Fix trait upcasting to dyn type with no principal when there are projections

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1090,12 +1090,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             {
                 // See `assemble_candidates_for_unsizing` for more info.
                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
-                let iter = data_a
+                let existential_predicates = if data_b.principal().is_some() {
+                    tcx.mk_poly_existential_predicates_from_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(
@@ -1108,8 +1106,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     .auto_traits()
                                     .map(ty::ExistentialPredicate::AutoTrait)
                                     .map(ty::Binder::dummy),
-                    );
+                            ),
-                let existential_predicates = tcx.mk_poly_existential_predicates_from_iter(iter);
+                    )
+                } else {
+                    // If we're unsizing to a dyn type that has no principal, then drop
+                    // the principal and projections from the type. We use the auto traits
+                    // from the RHS type since as we noted that we've checked for auto
+                    // trait compatibility during unsizing.
+                    tcx.mk_poly_existential_predicates_from_iter(
+                        data_b
+                            .auto_traits()
+                            .map(ty::ExistentialPredicate::AutoTrait)
+                            .map(ty::Binder::dummy),
+                    )
+                };
                 let source_trait = Ty::new_dynamic(tcx, existential_predicates, r_b, dyn_a);
 
                 // Require that the traits involved in this upcast are **equal**;

tests/ui/traits/dyn-drop-principal-with-projections.rs

@@ -0,0 +1,13 @@
+//@ check-pass
+
+trait Tr {
+    type Assoc;
+}
+
+impl Tr for () {
+    type Assoc = ();
+}
+
+fn main() {
+    let x = &() as &(dyn Tr<Assoc = ()> + Send) as &dyn Send;
+}