Consider a goal as NOT changed if its response is identity modulo regions

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -344,7 +344,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             Ok(response) => response,
         };
 
-        let has_changed = !canonical_response.value.var_values.is_identity()
+        let has_changed = !canonical_response.value.var_values.is_identity_modulo_regions()
             || !canonical_response.value.external_constraints.opaque_types.is_empty();
         let (certainty, nested_goals) = match self.instantiate_and_apply_query_response(
             goal.param_env,

tests/ui/impl-trait/autoderef.rs

@@ -1,5 +1,5 @@
 // revisions: current next
-//[next] compile-flag: -Ztrait-solver=next
+//[next] compile-flags: -Ztrait-solver=next
 // check-pass
 
 use std::path::Path;

tests/ui/traits/new-solver/dont-loop-fulfill-on-region-constraints.rs

@@ -0,0 +1,32 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+trait Eq<'a, 'b, T> {}
+
+trait Ambig {}
+impl Ambig for () {}
+
+impl<'a, T> Eq<'a, 'a, T> for () where T: Ambig {}
+
+fn eq<'a, 'b, T>(t: T)
+where
+    (): Eq<'a, 'b, T>,
+{
+}
+
+fn test<'r>() {
+    let mut x = Default::default();
+
+    // When we evaluate `(): Eq<'r, 'r, ?0>` we uniquify the regions.
+    // That leads us to evaluate `(): Eq<'?0, '?1, ?0>`. The response of this
+    // will be ambiguous (because `?0: Ambig` is ambig) and also not an "identity"
+    // response, since the region constraints will contain `'?0 == '?1` (so
+    // `is_changed` will return true). Since it's both ambig and changed,
+    // fulfillment will both re-register the goal AND loop again. This hits the
+    // overflow limit. This should neither be considered overflow, nor ICE.
+    eq::<'r, 'r, _>(x);
+
+    x = ();
+}
+
+fn main() {}