[PEP 695] Generate error if new-style type alias used as base class (#…

…17789)

It doesn't work at runtime.

mypy/semanal.py

@@ -1824,6 +1824,8 @@ def analyze_class(self, defn: ClassDef) -> None:
             defn, bases, context=defn
         )
 
+        self.check_type_alias_bases(bases)
+
         for tvd in tvar_defs:
             if isinstance(tvd, TypeVarType) and any(
                 has_placeholder(t) for t in [tvd.upper_bound] + tvd.values
@@ -1895,6 +1897,19 @@ def analyze_class(self, defn: ClassDef) -> None:
 
             self.analyze_class_body_common(defn)
 
+    def check_type_alias_bases(self, bases: list[Expression]) -> None:
+        for base in bases:
+            if isinstance(base, IndexExpr):
+                base = base.base
+            if (
+                isinstance(base, RefExpr)
+                and isinstance(base.node, TypeAlias)
+                and base.node.python_3_12_type_alias
+            ):
+                self.fail(
+                    'Type alias defined using "type" statement not valid as base class', base
+                )
+
     def setup_type_vars(self, defn: ClassDef, tvar_defs: list[TypeVarLikeType]) -> None:
         defn.type_vars = tvar_defs
         defn.info.type_vars = []

test-data/unit/check-python312.test

@@ -591,6 +591,40 @@ a4: A4
 reveal_type(a4)  # N: Revealed type is "Union[builtins.int, builtins.str]"
 [builtins fixtures/type.pyi]
 
+[case testPEP695TypeAliasNotValidAsBaseClass]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from typing import TypeAlias
+
+import m
+
+type A1 = int
+class Bad1(A1):  # E: Type alias defined using "type" statement not valid as base class
+    pass
+
+type A2[T] = list[T]
+class Bad2(A2[int]):  # E: Type alias defined using "type" statement not valid as base class
+    pass
+
+class Bad3(m.A1):  # E: Type alias defined using "type" statement not valid as base class
+    pass
+
+class Bad4(m.A2[int]):  # E: Type alias defined using "type" statement not valid as base class
+    pass
+
+B1 = int
+B2 = list
+B3: TypeAlias = int
+class Good1(B1): pass
+class Good2(B2[int]): pass
+class Good3(list[A1]): pass
+class Good4(list[A2[int]]): pass
+class Good5(B3): pass
+
+[file m.py]
+type A1 = str
+type A2[T] = list[T]
+[typing fixtures/typing-medium.pyi]
+
 [case testPEP695TypeAliasWithUnusedTypeParams]
 # flags: --enable-incomplete-feature=NewGenericSyntax
 type A[T] = int
@@ -637,9 +671,7 @@ class D: pass
 
 type A = C
 
-# Note that this doesn't actually work at runtime, but we currently don't
-# keep track whether a type alias is valid in various runtime type contexts.
-class D(A):
+class D(A):  # E: Type alias defined using "type" statement not valid as base class
     pass
 
 class C: pass