[PEP 695] Detect invalid number of constrained types

At least two are required, according do PEP 695.

Work on #15238.

mypy/fastparse.py

@@ -1185,8 +1185,12 @@ def translate_type_params(self, type_params: list[Any]) -> list[TypeParam]:
                 explicit_type_params.append(TypeParam(p.name, TYPE_VAR_TUPLE_KIND, None, []))
             else:
                 if isinstance(p.bound, ast3.Tuple):
-                    conv = TypeConverter(self.errors, line=p.lineno)
-                    values = [conv.visit(t) for t in p.bound.elts]
+                    if len(p.bound.elts) < 2:
+                        self.fail(message_registry.TYPE_VAR_TOO_FEW_CONSTRAINED_TYPES,
+                                  p.lineno, p.col_offset, blocker=False)
+                    else:
+                        conv = TypeConverter(self.errors, line=p.lineno)
+                        values = [conv.visit(t) for t in p.bound.elts]
                 elif p.bound is not None:
                     bound = TypeConverter(self.errors, line=p.lineno).visit(p.bound)
                 explicit_type_params.append(TypeParam(p.name, TYPE_VAR_KIND, bound, values))

mypy/message_registry.py

@@ -330,3 +330,6 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 NARROWED_TYPE_NOT_SUBTYPE: Final = ErrorMessage(
     "Narrowed type {} is not a subtype of input type {}", codes.NARROWED_TYPE_NOT_SUBTYPE
 )
+TYPE_VAR_TOO_FEW_CONSTRAINED_TYPES: Final = ErrorMessage(
+    "Type variable must have at least two constrained types", codes.MISC
+)

test-data/unit/check-python312.test

@@ -1494,3 +1494,13 @@ reveal_type(a)  # N: Revealed type is "builtins.list[builtins.int]"
 # flags: --enable-incomplete-feature=NewGenericSyntax
 def f[T](x: foobar, y: T) -> T: ...  # E: Name "foobar" is not defined
 reveal_type(f)  # N: Revealed type is "def [T] (x: Any, y: T`-1) -> T`-1"
+
+[case testPEP695WrongNumberOfConstrainedTypes]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+type A[T: ()] = list[T]  # E: Type variable must have at least two constrained types
+a: A[int]
+reveal_type(a)  # N: Revealed type is "builtins.list[builtins.int]"
+
+type B[T: (int,)] = list[T]  # E: Type variable must have at least two constrained types
+b: B[str]
+reveal_type(b)  # N: Revealed type is "builtins.list[builtins.str]"