Correctly narrow types for tuple[type[X], ...] (#15691)

`flatten_types` forgot about the second way we represent `tuple` inside.

Closes #15443

---------

Co-authored-by: Ilya Priven <ilya.konstantinov@gmail.com>

Author: sobolevn

mypy/checker.py

@@ -7146,6 +7146,8 @@ def flatten_types(t: Type) -> list[Type]:
     t = get_proper_type(t)
     if isinstance(t, TupleType):
         return [b for a in t.items for b in flatten_types(a)]
+    elif is_named_instance(t, "builtins.tuple"):
+        return [t.args[0]]
     else:
         return [t]
 

test-data/unit/check-narrowing.test

@@ -1261,3 +1261,50 @@ def g() -> None:
         def foo(): ...
     foo()
 [builtins fixtures/dict.pyi]
+
+
+[case testNarrowingWithTupleOfTypes]
+from typing import Tuple, Type
+
+class Base: ...
+
+class Impl1(Base): ...
+class Impl2(Base): ...
+
+impls: Tuple[Type[Base], ...] = (Impl1, Impl2)
+some: object
+
+if isinstance(some, impls):
+    reveal_type(some)  # N: Revealed type is "__main__.Base"
+else:
+    reveal_type(some)  # N: Revealed type is "builtins.object"
+
+raw: Tuple[type, ...]
+if isinstance(some, raw):
+    reveal_type(some)  # N: Revealed type is "builtins.object"
+else:
+    reveal_type(some)  # N: Revealed type is "builtins.object"
+[builtins fixtures/dict.pyi]
+
+
+[case testNarrowingWithTupleOfTypesPy310Plus]
+# flags: --python-version 3.10
+class Base: ...
+
+class Impl1(Base): ...
+class Impl2(Base): ...
+
+some: int | Base
+
+impls: tuple[type[Base], ...] = (Impl1, Impl2)
+if isinstance(some, impls):
+    reveal_type(some)  # N: Revealed type is "__main__.Base"
+else:
+    reveal_type(some)  # N: Revealed type is "Union[builtins.int, __main__.Base]"
+
+raw: tuple[type, ...]
+if isinstance(some, raw):
+    reveal_type(some)  # N: Revealed type is "Union[builtins.int, __main__.Base]"
+else:
+    reveal_type(some)  # N: Revealed type is "Union[builtins.int, __main__.Base]"
+[builtins fixtures/dict.pyi]