Replace "try to extract type from the first overload" with simply using the underlying type

Author: sterliakov

mypy/checker.py

@@ -7691,9 +7691,8 @@ def get_isinstance_type(self, expr: Expression) -> list[TypeRange] | None:
         types: list[TypeRange] = []
         for typ in all_types:
             if isinstance(typ, FunctionLike) and typ.is_type_obj():
-                # Type variables may be present -- erase them, which is the best
-                # we can do (outside disallowing them here).
-                erased_type = erase_typevars(typ.items[0].ret_type)
+                # If a type is generic, `isinstance` can only narrow its variables to Any.
+                erased_type = fill_typevars_with_any(typ.type_object())
                 types.append(TypeRange(erased_type, is_upper_bound=False))
             elif isinstance(typ, TypeType):
                 # Type[A] means "any type that is a subtype of A" rather than "precisely type A"

test-data/unit/check-narrowing.test

@@ -2463,3 +2463,50 @@ def test(x: T) -> T:
     reveal_type(x.x)  # N: Revealed type is "builtins.str"
     return x
 [builtins fixtures/isinstance.pyi]
+
+[case testIsinstanceNarrowingWithSelfTypes]
+from typing import Generic, TypeVar, overload
+
+T = TypeVar("T")
+
+class A(Generic[T]):
+    def __init__(self: A[int]) -> None:
+        pass
+
+def check_a(obj: "A[T] | str") -> None:
+    reveal_type(obj)  # N: Revealed type is "Union[__main__.A[T`-1], builtins.str]"
+    if isinstance(obj, A):
+        reveal_type(obj)  # N: Revealed type is "__main__.A[T`-1]"
+    else:
+        reveal_type(obj)  # N: Revealed type is "builtins.str"
+
+class B(Generic[T]):
+    @overload
+    def __init__(self, x: T) -> None: ...
+    @overload
+    def __init__(self: B[int]) -> None: ...
+    def __init__(self, x: "T | None" = None) -> None:
+        pass
+
+def check_b(obj: "B[T] | str") -> None:
+    reveal_type(obj)  # N: Revealed type is "Union[__main__.B[T`-1], builtins.str]"
+    if isinstance(obj, B):
+        reveal_type(obj)  # N: Revealed type is "__main__.B[T`-1]"
+    else:
+        reveal_type(obj)  # N: Revealed type is "builtins.str"
+
+class C(Generic[T]):
+    @overload
+    def __init__(self: C[int]) -> None: ...
+    @overload
+    def __init__(self, x: T) -> None: ...
+    def __init__(self, x: "T | None" = None) -> None:
+        pass
+
+def check_c(obj: "C[T] | str") -> None:
+    reveal_type(obj)  # N: Revealed type is "Union[__main__.C[T`-1], builtins.str]"
+    if isinstance(obj, C):
+        reveal_type(obj)  # N: Revealed type is "__main__.C[T`-1]"
+    else:
+        reveal_type(obj)  # N: Revealed type is "builtins.str"
+[builtins fixtures/isinstance.pyi]

test-data/unit/check-typeddict.test

@@ -810,7 +810,7 @@ from typing import TypedDict
 D = TypedDict('D', {'x': int})
 d: object
 if isinstance(d, D):   # E: Cannot use isinstance() with TypedDict type
-    reveal_type(d)     # N: Revealed type is "TypedDict('__main__.D', {'x': builtins.int})"
+    reveal_type(d)     # N: Revealed type is "__main__.D"
 issubclass(object, D)  # E: Cannot use issubclass() with TypedDict type
 [builtins fixtures/isinstancelist.pyi]
 [typing fixtures/typing-typeddict.pyi]