Fix type object with type var default in union context (#17991)

Union type context wasn't handled previously, and it triggered false
positives, but apparently only if a type object had type var defaults.

Fixes #17942.

mypy/checkexpr.py

@@ -396,8 +396,8 @@ def analyze_ref_expr(self, e: RefExpr, lvalue: bool = False) -> Type:
                 # TODO: always do this in type_object_type by passing the original context
                 result.ret_type.line = e.line
                 result.ret_type.column = e.column
-            if isinstance(get_proper_type(self.type_context[-1]), TypeType):
-                # This is the type in a Type[] expression, so substitute type
+            if is_type_type_context(self.type_context[-1]):
+                # This is the type in a type[] expression, so substitute type
                 # variables with Any.
                 result = erasetype.erase_typevars(result)
         elif isinstance(node, MypyFile):
@@ -6592,3 +6592,12 @@ def get_partial_instance_type(t: Type | None) -> PartialType | None:
     if t is None or not isinstance(t, PartialType) or t.type is None:
         return None
     return t
+
+
+def is_type_type_context(context: Type | None) -> bool:
+    context = get_proper_type(context)
+    if isinstance(context, TypeType):
+        return True
+    if isinstance(context, UnionType):
+        return any(is_type_type_context(item) for item in context.items)
+    return False

test-data/unit/check-typevar-defaults.test

@@ -717,3 +717,15 @@ def func_d3(
     reveal_type(c)  # N: Revealed type is "__main__.B[__main__.A[builtins.dict[builtins.int, builtins.float]]]"
     reveal_type(d)  # N: Revealed type is "__main__.B[builtins.int]"
 [builtins fixtures/dict.pyi]
+
+[case testTypeVarDefaultsAndTypeObjectTypeInUnion]
+from __future__ import annotations
+from typing import Generic
+from typing_extensions import TypeVar
+
+_I = TypeVar("_I", default=int)
+
+class C(Generic[_I]): pass
+
+t: type[C] | int = C
+[builtins fixtures/tuple.pyi]