Handle tuples specially

Author: sterliakov

mypy/checker.py

@@ -7693,6 +7693,9 @@ def get_isinstance_type(self, expr: Expression) -> list[TypeRange] | None:
             if isinstance(typ, FunctionLike) and typ.is_type_obj():
                 # If a type is generic, `isinstance` can only narrow its variables to Any.
                 erased_type = fill_typevars_with_any(typ.type_object())
+                # Tuples may have unattended type variables among their items
+                if isinstance(erased_type, TupleType):
+                    erased_type = erase_typevars(erased_type)
                 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

@@ -2480,6 +2480,7 @@ def check_a(obj: "A[T] | str") -> None:
     else:
         reveal_type(obj)  # N: Revealed type is "builtins.str"
 
+
 class B(Generic[T]):
     @overload
     def __init__(self, x: T) -> None: ...
@@ -2495,6 +2496,7 @@ def check_b(obj: "B[T] | str") -> None:
     else:
         reveal_type(obj)  # N: Revealed type is "builtins.str"
 
+
 class C(Generic[T]):
     @overload
     def __init__(self: C[int]) -> None: ...
@@ -2509,4 +2511,12 @@ def check_c(obj: "C[T] | str") -> None:
         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]
+
+
+class D(tuple[T], Generic[T]): ...
+
+def check_d(arg: D[T]) -> None:
+    if not isinstance(arg, D):
+        return
+    reveal_type(arg)  # N: Revealed type is "tuple[T`-1, fallback=__main__.D[Any]]"
+[builtins fixtures/tuple.pyi]