Disallow ClassVar in type aliases (#19263)

Author: brianschubert

mypy/typeanal.py

@@ -675,6 +675,10 @@ def try_analyze_special_unbound_type(self, t: UnboundType, fullname: str) -> Typ
                     t,
                     code=codes.VALID_TYPE,
                 )
+            if self.defining_alias:
+                self.fail(
+                    "ClassVar[...] can't be used inside a type alias", t, code=codes.VALID_TYPE
+                )
             if len(t.args) == 0:
                 return AnyType(TypeOfAny.from_omitted_generics, line=t.line, column=t.column)
             if len(t.args) != 1:

test-data/unit/check-type-aliases.test

@@ -1311,3 +1311,10 @@ class Bar(Generic[T]):
 x: Bar[int]
 reveal_type(x.var.bar)  # N: Revealed type is "__main__.Bar[builtins.int]"
 [builtins fixtures/tuple.pyi]
+
+[case testExplicitTypeAliasClassVarProhibited]
+from typing import ClassVar
+from typing_extensions import TypeAlias
+
+Foo: TypeAlias = ClassVar[int]  # E: ClassVar[...] can't be used inside a type alias
+[builtins fixtures/tuple.pyi]