Error message for ambiguous RTN from super bounds

compiler/rustc_hir_analysis/messages.ftl

@@ -194,6 +194,10 @@ hir_analysis_return_type_notation_equality_bound =
 hir_analysis_return_type_notation_missing_method =
     cannot find associated function `{$assoc_name}` for `{$ty_name}`
 
+hir_analysis_return_type_notation_conflicting_bound =
+    ambiguous associated function `{$assoc_name}` for `{$ty_name}`
+    .note = `{$assoc_name}` is declared in two supertraits: `{$first_bound}` and `{$second_bound}`
+
 hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is not allowed within types on item signatures for {$kind}
     .label = not allowed in type signatures
 

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2083,8 +2083,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         };
 
-        if let Some(_conflicting_candidate) = matching_candidates.next() {
-            todo!()
+        if let Some(conflicting_candidate) = matching_candidates.next() {
+            return Err(self.tcx().sess.emit_err(
+                crate::errors::ReturnTypeNotationConflictingBound {
+                    span,
+                    ty_name: ty_name.to_string(),
+                    assoc_name: assoc_name.name,
+                    first_bound: candidate.print_only_trait_path(),
+                    second_bound: conflicting_candidate.print_only_trait_path(),
+                },
+            ));
         }
 
         Ok(candidate)

compiler/rustc_hir_analysis/src/errors.rs

@@ -6,7 +6,7 @@ use rustc_errors::{
     MultiSpan,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, print::TraitRefPrintOnlyTraitPath, Ty};
 use rustc_span::{symbol::Ident, Span, Symbol};
 
 #[derive(Diagnostic)]
@@ -516,6 +516,18 @@ pub(crate) struct ReturnTypeNotationMissingMethod {
     pub assoc_name: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_return_type_notation_conflicting_bound)]
+#[note]
+pub(crate) struct ReturnTypeNotationConflictingBound<'tcx> {
+    #[primary_span]
+    pub span: Span,
+    pub ty_name: String,
+    pub assoc_name: Symbol,
+    pub first_bound: ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>>,
+    pub second_bound: ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>>,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_placeholder_not_allowed_item_signatures, code = "E0121")]
 pub(crate) struct PlaceholderNotAllowedItemSignatures {