don't treat inherited opaques as defining

compiler/rustc_ty_utils/src/opaque_types.rs

@@ -33,8 +33,7 @@ enum CollectionMode {
     ImplTraitInAssocTypes,
     /// When collecting for an explicit `#[define_opaque]` attribute, find all TAITs
     Taits,
-    /// The default case, only collect RPITs and AsyncFn return types, as these are
-    /// always defined by the current item.
+    /// By default collect RPITs and async function return types defined by this item
     RpitAndAsyncFnOnly,
 }
 
@@ -323,6 +322,12 @@ fn opaque_types_defined_by<'tcx>(
     if tcx.is_typeck_child(item.to_def_id()) {
         return tcx.opaque_types_defined_by(tcx.local_parent(item));
     }
+
+    // delegation inherits its signature without defining its opaque types
+    if tcx.hir_opt_delegation_sig_id(item).is_some() {
+        return tcx.mk_local_def_ids(&[]);
+    }
+
     let kind = tcx.def_kind(item);
     trace!(?kind);
     let mut collector = OpaqueTypeCollector::new(tcx, item);

tests/ui/delegation/fn-header.rs

@@ -1,8 +1,7 @@
 //@ revisions: old next
 //@ ignore-compare-mode-next-solver (explicit revisions)
 //@[next] compile-flags: -Znext-solver
-//@[next] known-bug: trait-system-refactor-initiative#304
-//@[old] check-pass
+//@ check-pass
 //@ edition:2018
 //@ aux-crate:fn_header_aux=fn-header-aux.rs
 //@ ignore-backends: gcc

tests/ui/delegation/impl-trait.rs

@@ -1,8 +1,7 @@
 //@ revisions: old next
 //@ ignore-compare-mode-next-solver (explicit revisions)
 //@[next] compile-flags: -Znext-solver
-//@[next] known-bug: trait-system-refactor-initiative#304
-//@[old] check-pass
+//@ check-pass
 
 #![feature(fn_delegation)]