Allow specializing on const trait bounds

compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs

@@ -423,13 +423,10 @@ fn trait_predicate_kind<'tcx>(
     predicate: ty::Predicate<'tcx>,
 ) -> Option<TraitSpecializationKind> {
     match predicate.kind().skip_binder() {
-        ty::PredicateKind::Trait(ty::TraitPredicate {
-            trait_ref,
-            constness: ty::BoundConstness::NotConst,
-            polarity: _,
-        }) => Some(tcx.trait_def(trait_ref.def_id).specialization_kind),
-        ty::PredicateKind::Trait(_)
-        | ty::PredicateKind::RegionOutlives(_)
+        ty::PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: _, polarity: _ }) => {
+            Some(tcx.trait_def(trait_ref.def_id).specialization_kind)
+        }
+        ty::PredicateKind::RegionOutlives(_)
         | ty::PredicateKind::TypeOutlives(_)
         | ty::PredicateKind::Projection(_)
         | ty::PredicateKind::WellFormed(_)

src/test/ui/specialization/const_trait_impl.rs

@@ -0,0 +1,55 @@
+// check-pass
+#![feature(const_trait_impl, min_specialization, rustc_attrs)]
+
+#[rustc_specialization_trait]
+#[const_trait]
+pub unsafe trait Sup {
+    fn foo() -> u32;
+}
+
+#[rustc_specialization_trait]
+#[const_trait]
+pub unsafe trait Sub: ~const Sup {}
+
+unsafe impl const Sup for u8 {
+    default fn foo() -> u32 {
+        1
+    }
+}
+
+unsafe impl const Sup for () {
+    fn foo() -> u32 {
+        42
+    }
+}
+
+unsafe impl const Sub for () {}
+
+#[const_trait]
+pub trait A {
+    fn a() -> u32;
+}
+
+impl<T: ~const Default> const A for T {
+    default fn a() -> u32 {
+        2
+    }
+}
+
+impl<T: ~const Default + ~const Sup> const A for T {
+    default fn a() -> u32 {
+        3
+    }
+}
+
+impl<T: ~const Default + ~const Sub> const A for T {
+    fn a() -> u32 {
+        T::foo()
+    }
+}
+
+const _: () = assert!(<()>::a() == 42);
+const _: () = assert!(<u8>::a() == 3);
+const _: () = assert!(<u16>::a() == 2);
+
+fn main() {}