Use the fallback for ModuleSpec early if it can never be resolved (#20167)

Fixes #18237.

Author: sterliakov

mypy/semanal.py

@@ -750,7 +750,15 @@ def add_implicit_module_attrs(self, file_node: MypyFile) -> None:
                 else:
                     inst = self.named_type_or_none("importlib.machinery.ModuleSpec")
                 if inst is None:
-                    if self.final_iteration:
+                    if (
+                        self.final_iteration
+                        or self.options.clone_for_module("importlib.machinery").follow_imports
+                        == "skip"
+                    ):
+                        # If we are not allowed to resolve imports from `importlib.machinery`,
+                        # ModuleSpec will not be available at any iteration.
+                        # Use the fallback earlier.
+                        # (see https://github.com/python/mypy/issues/18237)
                         inst = self.named_type_or_none("builtins.object")
                         assert inst is not None, "Cannot find builtins.object"
                     else:

test-data/unit/cmdline.test

@@ -1393,3 +1393,21 @@ Ts = TypeVarTuple("Ts")
 Warning: TypeVarTuple is already enabled by default
 Warning: Unpack is already enabled by default
 == Return code: 0
+
+[case testImportlibImportCannotBeResolved]
+# cmd: mypy a.py
+# This test is here because it needs use_builtins_fixtures off.
+
+[file a.py]
+from typing import NamedTuple
+
+class CodecKey(NamedTuple):
+    def foo(self) -> "CodecKey":
+        ...
+
+[file mypy.ini]
+\[mypy]
+
+\[mypy-importlib.*]
+follow_imports = skip
+follow_imports_for_stubs = True