Un-shadow object bound candidate in NormalizesTo goal

Author: adwinwhite

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -1143,6 +1143,12 @@ where
                 // See `tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs`.
                 if candidates.iter().any(|c| matches!(c.source, CandidateSource::ParamEnv(_))) {
                     candidates.retain(|c| matches!(c.source, CandidateSource::ParamEnv(_)));
+                } else if matches!(goal.predicate.self_ty().kind(), ty::Dynamic(..)) {
+                    // Object candidate may be shadowed by where-bound for the trait goal, see
+                    // `tests/ui/traits/next-solver/normalization-shadowing/use_object_if_empty_env.rs`.
+                    // Trait objects always have their associated types specified so `candidates`
+                    // won't be empty.
+                    self.assemble_object_bound_candidates(goal, &mut candidates);
                 } else if candidates.is_empty() {
                     // If the trait goal has been proven by using the environment, we want to treat
                     // aliases as rigid if there are no applicable projection bounds in the environment.

tests/ui/traits/next-solver/normalization-shadowing/use_object_if_empty_env.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+trait Trait {
+    type Assoc;
+}
+
+// We have param env candidate for the trait goal but not the projection.
+// Under such circumstance, consider object candidate if the self_ty is trait object.
+fn foo<T>(x: <dyn Trait<Assoc = T> as Trait>::Assoc) -> T
+where
+    dyn Trait<Assoc = T>: Trait,
+{
+    x
+}
+
+fn main() {}