Only select true errors in impossible_predicates

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -701,9 +701,15 @@ pub fn impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, predicates: Vec<ty::Clause
         let obligation = Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate);
         ocx.register_obligation(obligation);
     }
-    let errors = ocx.select_all_or_error();
 
-    if !errors.is_empty() {
+    // Use `select_where_possible` to only return impossible for true errors,
+    // and not ambiguities or overflows. Since the new trait solver forces
+    // some currently undetected overlap between `dyn Trait: Trait` built-in
+    // vs user-written impls to AMBIGUOUS, this may return ambiguity even
+    // with no infer vars. There may also be ways to encounter ambiguity due
+    // to post-mono overflow.
+    let true_errors = ocx.select_where_possible();
+    if !true_errors.is_empty() {
         return true;
     }
 

tests/ui/traits/object/ambiguity-vtable-segfault.rs

@@ -0,0 +1,37 @@
+// In this example below, we have two overlapping candidates for `dyn Q: Q`.
+// Specifically, the user written impl for `<dyn Q as Mirror>::Assoc` and the
+// built-in impl for object types. Since they differ by their region responses,
+// the goal is ambiguous. This affects codegen since impossible obligations
+// for method dispatch will lead to a segfault, since we end up emitting dummy
+// call vtable offsets due to <https://github.com/rust-lang/rust/pull/136311>.
+
+// Test for <https://github.com/rust-lang/rust/issues/141119>.
+
+//@ run-pass
+
+trait Mirror {
+    type Assoc: ?Sized;
+}
+impl<T: ?Sized> Mirror for T {
+    type Assoc = T;
+}
+
+trait Q: 'static {
+    fn q(&self);
+}
+
+impl Q for i32 {
+    fn q(&self) { println!("i32"); }
+}
+
+impl Q for <dyn Q as Mirror>::Assoc where Self: 'static {
+    fn q(&self) { println!("dyn Q"); }
+}
+
+fn foo<T: Q + ?Sized>(t: &T) {
+    t.q();
+}
+
+fn main() {
+    foo(&1 as &dyn Q);
+}