Fix type of forward reference to a decorated class method (#4486)

The case wasn't properly handled before when accessing through a type
object -- the type of the forward reference defaulted to `Any`.

The fix doesn't work at module top level since module top levels can't
be deferred, but the reference would generally fail at runtime anyway.

Fixes #4485.

Author: JukkaL

mypy/checkmember.py

@@ -467,8 +467,12 @@ def analyze_class_attribute_access(itype: Instance,
         return builtin_type('types.ModuleType')
 
     if is_decorated:
-        # TODO: Return type of decorated function. This is quick hack to work around #998.
-        return AnyType(TypeOfAny.special_form)
+        assert isinstance(node.node, Decorator)
+        if node.node.type:
+            return node.node.type
+        else:
+            not_ready_callback(name, context)
+            return AnyType(TypeOfAny.from_error)
     else:
         return function_type(cast(FuncBase, node.node), builtin_type('builtins.function'))
 

test-data/unit/check-inference.test

@@ -1786,6 +1786,31 @@ y = 0
 tmp/m.py:2: error: "int" not callable
 main:3: error: "int" not callable
 
+[case testForwardReferenceToDecoratedClassMethod]
+from typing import TypeVar, Callable
+
+T = TypeVar('T')
+def dec() -> Callable[[T], T]: pass
+
+A.g  # E: Cannot determine type of 'g'
+
+class A:
+    @classmethod
+    def f(cls) -> None:
+        reveal_type(cls.g)  # E: Revealed type is 'def (x: builtins.str)'
+
+    @classmethod
+    @dec()
+    def g(cls, x: str) -> None:
+        pass
+
+    @classmethod
+    def h(cls) -> None:
+        reveal_type(cls.g)  # E: Revealed type is 'def (x: builtins.str)'
+
+reveal_type(A.g)  # E: Revealed type is 'def (x: builtins.str)'
+[builtins fixtures/classmethod.pyi]
+
 
 -- Tests for special cases of unification
 -- --------------------------------------