Rollup merge of #122360 - veera-sivarajan:bugfix-121941, r=compiler-errors

 Don't Create `ParamCandidate` When Obligation Contains Errors

Fixes #121941

I'm not sure if I understand this correctly but this bug was caused by an error type incorrectly matching against `ParamCandidate`. This was introduced by the changes made in #72621 (figured using cargo-bisect-rustc).

This PR fixes it by skipping `ParamCandidate` generation when an error type is involved. Also, this is similar to #73005 but addresses `ParamCandidate` instead of `ImplCandidate`.

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 66f740b761d..89654ed61ae 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -219,6 +219,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<(), SelectionError<'tcx>> {
         debug!(?stack.obligation);
 
+        // An error type will unify with anything. So, avoid
+        // matching an error type with `ParamCandidate`.
+        // This helps us avoid spurious errors like issue #121941.
+        if stack.obligation.predicate.references_error() {
+            return Ok(());
+        }
+
         let all_bounds = stack
             .obligation
             .param_env