Narrow Callable generic return types (#20868)

Requires #20864 to be merged to work
Fixes #20628

Author: hauntsaninja

mypy/meet.py

@@ -229,6 +229,15 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
         ):
             return original_declared
         return meet_types(original_declared, original_narrowed)
+    elif (
+        isinstance(declared, CallableType)
+        and isinstance(narrowed, CallableType)
+        and has_type_vars(declared.ret_type)
+    ):
+        return narrowed.copy_modified(
+            ret_type=narrow_declared_type(declared.ret_type, narrowed.ret_type)
+        )
+
     return original_narrowed
 
 

test-data/unit/check-narrowing.test

@@ -3912,3 +3912,26 @@ def get_owner(uid: UserId, tid: TeamId):
             reveal_type(INVALID)  # N: Revealed type is "builtins.int"
             return None
 [builtins fixtures/primitives.pyi]
+
+[case testNarrowCallableTypeVarByEquality]
+from typing import Callable, TypeVar
+
+T = TypeVar("T")
+
+def remove(path: str) -> None: ...
+def unlink(path: str) -> None: ...
+
+def f1(func: Callable[..., T], arg: str) -> T:
+    if func == remove:
+        reveal_type(func)  # N: Revealed type is "def (path: builtins.str) -> T`-1"
+        reveal_type(func(arg))  # N: Revealed type is "T`-1"
+        return func(arg)
+    return func(arg)
+
+def f2(func: Callable[..., T], arg: str) -> T:
+    if func in [unlink, remove]:
+        reveal_type(func)  # N: Revealed type is "def (path: builtins.str) -> T`-1"
+        reveal_type(func(arg))  # N: Revealed type is "T`-1"
+        return func(arg)
+    return func(arg)
+[builtins fixtures/primitives.pyi]