rewriter: Accept UnknownValue() in more places

Fixes #14840

4 files changed, 40 insertions, 6 deletions

diff --git a/mesonbuild/ast/interpreter.py b/mesonbuild/ast/interpreter.py
index 62c4839..86b6442 100644
--- a/mesonbuild/ast/interpreter.py
+++ b/mesonbuild/ast/interpreter.py
@@ -692,8 +692,11 @@ class AstInterpreter(InterpreterBase):
     def func_get_variable(self, node: BaseNode, args: T.List[TYPE_var], kwargs: T.Dict[str, TYPE_var]) -> T.Any:
         assert isinstance(node, FunctionNode)
         var_name = args[0]
-        assert isinstance(var_name, str)
-        val = self.get_cur_value(var_name)
+        if isinstance(var_name, UnknownValue):
+            val: T.Union[UnknownValue, BaseNode] = UnknownValue()
+        else:
+            assert isinstance(var_name, str)
+            val = self.get_cur_value(var_name)
         self.dataflow_dag.add_edge(val, node)
         return val
 
diff --git a/mesonbuild/ast/introspection.py b/mesonbuild/ast/introspection.py
index decce4b..6ffc6ad 100644
--- a/mesonbuild/ast/introspection.py
+++ b/mesonbuild/ast/introspection.py
@@ -203,9 +203,11 @@ class IntrospectionInterpreter(AstInterpreter):
         has_fallback = 'fallback' in kwargs
         required = kwargs.get('required', True)
         version = kwargs.get('version', [])
-        if not isinstance(version, UnknownValue):
-            if not isinstance(version, list):
-                version = [version]
+        if not isinstance(version, list):
+            version = [version]
+        if any(isinstance(el, UnknownValue) for el in version):
+            version = UnknownValue()
+        else:
             assert all(isinstance(el, str) for el in version)
             version = T.cast(T.List[str], version)
         assert isinstance(required, (bool, UnknownValue))
diff --git a/test cases/unit/56 introspection/meson.build b/test cases/unit/56 introspection/meson.build
index 568d5cc..c7856fe 100644
--- a/test cases/unit/56 introspection/meson.build
+++ b/test cases/unit/56 introspection/meson.build
@@ -58,6 +58,21 @@ test('test case 1', t1)
 test('test case 2', t2, depends: t3)
 benchmark('benchmark 1', t3, args: [cus1, cus2, cus3])
 
+### BEGIN: Stuff to test the handling of `UnknownValue`
+var_1 = 1
+var_2 = 2
+unknown_var = 'var_1'
+if host_machine.system() == 'windows'
+    unknown_var = 'var_2'
+endif
+# The IntrospectionInterpreter can't know the value of unknown_var and use the `UnknownValue` class.
+
+message(get_variable(unknown_var))
+
+dependency(unknown_var, version: [unknown_var], required: false)
+dependency(unknown_var, version: unknown_var, required: false)
+### END: Stuff to test the handling of `UnknownValue`
+
 ### Stuff to test the AST JSON printer
 foreach x : ['a', 'b', 'c']
   if x == 'a'
diff --git a/unittests/allplatformstests.py b/unittests/allplatformstests.py
index f158be7..04cbfc6 100644
--- a/unittests/allplatformstests.py
+++ b/unittests/allplatformstests.py
@@ -3834,7 +3834,21 @@ class AllPlatformTests(BasePlatformTests):
                 'version': ['>=1.0.0', '<=99.9.9'],
                 'has_fallback': True,
                 'conditional': True
-            }
+            },
+            {
+                'conditional': False,
+                'has_fallback': False,
+                'name': 'unknown',
+                'required': False,
+                'version': 'unknown'
+            },
+            {
+                'conditional': False,
+                'has_fallback': False,
+                'name': 'unknown',
+                'required': False,
+                'version': 'unknown'
+            },
         ]
         self.maxDiff = None
         self.assertListEqual(res_nb, expected)