delay a bug when encountering an ambiguity in MIR typeck

compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -157,10 +157,18 @@ where
         }
 
         let mut region_constraints = QueryRegionConstraints::default();
-        let (output, error_info, mut obligations, _) =
-            Q::fully_perform_into(self, infcx, &mut region_constraints).map_err(|_| {
-                infcx.tcx.sess.delay_span_bug(span, format!("error performing {self:?}"))
-            })?;
+        let (output, error_info, mut obligations) =
+            Q::fully_perform_into(self, infcx, &mut region_constraints)
+                .map_err(|_| {
+                    infcx.tcx.sess.delay_span_bug(span, format!("error performing {self:?}"))
+                })
+                .and_then(|(output, error_info, obligations, certainty)| match certainty {
+                    Certainty::Proven => Ok((output, error_info, obligations)),
+                    Certainty::Ambiguous => Err(infcx
+                        .tcx
+                        .sess
+                        .delay_span_bug(span, format!("ambiguity performing {self:?}"))),
+                })?;
 
         // Typically, instantiating NLL query results does not
         // create obligations. However, in some cases there

tests/ui/type-alias-impl-trait/rpit_tait_equality_in_canonical_query.current.stderr

@@ -0,0 +1,20 @@
+error: internal compiler error: no errors encountered even though `delay_span_bug` issued
+
+error: internal compiler error: ambiguity performing ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing }, value: ProvePredicate { predicate: Binder { value: ProjectionPredicate(AliasTy { args: [FnDef(DefId(get_rpit), []), ()], def_id: DefId(ops::function::FnOnce::Output) }, Term::Ty(Alias(Opaque, AliasTy { args: [], def_id: DefId(Opaque::{opaque#0}) }))), bound_vars: [] } } }
+  --> $DIR/rpit_tait_equality_in_canonical_query.rs:28:5
+   |
+LL |     query(get_rpit);
+   |     ^^^^^^^^^^^^^^^
+   |
+  --> $DIR/rpit_tait_equality_in_canonical_query.rs:28:5
+   |
+LL |     query(get_rpit);
+   |     ^^^^^^^^^^^^^^^
+
+
+
+
+query stack during panic:
+end of query stack
+error: aborting due to 2 previous errors
+

tests/ui/type-alias-impl-trait/rpit_tait_equality_in_canonical_query.rs

@@ -7,7 +7,14 @@
 
 // revisions: current next
 //[next] compile-flags: -Ztrait-solver=next
-// check-pass
+//[next] check-pass
+
+//[current] known-bug: #108498
+//[current] failure-status: 101
+//[current] normalize-stderr-test: "DefId\(.*?\]::" -> "DefId("
+//[current] normalize-stderr-test: "(?m)^note: .*\n" -> ""
+//[current] normalize-stderr-test: "(?m)^ *\d+: .*\n" -> ""
+//[current] normalize-stderr-test: "(?m)^ *at .*\n" -> ""
 
 #![feature(type_alias_impl_trait)]