functools plugin: Use get_proper_type before isinstance checks in lru_cache callback

Prevents mypy error about applying isinstance to unexpanded types by ensuring
first_arg_type is expanded before checking for

Author: machin0r

mypy/plugins/functools.py

@@ -410,9 +410,9 @@ def functools_lru_cache_callback(ctx: mypy.plugin.FunctionContext) -> Type:
         first_arg_type = ctx.arg_types[0][0]
 
         # Explicitly check that this is NOT a literal or other non-function type
-        from mypy.types import Instance, LiteralType
-
-        if isinstance(first_arg_type, (LiteralType, Instance)):
+        from mypy.types import LiteralType, Instance
+        proper_first_arg_type = get_proper_type(first_arg_type)
+        if isinstance(proper_first_arg_type, (LiteralType, Instance)):
             # This is likely maxsize=128 or similar - let MyPy handle it
             return ctx.default_return_type