Allow ~const bounds on inherent impls

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1655,7 +1655,9 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 walk_list!(self, visit_ty, ty);
             }
             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, ref body))
-                if self.in_const_trait_impl || ctxt == AssocCtxt::Trait =>
+                if self.in_const_trait_impl
+                    || ctxt == AssocCtxt::Trait
+                    || matches!(sig.header.constness, Const::Yes(_)) =>
             {
                 self.visit_vis(&item.vis);
                 self.visit_ident(item.ident);

src/test/ui/rfc-2632-const-trait-impl/inherent-impl-const-bounds.rs

@@ -0,0 +1,21 @@
+// check-pass
+#![feature(const_trait_impl)]
+#![feature(const_fn_trait_bound)]
+
+struct S;
+
+trait A {}
+trait B {}
+
+impl const A for S {}
+impl const B for S {}
+
+impl S {
+    const fn a<T: ~const A>() where T: ~const B {
+
+    }
+}
+
+const _: () = S::a::<S>();
+
+fn main() {}