From 7ce50f87c0439e7352680fd76bdaa5ae02cfecfc Mon Sep 17 00:00:00 2001
From: Lucas Sunsi Abreu <lsunsi@proton.me>
Date: Tue, 8 Sep 2026 07:05:59 -0300
Subject: [PATCH 1/2] Add test for soundness issue on new solver

---
 tests/ui/const-generics/dyn-trait-ill-typed.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 tests/ui/const-generics/dyn-trait-ill-typed.rs

diff --git a/tests/ui/const-generics/dyn-trait-ill-typed.rs b/tests/ui/const-generics/dyn-trait-ill-typed.rs
new file mode 100644
index 0000000000000..d6c1fdc29fb53
--- /dev/null
+++ b/tests/ui/const-generics/dyn-trait-ill-typed.rs
@@ -0,0 +1,14 @@
+//! Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/296>.
+//@ compile-flags: -Znext-solver=globally
+//@ check-pass
+
+// CHECK PASS TO SHOW IT PASSES, BUT IT SHOULD NOT
+// THIS CODE SEGFAULTS, WITH REASON
+
+fn foo<const M: u32>() -> Box<dyn Tr<M>> {
+    loop {}
+}
+
+trait Tr<const N: usize> {}
+
+fn main() {}

From 265ca102b5ec5755a92bfa50c246df39a4943732 Mon Sep 17 00:00:00 2001
From: Lucas Sunsi Abreu <lsunsi@proton.me>
Date: Mon, 7 Sep 2026 11:04:13 -0300
Subject: [PATCH 2/2] Make ConstKind Placeholder also check ConstArgHasType

---
 compiler/rustc_trait_selection/src/traits/wf.rs    |  5 ++++-
 tests/ui/const-generics/dyn-trait-ill-typed.rs     |  3 ++-
 tests/ui/const-generics/dyn-trait-ill-typed.stderr | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/const-generics/dyn-trait-ill-typed.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 5fc9e57795b72..1de2318fe77f4 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -1003,7 +1003,10 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                         let kind = obligation.predicate.kind().skip_binder();
                         let keep = match kind {
                             ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _))
-                                if matches!(ct.kind(), ty::ConstKind::Param(..)) =>
+                                if matches!(
+                                    ct.kind(),
+                                    ty::ConstKind::Param(..) | ty::ConstKind::Placeholder(..)
+                                ) =>
                             {
                                 // ConstArgHasType clauses are not higher kinded. Assert as
                                 // such so we can fix this up if that ever changes.
diff --git a/tests/ui/const-generics/dyn-trait-ill-typed.rs b/tests/ui/const-generics/dyn-trait-ill-typed.rs
index d6c1fdc29fb53..3dba700a7fa39 100644
--- a/tests/ui/const-generics/dyn-trait-ill-typed.rs
+++ b/tests/ui/const-generics/dyn-trait-ill-typed.rs
@@ -1,10 +1,11 @@
 //! Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/296>.
 //@ compile-flags: -Znext-solver=globally
-//@ check-pass
+//@ check-fail
 
 // CHECK PASS TO SHOW IT PASSES, BUT IT SHOULD NOT
 // THIS CODE SEGFAULTS, WITH REASON
 
+//~v ERROR: the constant `M` is not of type `usize`
 fn foo<const M: u32>() -> Box<dyn Tr<M>> {
     loop {}
 }
diff --git a/tests/ui/const-generics/dyn-trait-ill-typed.stderr b/tests/ui/const-generics/dyn-trait-ill-typed.stderr
new file mode 100644
index 0000000000000..8917b53d4d826
--- /dev/null
+++ b/tests/ui/const-generics/dyn-trait-ill-typed.stderr
@@ -0,0 +1,14 @@
+error: the constant `M` is not of type `usize`
+  --> $DIR/dyn-trait-ill-typed.rs:9:27
+   |
+LL | fn foo<const M: u32>() -> Box<dyn Tr<M>> {
+   |                           ^^^^^^^^^^^^^^ expected `usize`, found `u32`
+   |
+note: required by a const generic parameter in `Tr`
+  --> $DIR/dyn-trait-ill-typed.rs:13:10
+   |
+LL | trait Tr<const N: usize> {}
+   |          ^^^^^^^^^^^^^^ required by this const generic parameter in `Tr`
+
+error: aborting due to 1 previous error
+