Rollup merge of #140711 - compiler-errors:combine-maybes, r=lcnr

Do not discard constraints on overflow if there was candidate ambiguity

Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/201.

There's a pretty chunky justification in the test.

r? lcnr

2 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs b/compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs
index 2d445dd0790..f64cd5ffebe 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs
@@ -99,7 +99,13 @@ pub(super) fn fulfillment_error_for_stalled<'tcx>(
             Ok((_, Certainty::Maybe(MaybeCause::Ambiguity))) => {
                 (FulfillmentErrorCode::Ambiguity { overflow: None }, true)
             }
-            Ok((_, Certainty::Maybe(MaybeCause::Overflow { suggest_increasing_limit }))) => (
+            Ok((
+                _,
+                Certainty::Maybe(MaybeCause::Overflow {
+                    suggest_increasing_limit,
+                    keep_constraints: _,
+                }),
+            )) => (
                 FulfillmentErrorCode::Ambiguity { overflow: Some(suggest_increasing_limit) },
                 // Don't look into overflows because we treat overflows weirdly anyways.
                 // We discard the inference constraints from overflowing goals, so
diff --git a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
index 24b87000e32..9795655e842 100644
--- a/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
+++ b/compiler/rustc_trait_selection/src/solve/inspect/analyse.rs
@@ -382,7 +382,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
             if let Some(term_hack) = normalizes_to_term_hack {
                 infcx
                     .probe(|_| term_hack.constrain(infcx, DUMMY_SP, uncanonicalized_goal.param_env))
-                    .map(|certainty| ok.value.certainty.unify_with(certainty))
+                    .map(|certainty| ok.value.certainty.and(certainty))
             } else {
                 Ok(ok.value.certainty)
             }