diff --git a/src/librustc_middle/mir/interpret/queries.rs b/src/librustc_middle/mir/interpret/queries.rs
index 0a9c2ac9475..d7c0be05859 100644
--- a/src/librustc_middle/mir/interpret/queries.rs
+++ b/src/librustc_middle/mir/interpret/queries.rs
@@ -39,7 +39,7 @@ impl<'tcx> TyCtxt<'tcx> {
         promoted: Option<mir::Promoted>,
         span: Option<Span>,
     ) -> ConstEvalResult<'tcx> {
-        match ty::Instance::resolve_const_arg(self, param_env, def, substs) {
+        match ty::Instance::resolve_opt_const_arg(self, param_env, def, substs) {
             Ok(Some(instance)) => {
                 let cid = GlobalId { instance, promoted };
                 self.const_eval_global_id(param_env, cid, span)
diff --git a/src/librustc_middle/ty/instance.rs b/src/librustc_middle/ty/instance.rs
index 1a5c7fe4859..f627d05d3e9 100644
--- a/src/librustc_middle/ty/instance.rs
+++ b/src/librustc_middle/ty/instance.rs
@@ -338,6 +338,21 @@ impl<'tcx> Instance<'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         def_id: DefId,
         substs: SubstsRef<'tcx>,
+    ) -> Result<Option<Instance<'tcx>>, ErrorReported> {
+        Instance::resolve_opt_const_arg(
+            tcx,
+            param_env,
+            ty::WithOptConstParam::unknown(def_id),
+            substs,
+        )
+    }
+
+    // This should be kept up to date with `resolve`.
+    pub fn resolve_opt_const_arg(
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        def: ty::WithOptConstParam<DefId>,
+        substs: SubstsRef<'tcx>,
     ) -> Result<Option<Instance<'tcx>>, ErrorReported> {
         // All regions in the result of this query are erased, so it's
         // fine to erase all of the input regions.
@@ -348,18 +363,6 @@ impl<'tcx> Instance<'tcx> {
         let substs = tcx.erase_regions(&substs);
 
         // FIXME(eddyb) should this always use `param_env.with_reveal_all()`?
-        tcx.resolve_instance(tcx.erase_regions(&param_env.and((def_id, substs))))
-    }
-
-    // This should be kept up to date with `resolve`.
-    pub fn resolve_const_arg(
-        tcx: TyCtxt<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-        def: ty::WithOptConstParam<DefId>,
-        substs: SubstsRef<'tcx>,
-    ) -> Result<Option<Instance<'tcx>>, ErrorReported> {
-        let substs = tcx.erase_regions(&substs);
-
         if let Some((did, param_did)) = def.as_const_arg() {
             tcx.resolve_instance_of_const_arg(
                 tcx.erase_regions(&param_env.and((did, param_did, substs))),