use is_same_type when resolving decorators in suggestions

mypy/suggestions.py

@@ -58,6 +58,7 @@
 from mypy.plugin import FunctionContext, MethodContext, Plugin
 from mypy.server.update import FineGrainedBuildManager
 from mypy.state import state
+from mypy.subtypes import is_same_type
 from mypy.traverser import TraverserVisitor
 from mypy.typeops import bind_self, make_simplified_union
 from mypy.types import (
@@ -671,7 +672,7 @@ def extract_from_decorator(self, node: Decorator) -> FuncDef | None:
                 if not (
                     len(ct.arg_types) == 1
                     and _arg_accepts_function(get_proper_type(ct.arg_types[0]))
-                    and ct.arg_types[0] == ct.ret_type
+                    and is_same_type(ct.arg_types[0], ct.ret_type)
                 ):
                     return None
 

test-data/unit/fine-grained-suggest.test

@@ -683,6 +683,33 @@ f('hi')
 (str) -> str
 ==
 
+
+[case testSuggestInferFuncDecorator7]
+# suggest: foo.f
+[file foo.py]
+from __future__ import annotations
+
+from typing import Callable, Protocol, TypeVar
+from typing_extensions import ParamSpec, TypeAlias
+
+P = ParamSpec('P')
+R = TypeVar('R')
+Alias: TypeAlias = Callable[P, R]
+
+def dec(f: Callable[P, R]) -> Alias[P, R]:
+    return f
+
+@dec
+def f(x):
+    return x
+
+f('hi')
+
+[builtins fixtures/isinstancelist.pyi]
+[out]
+(str) -> str
+==
+
 [case testSuggestFlexAny1]
 # suggest: --flex-any=0.4 m.foo
 # suggest: --flex-any=0.7 m.foo