Fix reassignment of generic TypeAliasType treated as type expression

mypy/semanal.py

@@ -3493,13 +3493,24 @@ 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
 

test-data/unit/check-python312.test

@@ -2237,3 +2237,14 @@ class D[*Ts](Generic[Unpack[Us]]):  # E: Generic[...] base class is redundant \
                                     # E: Can only use one type var tuple in a class def
     pass
 [builtins fixtures/tuple.pyi]
+
+[case testPep695GenericTypeAliasReassignment]
+from typing import reveal_type
+
+type A[T] = T | str
+B = A
+reveal_type(B)
+[out]
+main:5: note: Revealed type is "typing.TypeAliasType"
+[builtins fixtures/tuple.pyi]
+[typing fixtures/typing-full.pyi]