refactor: Refactor BothLibraries logic

This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.

1 files changed, 3 insertions, 1 deletions

diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index d82aad2..e65e4cb 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -14,7 +14,7 @@
 
 from .. import mparser
 from .exceptions import InvalidCode
-from .helpers import flatten
+from .helpers import flatten, resolve_second_level_holders
 from ..mesonlib import HoldableObject
 
 import typing as T
@@ -57,6 +57,8 @@ class InterpreterObject:
             method = self.methods[method_name]
             if not getattr(method, 'no-args-flattening', False):
                 args = flatten(args)
+            if not getattr(method, 'no-second-level-holder-flattening', False):
+                args, kwargs = resolve_second_level_holders(args, kwargs)
             return method(args, kwargs)
         raise InvalidCode(f'Unknown method "{method_name}" in object {self} of type {type(self).__name__}.')