fix ice on unknown const arg parent

Author: lcnr

src/librustc_typeck/collect/type_of.rs

@@ -108,7 +108,11 @@ pub(super) fn const_param_of(tcx: TyCtxt<'_>, def_id: DefId) -> Option<DefId> {
                         tcx.generics_of(tcx.parent(def_id).unwrap())
                     }
                     Res::Def(_, def_id) => tcx.generics_of(def_id),
-                    res => span_bug!(
+                    Res::Err => {
+                        tcx.sess.delay_span_bug(tcx.def_span(def_id), "anon const with Res::Err");
+                        return None;
+                    }
+                    _ => span_bug!(
                         DUMMY_SP,
                         "unexpected anon const res {:?} in path: {:?}",
                         res,

src/test/ui/const-generics/unknown_adt.rs

@@ -0,0 +1,7 @@
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+fn main() {
+    let _: UnknownStruct<7>;
+    //~^ ERROR cannot find type `UnknownStruct`
+}

src/test/ui/const-generics/unknown_adt.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `UnknownStruct` in this scope
+  --> $DIR/unknown_adt.rs:5:12
+   |
+LL |     let _: UnknownStruct<7>;
+   |            ^^^^^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.