Fix: Treat bare generic TypeAliasType assignment as variable (Fixes #20308)

Author: pareshjoshij

mypy/semanal.py

@@ -3493,13 +3493,28 @@ def is_type_ref(self, rv: Expression, bare: bool = False) -> bool:
         else:
             valid_refs = type_constructors
 
-        if isinstance(rv.node, TypeAlias) or rv.fullname in valid_refs:
+        if isinstance(rv.node, TypeAlias):
+            # Fix for #20308: Reassignment of a generic TypeAliasType (PEP 695)
+            # should be evaluated as a variable assignment, not a type expression.
+            # This allows usage like `B = A` where `type A[T] = ...` without
+            # triggering "Missing type parameters" error.
+            if (
+                bare
+                and rv.node.python_3_12_type_alias
+                and rv.node.alias_tvars
+            ):
+                return False
             return True
+
+        if rv.fullname in valid_refs:
+            return True
+
         if isinstance(rv.node, TypeInfo):
             if bare:
                 return True
             # Assignment color = Color['RED'] defines a variable, not an alias.
             return not rv.node.is_enum
+
         if isinstance(rv.node, Var):
             return rv.node.fullname in NEVER_NAMES