Fix edge case

Author: ilevkivskyi

mypy/binder.py

@@ -247,6 +247,9 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                     type = possible_types[0]
                 else:
                     type = make_simplified_union(possible_types)
+                    # Legacy guard for corner case, e.g. when the original type is TypeVarType.
+                    if declaration_type is not None and not is_subtype(type, declaration_type):
+                        type = declaration_type
                     # Try simplifying resulting type for unions involving variadic tuples.
                     # Technically, everything is still valid without this step, but if we do
                     # not do this, this may create long unions after exiting an if check like:

test-data/unit/check-narrowing.test

@@ -2416,3 +2416,20 @@ while x is not None and b():
     x = f()
 
 [builtins fixtures/primitives.pyi]
+
+[case testNarrowingTypeVarMultiple]
+from typing import TypeVar
+
+class A: ...
+class B: ...
+
+T = TypeVar("T")
+def foo(x: T) -> T:
+    if isinstance(x, A):
+        pass
+    elif isinstance(x, B):
+        pass
+    else:
+        raise
+    return x
+[builtins fixtures/isinstance.pyi]