Rollup merge of #141121 - compiler-errors:ambig-is-not-err, r=lcnr

Only select true errors in `impossible_predicates`

See description in test.

Fixes #141119

r? lcnr

1 files changed, 37 insertions, 0 deletions

diff --git a/tests/ui/traits/object/ambiguity-vtable-segfault.rs b/tests/ui/traits/object/ambiguity-vtable-segfault.rs
new file mode 100644
index 00000000000..dff7d26ae93
--- /dev/null
+++ b/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);
+}