Still-further-specializable projections are ambiguous

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -174,7 +174,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 goal.predicate.def_id(),
                 impl_def_id
             )? else {
-                return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
+                return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
             };
 
             if !assoc_def.item.defaultness(tcx).has_value() {

tests/ui/traits/new-solver/specialization-unconstrained.rs

@@ -0,0 +1,22 @@
+// compile-flags: -Ztrait-solver=next
+
+#![feature(specialization)]
+//~^ WARN the feature `specialization` is incomplete
+
+// Do not treat the RHS of a projection-goal as an unconstrained `Certainty::Yes` response
+// if the impl is still further specializable.
+
+trait Default {
+   type Id;
+}
+
+impl<T> Default for T {
+   default type Id = T;
+}
+
+fn test<T: Default<Id = U>, U>() {}
+
+fn main() {
+    test::<u32, ()>();
+    //~^ ERROR cannot satisfy `<u32 as Default>::Id == ()`
+}

tests/ui/traits/new-solver/specialization-unconstrained.stderr

@@ -0,0 +1,25 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/specialization-unconstrained.rs:3:12
+   |
+LL | #![feature(specialization)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+   = help: consider using `min_specialization` instead, which is more stable and complete
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0284]: type annotations needed: cannot satisfy `<u32 as Default>::Id == ()`
+  --> $DIR/specialization-unconstrained.rs:20:5
+   |
+LL |     test::<u32, ()>();
+   |     ^^^^^^^^^^^^^^^ cannot satisfy `<u32 as Default>::Id == ()`
+   |
+note: required by a bound in `test`
+  --> $DIR/specialization-unconstrained.rs:17:20
+   |
+LL | fn test<T: Default<Id = U>, U>() {}
+   |                    ^^^^^^ required by this bound in `test`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0284`.