expose default substs in param_env

(cherry picked from commit c6b6901)

Author: b-naber

compiler/rustc_ty_utils/src/ty.rs

@@ -248,6 +248,7 @@ fn trait_of_item(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
 }
 
 /// See `ParamEnv` struct definition for details.
+#[instrument(level = "debug", skip(tcx))]
 fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
     // The param_env of an impl Trait type is its defining function's param_env
     if let Some(parent) = ty::is_impl_trait_defn(tcx, def_id) {
@@ -275,9 +276,20 @@ fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
         predicates.extend(environment);
     }
 
+    // It's important that we include the default substs in unevaluated
+    // constants, since `Unevaluated` instances in predicates whose substs are None
+    // can lead to "duplicate" caller bounds candidates during trait selection,
+    // duplicate in the sense that both have their default substs, but the
+    // candidate that resulted from a superpredicate still uses `None` in its
+    // `substs_` field of `Unevaluated` to indicate that it has its default substs,
+    // whereas the other candidate has `substs_: Some(default_substs)`, see
+    // issue #89334
+    predicates = tcx.expose_default_const_substs(predicates);
+
     let unnormalized_env =
         ty::ParamEnv::new(tcx.intern_predicates(&predicates), traits::Reveal::UserFacing);
 
+    debug!("unnormalized_env caller bounds: {:?}", unnormalized_env.caller_bounds());
     let body_id = def_id
         .as_local()
         .map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))