Don't create projection ty for const projection

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1272,14 +1272,29 @@ fn project<'cx, 'tcx>(
         ProjectionCandidateSet::Single(candidate) => {
             Ok(Projected::Progress(confirm_candidate(selcx, obligation, candidate)))
         }
-        ProjectionCandidateSet::None => Ok(Projected::NoProgress(
+        ProjectionCandidateSet::None => {
-            // FIXME(associated_const_generics): this may need to change in the future?
+            let tcx = selcx.tcx();
-            // need to investigate whether or not this is fine.
+            let term = match tcx.def_kind(obligation.predicate.def_id) {
-            selcx
+                DefKind::AssocTy | DefKind::ImplTraitPlaceholder => tcx
-                .tcx()
+                    .mk_projection(obligation.predicate.def_id, obligation.predicate.substs)
-                .mk_projection(obligation.predicate.def_id, obligation.predicate.substs)
+                    .into(),
-                .into(),
+                DefKind::AssocConst => tcx
-        )),
+                    .mk_const(
+                        ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(
+                            obligation.predicate.def_id,
+                            obligation.predicate.substs,
+                        )),
+                        tcx.type_of(obligation.predicate.def_id)
+                            .subst(tcx, obligation.predicate.substs),
+                    )
+                    .into(),
+                kind => {
+                    bug!("unknown projection def-id: {}", kind.descr(obligation.predicate.def_id))
+                }
+            };
+
+            Ok(Projected::NoProgress(term))
+        }
         // Error occurred while trying to processing impls.
         ProjectionCandidateSet::Error(e) => Err(ProjectionError::TraitSelectionError(e)),
         // Inherent ambiguity that prevents us from even enumerating the

tests/ui/associated-consts/projection-unspecified-but-bounded.rs

@@ -0,0 +1,16 @@
+#![feature(associated_const_equality)]
+
+// Issue 110549
+
+pub trait TraitWAssocConst {
+    const A: usize;
+}
+
+fn foo<T: TraitWAssocConst<A = 32>>() {}
+
+fn bar<T: TraitWAssocConst>() {
+    foo::<T>();
+    //~^ ERROR type mismatch resolving `<T as TraitWAssocConst>::A == 32`
+}
+
+fn main() {}

tests/ui/associated-consts/projection-unspecified-but-bounded.stderr

@@ -0,0 +1,17 @@
+error[E0271]: type mismatch resolving `<T as TraitWAssocConst>::A == 32`
+  --> $DIR/projection-unspecified-but-bounded.rs:12:11
+   |
+LL |     foo::<T>();
+   |           ^ expected `32`, found `<T as TraitWAssocConst>::A`
+   |
+   = note: expected constant `32`
+              found constant `<T as TraitWAssocConst>::A`
+note: required by a bound in `foo`
+  --> $DIR/projection-unspecified-but-bounded.rs:9:28
+   |
+LL | fn foo<T: TraitWAssocConst<A = 32>>() {}
+   |                            ^^^^^^ required by this bound in `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.