Report correct SelectionError for ConstArgHasType in new solver fulfill

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -10,9 +10,9 @@ use rustc_infer::traits::{
     self, FromSolverError, MismatchedProjectionTypes, Obligation, ObligationCause,
     ObligationCauseCode, PredicateObligation, PredicateObligations, SelectionError, TraitEngine,
 };
-use rustc_middle::bug;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::{bug, span_bug};
 use rustc_next_trait_solver::solve::{GenerateProofTree, HasChanged, SolverDelegateEvalExt as _};
 use tracing::{instrument, trace};
 
@@ -258,6 +258,23 @@ fn fulfillment_error_for_no_solution<'tcx>(
                 MismatchedProjectionTypes { err: TypeError::Mismatch },
             )
         }
+        ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, expected_ty)) => {
+            let ct_ty = match ct.kind() {
+                ty::ConstKind::Unevaluated(uv) => {
+                    infcx.tcx.type_of(uv.def).instantiate(infcx.tcx, uv.args)
+                }
+                ty::ConstKind::Param(param_ct) => param_ct.find_ty_from_env(obligation.param_env),
+                _ => span_bug!(
+                    obligation.cause.span,
+                    "ConstArgHasWrongType failed but we don't know how to compute type"
+                ),
+            };
+            FulfillmentErrorCode::Select(SelectionError::ConstArgHasWrongType {
+                ct,
+                ct_ty,
+                expected_ty,
+            })
+        }
         ty::PredicateKind::NormalizesTo(..) => {
             FulfillmentErrorCode::Project(MismatchedProjectionTypes { err: TypeError::Mismatch })
         }

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.stderr → tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.current.stderr

@@ -1,11 +1,11 @@
 error[E0308]: mismatched types
-  --> $DIR/const-in-impl-fn-return-type.rs:15:39
+  --> $DIR/const-in-impl-fn-return-type.rs:20:39
    |
 LL |     fn func<const N: u32>() -> [(); { () }] {
    |                                       ^^ expected `usize`, found `()`
 
 error: the constant `N` is not of type `usize`
-  --> $DIR/const-in-impl-fn-return-type.rs:7:32
+  --> $DIR/const-in-impl-fn-return-type.rs:12:32
    |
 LL |     fn func<const N: u32>() -> [(); N];
    |                                ^^^^^^^ expected `usize`, found `u32`

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.next.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/const-in-impl-fn-return-type.rs:20:39
+   |
+LL |     fn func<const N: u32>() -> [(); { () }] {
+   |                                       ^^ expected `usize`, found `()`
+
+error: the constant `N` is not of type `usize`
+  --> $DIR/const-in-impl-fn-return-type.rs:12:32
+   |
+LL |     fn func<const N: u32>() -> [(); N];
+   |                                ^^^^^^^ expected `usize`, found `u32`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/issue-114918/const-in-impl-fn-return-type.rs

@@ -1,4 +1,9 @@
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
 // Regression test for #114918
+
 // Test that a const generic enclosed in a block within the return type
 // of an impl fn produces a type mismatch error instead of triggering
 // a const eval cycle