fix: handle union-bound TypeVar in type[T] callable analysis

When a TypeVar T has a union bound (e.g. T: bool|int|float|str),
calling a value of type type[T] would incorrectly infer the return
type as the union rather than T. This happened because
analyze_type_type_callee only replaced return types for CallableType
and Overloaded, not for UnionType.

Additionally, CallableType.type_object() would crash with an assertion
error when encountering a TypeVarType whose upper_bound is a union,
since it expected all paths to resolve to an Instance.

Fixes #21106

Signed-off-by: bahtya <bahtyar153@qq.com>

Author: Bahtya

mypy/checkexpr.py

@@ -1900,6 +1900,15 @@ def can_return_none(self, type: TypeInfo, attr_name: str) -> bool:
                     return is_subtype(NoneType(), node.type.ret_type)
         return False
 
+    def _replace_callable_return_type(self, tp: Type, replacement: Type) -> ProperType:
+        """Replace the return type of a callable or overloaded type with replacement."""
+        ptp = get_proper_type(tp)
+        if isinstance(ptp, CallableType):
+            return ptp.copy_modified(ret_type=replacement)
+        if isinstance(ptp, Overloaded):
+            return Overloaded([c.copy_modified(ret_type=replacement) for c in ptp.items])
+        return ptp
+
     def analyze_type_type_callee(self, item: ProperType, context: Context) -> Type:
         """Analyze the callee X in X(...) where X is Type[item].
 
@@ -1936,6 +1945,14 @@ def analyze_type_type_callee(self, item: ProperType, context: Context) -> Type:
                 callee = callee.copy_modified(ret_type=item)
             elif isinstance(callee, Overloaded):
                 callee = Overloaded([c.copy_modified(ret_type=item) for c in callee.items])
+            elif isinstance(callee, UnionType):
+                callee = UnionType(
+                    [
+                        self._replace_callable_return_type(tp, item)
+                        for tp in callee.relevant_items()
+                    ],
+                    callee.line,
+                )
             return callee
         # We support Type of namedtuples but not of tuples in general
         if isinstance(item, TupleType) and tuple_fallback(item).type.fullname != "builtins.tuple":

mypy/types.py

@@ -2311,6 +2311,15 @@ def type_object(self) -> mypy.nodes.TypeInfo:
         ret = get_proper_type(self.ret_type)
         if isinstance(ret, TypeVarType):
             ret = get_proper_type(ret.upper_bound)
+        if isinstance(ret, UnionType):
+            # When the TypeVar has a union bound, pick the first item's
+            # fallback. This is only used for is_protocol checks, which
+            # are not applicable to union-bound typevars.
+            first = get_proper_type(ret.items[0])
+            if isinstance(first, Instance):
+                ret = first
+            else:
+                ret = self.fallback
         if isinstance(ret, TupleType):
             ret = ret.partial_fallback
         if isinstance(ret, TypedDictType):