Don't add a note for implementing a trait if its inner type is erroneous

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

@@ -371,23 +371,28 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
                     if !candidate_set.ambiguous && no_candidates_apply {
                         let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
-                        let self_ty = trait_ref.self_ty();
-                        let (trait_desc, self_desc) = with_no_trimmed_paths!({
-                            let trait_desc = trait_ref.print_only_trait_path().to_string();
-                            let self_desc = if self_ty.has_concrete_skeleton() {
-                                Some(self_ty.to_string())
+                        if !trait_ref.references_error() {
+                            let self_ty = trait_ref.self_ty();
+                            let (trait_desc, self_desc) = with_no_trimmed_paths!({
+                                let trait_desc = trait_ref.print_only_trait_path().to_string();
+                                let self_desc = if self_ty.has_concrete_skeleton() {
+                                    Some(self_ty.to_string())
+                                } else {
+                                    None
+                                };
+                                (trait_desc, self_desc)
+                            });
+                            let cause = if let Conflict::Upstream = conflict {
+                                IntercrateAmbiguityCause::UpstreamCrateUpdate {
+                                    trait_desc,
+                                    self_desc,
+                                }
                             } else {
-                                None
+                                IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
                             };
-                            (trait_desc, self_desc)
-                        });
-                        let cause = if let Conflict::Upstream = conflict {
-                            IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
-                        } else {
-                            IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
-                        };
-                        debug!(?cause, "evaluate_stack: pushing cause");
-                        self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
+                            debug!(?cause, "evaluate_stack: pushing cause");
+                            self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
+                        }
                     }
                 }
             }

src/test/ui/trait-bounds/impl-bound-with-references-error.rs

@@ -0,0 +1,20 @@
+// Regression test for #105138.
+// This test ensures that the compiler does not add note
+// for implementation of trait whose inner type is erroneous.
+
+pub enum LabelText {
+    Plain,
+}
+
+impl<T> From<T> for LabelText
+//~^ ERROR conflicting implementations of trait `From<LabelText>` for type `LabelText` [E0119]
+where
+    T: Into<Cow<'static, str>>,
+    //~^ ERROR cannot find type `Cow` in this scope [E0412]
+{
+    fn from(text: T) -> Self {
+        LabelText::Plain(text.into())
+    }
+}
+
+fn main() {}

src/test/ui/trait-bounds/impl-bound-with-references-error.stderr

@@ -0,0 +1,24 @@
+error[E0412]: cannot find type `Cow` in this scope
+  --> $DIR/impl-bound-with-references-error.rs:12:13
+   |
+LL |     T: Into<Cow<'static, str>>,
+   |             ^^^ not found in this scope
+   |
+help: consider importing this enum
+   |
+LL | use std::borrow::Cow;
+   |
+
+error[E0119]: conflicting implementations of trait `From<LabelText>` for type `LabelText`
+  --> $DIR/impl-bound-with-references-error.rs:9:1
+   |
+LL | impl<T> From<T> for LabelText
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<T> From<T> for T;
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0412.
+For more information about an error, try `rustc --explain E0119`.