Fix type resolution of associated const equality bounds (take 2)

compiler/rustc_hir_analysis/src/astconv/bounds.rs

@@ -408,7 +408,7 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
             // Create the generic arguments for the associated type or constant by joining the
             // parent arguments (the arguments of the trait) and the own arguments (the ones of
             // the associated item itself) and construct an alias type using them.
-            candidate.map_bound(|trait_ref| {
+            let alias_ty = candidate.map_bound(|trait_ref| {
                 let ident = Ident::new(assoc_item.name, binding.ident.span);
                 let item_segment = hir::PathSegment {
                     ident,
@@ -430,7 +430,27 @@ impl<'tcx> dyn AstConv<'tcx> + '_ {
                 // *constants* to represent *const projections*. Alias *term* would be a more
                 // appropriate name but alas.
                 ty::AliasTy::new(tcx, assoc_item.def_id, alias_args)
-            })
+            });
+
+            // Provide the resolved type of the associated constant to `type_of(AnonConst)`.
+            if !speculative && let ty::AssocKind::Const = assoc_kind {
+                let hir::TypeBindingKind::Equality { term: hir::Term::Const(anon_const) } =
+                    binding.kind
+                else {
+                    bug!()
+                };
+                let ty = alias_ty.map_bound(|ty| tcx.type_of(ty.def_id).instantiate(tcx, ty.args));
+                // Since the arguments passed to the alias type above may contain early-bound
+                // generic parameters, the instantiated type may contain some as well.
+                // Therefore wrap it in `EarlyBinder`.
+                // FIXME(fmease): Reject escaping late-bound vars.
+                tcx.feed_anon_const_type(
+                    anon_const.def_id,
+                    ty::EarlyBinder::bind(ty.skip_binder()),
+                );
+            }
+
+            alias_ty
         };
 
         match binding.kind {

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -78,37 +78,6 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
                 .expect("const parameter types cannot be generic");
         }
 
-        Node::TypeBinding(binding @ &TypeBinding { hir_id: binding_id, .. })
-            if let Node::TraitRef(trait_ref) = tcx.parent_hir_node(binding_id) =>
-        {
-            let Some(trait_def_id) = trait_ref.trait_def_id() else {
-                return Ty::new_error_with_message(
-                    tcx,
-                    tcx.def_span(def_id),
-                    "Could not find trait",
-                );
-            };
-            let assoc_items = tcx.associated_items(trait_def_id);
-            let assoc_item = assoc_items.find_by_name_and_kind(
-                tcx,
-                binding.ident,
-                ty::AssocKind::Const,
-                def_id.to_def_id(),
-            );
-            return if let Some(assoc_item) = assoc_item {
-                tcx.type_of(assoc_item.def_id)
-                    .no_bound_vars()
-                    .expect("const parameter types cannot be generic")
-            } else {
-                // FIXME(associated_const_equality): add a useful error message here.
-                Ty::new_error_with_message(
-                    tcx,
-                    tcx.def_span(def_id),
-                    "Could not find associated const on trait",
-                )
-            };
-        }
-
         // This match arm is for when the def_id appears in a GAT whose
         // path can't be resolved without typechecking e.g.
         //

tests/ui/associated-consts/assoc-const-eq-supertraits.rs

@@ -0,0 +1,16 @@
+// Regression test for issue #118040.
+// Ensure that we support assoc const eq bounds where the assoc const comes from a supertrait.
+
+//@ check-pass
+
+#![feature(associated_const_equality)]
+
+trait Trait: SuperTrait {}
+trait SuperTrait: SuperSuperTrait<i32> {}
+trait SuperSuperTrait<T> {
+    const K: T;
+}
+
+fn take(_: impl Trait<K = 0>) {}
+
+fn main() {}

tests/ui/associated-consts/assoc-const-eq-ty-alias-noninteracting.rs

@@ -1,21 +1,25 @@
 // Regression test for issue #112560.
 // Respect the fact that (associated) types and constants live in different namespaces and
 // therefore equality bounds involving identically named associated items don't conflict if
-// their kind (type vs. const) differs.
-
-// FIXME(fmease): Extend this test to cover supertraits again
-// once #118040 is fixed. See initial version of PR #118360.
+// their kind (type vs. const) differs. This obviously extends to supertraits.
 
 //@ check-pass
 
 #![feature(associated_const_equality)]
 
-trait Trait {
+trait Trait: SuperTrait {
     type N;
+    type Q;
 
     const N: usize;
 }
 
-fn take(_: impl Trait<N = 0, N = ()>) {}
+trait SuperTrait {
+    const Q: &'static str;
+}
+
+fn take0(_: impl Trait<N = 0, N = ()>) {}
+
+fn take1(_: impl Trait<Q = "...", Q = [()]>) {}
 
 fn main() {}

tests/ui/generic-const-items/associated-const-equality.rs

@@ -1,22 +1,31 @@
 //@ check-pass
 
-#![feature(generic_const_items, associated_const_equality)]
+#![feature(generic_const_items, associated_const_equality, adt_const_params)]
 #![allow(incomplete_features)]
 
 trait Owner {
     const C<const N: u32>: u32;
     const K<const N: u32>: u32;
+    const Q<T>: Maybe<T>;
 }
 
 impl Owner for () {
     const C<const N: u32>: u32 = N;
     const K<const N: u32>: u32 = N + 1;
+    const Q<T>: Maybe<T> = Maybe::Nothing;
 }
 
 fn take0<const N: u32>(_: impl Owner<C<N> = { N }>) {}
 fn take1(_: impl Owner<K<99> = 100>) {}
+fn take2(_: impl Owner<Q<()> = { Maybe::Just(()) }>) {}
 
 fn main() {
     take0::<128>(());
     take1(());
 }
+
+#[derive(PartialEq, Eq, std::marker::ConstParamTy)]
+enum Maybe<T> {
+    Nothing,
+    Just(T),
+}