Delay a bug for stranded opaques

compiler/rustc_hir_analysis/src/check/check.rs

@@ -381,11 +381,17 @@ fn check_opaque_meets_bounds<'tcx>(
     match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
         Ok(()) => {}
         Err(ty_err) => {
+            // Some types may be left "stranded" if they can't be reached
+            // from an astconv'd bound but they're mentioned in the HIR. This
+            // will happen, e.g., when a nested opaque is inside of a non-
+            // existent associated type, like `impl Trait<Missing = impl Trait>`.
+            // See <tests/ui/impl-trait/stranded-opaque.rs>.
             let ty_err = ty_err.to_string(tcx);
-            tcx.dcx().span_bug(
+            let guar = tcx.dcx().span_delayed_bug(
                 span,
                 format!("could not unify `{hidden_ty}` with revealed type:\n{ty_err}"),
             );
+            return Err(guar);
         }
     }
 

tests/ui/impl-trait/stranded-opaque.rs

@@ -0,0 +1,13 @@
+trait Trait {}
+
+impl Trait for i32 {}
+
+// Since `Assoc` doesn't actually exist, it's "stranded", and won't show up in
+// the list of opaques that may be defined by the function. Make sure we don't
+// ICE in this case.
+fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+    //~^ ERROR associated type `Assoc` not found for `Trait`
+    16
+}
+
+fn main () {}

tests/ui/impl-trait/stranded-opaque.stderr

@@ -0,0 +1,9 @@
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded-opaque.rs:8:31
+   |
+LL | fn produce<T>() -> impl Trait<Assoc = impl Trait> {
+   |                               ^^^^^ associated type `Assoc` not found
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0220`.