
         (h_posargs, h_kwargs) = self.reduce_arguments(node.args)
         (posargs, kwargs) = self._unholder_args(h_posargs, h_kwargs)
@@ -626,14 +447,15 @@ class InterpreterBase:
                 func_args = flatten(posargs)
             if not getattr(func, 'no-second-level-holder-flattening', False):
                 func_args, kwargs = resolve_second_level_holders(func_args, kwargs)
-            return func(node, func_args, kwargs)
+            res = func(node, func_args, kwargs)
+            return self._holderify(res) if res is not None else None
         else:
             self.unknown_function_called(func_name)
             return None
 
-    def method_call(self, node: mparser.MethodNode) -> T.Optional[T.Union[TYPE_var, InterpreterObject]]:
+    def method_call(self, node: mparser.MethodNode) -> T.Optional[InterpreterObject]:
         invokable = node.source_object
-        obj: T.Union[TYPE_var, InterpreterObject]
+        obj: T.Optional[InterpreterObject]
         if isinstance(invokable, mparser.IdNode):
             object_name = invokable.value
             obj = self.get_variable(object_name)
@@ -644,16 +466,6 @@ class InterpreterBase:
         (args, kwargs) = self._unholder_args(h_args, h_kwargs)
         if is_disabled(args, kwargs):
             return Disabler()
-        if isinstance(obj, str):
-            raise mesonlib.MesonBugException('Strings are now wrapped in object holders!')
-        if isinstance(obj, bool):
-            raise mesonlib.MesonBugException('Booleans are now wrapped in object holders!')
-        if isinstance(obj, int):
-            raise mesonlib.MesonBugException('Integers are now wrapped in object holders!')
-        if isinstance(obj, list):
-            return self.array_method_call(obj, method_name, args, kwargs)
-        if isinstance(obj, dict):
-            return self.dict_method_call(obj, method_name, args, kwargs)
         if not isinstance(obj, InterpreterObject):
             raise InvalidArguments('Variable "%s" is not callable.' % object_name)
         # TODO: InterpreterBase **really** shouldn't be in charge of checking this
@@ -663,17 +475,11 @@ class InterpreterBase:
             elif not isinstance(obj, Disabler):
                 raise InvalidArguments(f'Invalid operation "extract_objects" on variable "{object_name}" of type {type(obj).__name__}')
         obj.current_node = node
-        return self._holderify(obj.method_call(method_name, args, kwargs))
+        res = obj.method_call(method_name, args, kwargs)
+        return self._holderify(res) if res is not None else None
 
-    def _holderify(self, res: T.Union[TYPE_var, InterpreterObject, None], *, permissive: bool = False) -> T.Union[TYPE_elementary, InterpreterObject]:
-        # TODO: remove `permissive` once all primitives are ObjectHolders
-        if res is None:
-            return None
-        elif isinstance(res, list):
-            return [self._holderify(x, permissive=permissive) for x in res]
-        elif isinstance(res, dict):
-            return {k: self._holderify(v, permissive=permissive) for k, v in res.items()}
-        elif isinstance(res, HoldableTypes):
+    def _holderify(self, res: T.Union[TYPE_var, InterpreterObject]) -> InterpreterObject:
+        if isinstance(res, HoldableTypes):
             # Always check for an exact match first.
             cls = self.holder_map.get(type(res), None)
             if cls is not None:
@@ -686,136 +492,42 @@ class InterpreterBase:
                     return cls(res, T.cast('Interpreter', self))
             raise mesonlib.MesonBugException(f'Object {res} of type {type(res).__name__} is neither in self.holder_map nor self.bound_holder_map.')
         elif isinstance(res, ObjectHolder):
-            if permissive:
-                return res
             raise mesonlib.MesonBugException(f'Returned object {res} of type {type(res).__name__} is an object holder.')
         elif isinstance(res, MesonInterpreterObject):
             return res
         raise mesonlib.MesonBugException(f'Unknown returned object {res} of type {type(res).__name__} in the parameters.')
 
     def _unholder_args(self,
-                       args: T.List[T.Union[TYPE_var, InterpreterObject]],
-                       kwargs: T.Dict[str, T.Union[TYPE_var, InterpreterObject]]) -> T.Tuple[T.List[TYPE_var], TYPE_kwargs]:
+                       args: T.List[InterpreterObject],
+                       kwargs: T.Dict[str, InterpreterObject]) -> T.Tuple[T.List[TYPE_var], TYPE_kwargs]:
         return [_unholder(x) for x in args], {k: _unholder(v) for k, v in kwargs.items()}
 
-    @staticmethod
-    def _get_one_string_posarg(posargs: T.List[TYPE_var], method_name: str) -> str:
-        if len(posargs) > 1:
-            raise InterpreterException(f'{method_name}() must have zero or one arguments')
-        elif len(posargs) == 1:
-            s = posargs[0]
-            if not isinstance(s, str):
-                raise InterpreterException(f'{method_name}() argument must be a string')
-            return s
-        return None
-
     def unknown_function_called(self, func_name: str) -> None:
         raise InvalidCode('Unknown function "%s".' % func_name)
 
-    @noKwargs
-    def array_method_call(self,
-                          obj: T.List[T.Union[TYPE_elementary, InterpreterObject]],
-                          method_name: str,
-                          posargs: T.List[TYPE_var],
-                          kwargs: TYPE_kwargs) -> T.Union[TYPE_var, InterpreterObject]:
-        if method_name == 'contains':
-            def check_contains(el: T.List[TYPE_var]) -> bool:
-                if len(posargs) != 1:
-                    raise InterpreterException('Contains method takes exactly one argument.')
-                item = posargs[0]
-                for element in el:
-                    if isinstance(element, list):
-                        found = check_contains(element)
-                        if found:
-                            return True
-                    if element == item:
-                        return True
-                return False
-            return self._holderify(check_contains([_unholder(x) for x in obj]))
-        elif method_name == 'length':
-            return self._holderify(len(obj))
-        elif method_name == 'get':
-            index = posargs[0]
-            fallback = None
-            if len(posargs) == 2:
-                fallback = self._holderify(posargs[1])
-            elif len(posargs) > 2:
-                m = 'Array method \'get()\' only takes two arguments: the ' \
-                    'index and an optional fallback value if the index is ' \
-                    'out of range.'
-                raise InvalidArguments(m)
-            if not isinstance(index, int):
-                raise InvalidArguments('Array index must be a number.')
-            if index < -len(obj) or index >= len(obj):
-                if fallback is None:
-                    m = 'Array index {!r} is out of bounds for array of size {!r}.'
-                    raise InvalidArguments(m.format(index, len(obj)))
-                if isinstance(fallback, mparser.BaseNode):
-                    return self.evaluate_statement(fallback)
-                return fallback
-            return obj[index]
-        raise InterpreterException(f'Arrays do not have a method called {method_name!r}.')
-
-    @noKwargs
-    def dict_method_call(self,
-                         obj: T.Dict[str, T.Union[TYPE_elementary, InterpreterObject]],
-                         method_name: str,
-                         posargs: T.List[TYPE_var],
-                         kwargs: TYPE_kwargs) -> T.Union[TYPE_var, InterpreterObject]:
-        if method_name in ('has_key', 'get'):
-            if method_name == 'has_key':
-                if len(posargs) != 1:
-                    raise InterpreterException('has_key() takes exactly one argument.')
-            else:
-                if len(posargs) not in (1, 2):
-                    raise InterpreterException('get() takes one or two arguments.')
-
-            key = posargs[0]
-            if not isinstance(key, (str)):
-                raise InvalidArguments('Dictionary key must be a string.')
-
-            has_key = key in obj
-
-            if method_name == 'has_key':
-                return self._holderify(has_key)
-
-            if has_key:
-                return obj[key]
-
-            if len(posargs) == 2:
-                fallback = self._holderify(posargs[1])
-                if isinstance(fallback, mparser.BaseNode):
-                    return self.evaluate_statement(fallback)
-                return fallback
-
-            raise InterpreterException(f'Key {key!r} is not in the dictionary.')
-
-        if method_name == 'keys':
-            if len(posargs) != 0:
-                raise InterpreterException('keys() takes no arguments.')
-            return sorted(obj.keys())
-
-        raise InterpreterException('Dictionaries do not have a method called "%s".' % method_name)
-
     def reduce_arguments(
                 self,
                 args: mparser.ArgumentNode,
                 key_resolver: T.Callable[[mparser.BaseNode], str] = default_resolve_key,
                 duplicate_key_error: T.Optional[str] = None,
             ) -> T.Tuple[
-                T.List[T.Union[TYPE_var, InterpreterObject]],
-                T.Dict[str, T.Union[TYPE_var, InterpreterObject]]
+                T.List[InterpreterObject],
+                T.Dict[str, InterpreterObject]
             ]:
         assert isinstance(args, mparser.ArgumentNode)
         if args.incorrect_order():
             raise InvalidArguments('All keyword arguments must be after positional arguments.')
         self.argument_depth += 1
-        reduced_pos: T.List[T.Union[TYPE_var, InterpreterObject]] = [self.evaluate_statement(arg) for arg in args.arguments]
-        reduced_kw: T.Dict[str, T.Union[TYPE_var, InterpreterObject]] = {}
+        reduced_pos = [self.evaluate_statement(arg) for arg in args.arguments]
+        if any(x is None for x in reduced_pos):
+            raise InvalidArguments(f'At least one value in the arguments is void.')
+        reduced_kw: T.Dict[str, InterpreterObject] = {}
         for key, val in args.kwargs.items():
             reduced_key = key_resolver(key)
             assert isinstance(val, mparser.BaseNode)
             reduced_val = self.evaluate_statement(val)
+            if reduced_val is None:
+                raise InvalidArguments(f'Value of key {reduced_key} is void.')
             if duplicate_key_error and reduced_key in reduced_kw:
                 raise InvalidArguments(duplicate_key_error.format(reduced_key))
             reduced_kw[reduced_key] = reduced_val
@@ -823,10 +535,10 @@ class InterpreterBase:
         final_kw = self.expand_default_kwargs(reduced_kw)
         return reduced_pos, final_kw
 
-    def expand_default_kwargs(self, kwargs: T.Dict[str, T.Union[TYPE_var, InterpreterObject]]) -> T.Dict[str, T.Union[TYPE_var, InterpreterObject]]:
+    def expand_default_kwargs(self, kwargs: T.Dict[str, T.Optional[InterpreterObject]]) -> T.Dict[str, T.Optional[InterpreterObject]]:
         if 'kwargs' not in kwargs:
             return kwargs
-        to_expand = kwargs.pop('kwargs')
+        to_expand = _unholder(kwargs.pop('kwargs'))
         if not isinstance(to_expand, dict):
             raise InterpreterException('Value of "kwargs" must be dictionary.')
         if 'kwargs' in to_expand:
@@ -834,20 +546,20 @@ class InterpreterBase:
         for k, v in to_expand.items():
             if k in kwargs:
                 raise InterpreterException(f'Entry "{k}" defined both as a keyword argument and in a "kwarg" entry.')
-            kwargs[k] = v
+            kwargs[k] = self._holderify(v)
         return kwargs
 
     def assignment(self, node: mparser.AssignmentNode) -> None:
         assert isinstance(node, mparser.AssignmentNode)
         if self.argument_depth != 0:
-            raise InvalidArguments('''Tried to assign values inside an argument list.
-To specify a keyword argument, use : instead of =.''')
+            raise InvalidArguments(textwrap.dedent('''\
+                Tried to assign values inside an argument list.
+                To specify a keyword argument, use : instead of =.
+            '''))
         var_name = node.var_name
         if not isinstance(var_name, str):
             raise InvalidArguments('Tried to assign value to a non-variable.')
         value = self.evaluate_statement(node.value)
-        if not self.is_assignable(value):
-            raise InvalidCode(f'Tried to assign the invalid value "{value}" of type {type(value).__name__} to variable.')
         # For mutable objects we need to make a copy on assignment
         if isinstance(value, MutableInterpreterObject):
             value = copy.deepcopy(value)
@@ -860,36 +572,23 @@ To specify a keyword argument, use : instead of =.''')
         if holderify:
             variable = self._holderify(variable)
         else:
-            # Ensure that we are never storing a HoldableObject
-            def check(x: T.Union[TYPE_var, InterpreterObject]) -> None:
-                if isinstance(x, mesonlib.HoldableObject):
-                    raise mesonlib.MesonBugException(f'set_variable in InterpreterBase called with a HoldableObject {x} of type {type(x).__name__}')
-                elif isinstance(x, list):
-                    for y in x:
-                        check(y)
-                elif isinstance(x, dict):
-                    for v in x.values():
-                        check(v)
-            check(variable)
+            # Ensure that we are always storing ObjectHolders
+            if not isinstance(variable, InterpreterObject):
+                raise mesonlib.MesonBugException(f'set_variable in InterpreterBase called with a non InterpreterObject {variable} of type {type(variable).__name__}')
         if not isinstance(varname, str):
             raise InvalidCode('First argument to set_variable must be a string.')
-        if not self.is_assignable(variable):
-            raise InvalidCode(f'Assigned value "{variable}" of type {type(variable).__name__} is not an assignable type.')
         if re.match('[_a-zA-Z][_0-9a-zA-Z]*$', varname) is None:
             raise InvalidCode('Invalid variable name: ' + varname)
         if varname in self.builtin:
             raise InvalidCode('Tried to overwrite internal variable "%s"' % varname)
         self.variables[varname] = variable
 
-    def get_variable(self, varname: str) -> T.Union[TYPE_var, InterpreterObject]:
+    def get_variable(self, varname: str) -> InterpreterObject:
         if varname in self.builtin:
             return self.builtin[varname]
         if varname in self.variables:
             return self.variables[varname]
         raise InvalidCode('Unknown variable "%s".' % varname)
 
-    def is_assignable(self, value: T.Any) -> bool:
-        return isinstance(value, (InterpreterObject, list, dict))
-
     def validate_extraction(self, buildtarget: mesonlib.HoldableObject) -> None:
         raise InterpreterException('validate_extraction is not implemented in this context (please file a bug)')