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

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs

@@ -132,12 +132,14 @@ where
                     (Certainty::Yes, NestedNormalizationGoals(goals))
                 }
                 _ => {
-                    let certainty = shallow_certainty.unify_with(goals_certainty);
+                    let certainty = shallow_certainty.and(goals_certainty);
                     (certainty, NestedNormalizationGoals::empty())
                 }
             };
 
-        if let Certainty::Maybe(cause @ MaybeCause::Overflow { .. }) = certainty {
+        if let Certainty::Maybe(cause @ MaybeCause::Overflow { keep_constraints: false, .. }) =
+            certainty
+        {
             // If we have overflow, it's probable that we're substituting a type
             // into itself infinitely and any partial substitutions in the query
             // response are probably not useful anyways, so just return an empty
@@ -193,6 +195,7 @@ where
                     debug!(?num_non_region_vars, "too many inference variables -> overflow");
                     return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Overflow {
                         suggest_increasing_limit: true,
+                        keep_constraints: false,
                     }));
                 }
             }

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -661,7 +661,7 @@ where
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {
                         self.nested_goals.push((source, with_resolved_vars));
-                        unchanged_certainty = unchanged_certainty.map(|c| c.unify_with(certainty));
+                        unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
                     }
                 }
             } else {
@@ -675,7 +675,7 @@ where
                     Certainty::Yes => {}
                     Certainty::Maybe(_) => {
                         self.nested_goals.push((source, goal));
-                        unchanged_certainty = unchanged_certainty.map(|c| c.unify_with(certainty));
+                        unchanged_certainty = unchanged_certainty.map(|c| c.and(certainty));
                     }
                 }
             }

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -253,16 +253,18 @@ where
     }
 
     fn bail_with_ambiguity(&mut self, responses: &[CanonicalResponse<I>]) -> CanonicalResponse<I> {
-        debug_assert!(!responses.is_empty());
-        if let Certainty::Maybe(maybe_cause) =
-            responses.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
-                certainty.unify_with(response.value.certainty)
-            })
-        {
-            self.make_ambiguous_response_no_constraints(maybe_cause)
-        } else {
-            panic!("expected flounder response to be ambiguous")
-        }
+        debug_assert!(responses.len() > 1);
+        let maybe_cause = responses.iter().fold(MaybeCause::Ambiguity, |maybe_cause, response| {
+            // Pull down the certainty of `Certainty::Yes` to ambiguity when combining
+            // these responses, b/c we're combining more than one response and this we
+            // don't know which one applies.
+            let candidate = match response.value.certainty {
+                Certainty::Yes => MaybeCause::Ambiguity,
+                Certainty::Maybe(candidate) => candidate,
+            };
+            maybe_cause.or(candidate)
+        });
+        self.make_ambiguous_response_no_constraints(maybe_cause)
     }
 
     /// If we fail to merge responses we flounder and return overflow or ambiguity.