Allow dropping dyn principal

2 files changed, 5 insertions, 1 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 52048ca79f9..aa313a526c1 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -1018,7 +1018,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // #2 (region bounds).
                 let principal_def_id_a = a_data.principal_def_id();
                 let principal_def_id_b = b_data.principal_def_id();
-                if principal_def_id_a == principal_def_id_b {
+                if principal_def_id_a == principal_def_id_b || principal_def_id_b.is_none() {
                     // We may upcast to auto traits that are either explicitly listed in
                     // the object type's bounds, or implied by the principal trait ref's
                     // supertraits.
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index cc5c7532b50..0ba3b4e6e55 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -1153,6 +1153,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
                 let iter = data_a
                     .principal()
+                    .filter(|_| {
+                        // optionally drop the principal, if we're unsizing to no principal
+                        data_b.principal().is_some()
+                    })
                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
                     .into_iter()
                     .chain(