Make assoc types work with ?const opt=out

src/test/ui/rfc-2632-const-trait-impl/assoc-type.rs

@@ -1,5 +1,7 @@
 // FIXME(fee1-dead): this should have a better error message
 #![feature(const_trait_impl)]
+#![feature(const_trait_bound_opt_out)]
+#![allow(incomplete_features)]
 
 struct NonConstAdd(i32);
 
@@ -20,4 +22,12 @@ impl const Foo for NonConstAdd {
     //~^ ERROR
 }
 
+trait Baz {
+    type Qux: ?const std::ops::Add;
+}
+
+impl const Baz for NonConstAdd {
+    type Qux = NonConstAdd; // OK
+}
+
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/assoc-type.stderr

@@ -1,5 +1,5 @@
 error[E0277]: cannot add `NonConstAdd` to `NonConstAdd`
-  --> $DIR/assoc-type.rs:19:5
+  --> $DIR/assoc-type.rs:21:5
    |
 LL |     type Bar: std::ops::Add;
    |               ------------- required by this bound in `Foo::Bar`