From 134d2ce56f81b96320f1ffb5473a3be41654e590 Mon Sep 17 00:00:00 2001
From: Yuki Okamoto <cord@x56.cx>
Date: Sun, 21 Jun 2026 16:50:37 +0900
Subject: [PATCH] fix: defer const normalize in coherence mode

---
 .../src/infer/relate/generalize.rs            | 13 +++++++-
 ...herence-overlap-const-projection-157937.rs | 33 +++++++++++++++++++
 ...nce-overlap-const-projection-157937.stderr | 12 +++++++
 3 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.rs
 create mode 100644 tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.stderr

diff --git a/compiler/rustc_infer/src/infer/relate/generalize.rs b/compiler/rustc_infer/src/infer/relate/generalize.rs
index ee6e13250e709..ae5db4ea177c2 100644
--- a/compiler/rustc_infer/src/infer/relate/generalize.rs
+++ b/compiler/rustc_infer/src/infer/relate/generalize.rs
@@ -797,11 +797,22 @@ impl<'tcx> TypeRelation<TyCtxt<'tcx>> for Generalizer<'_, 'tcx> {
             // We only need to be careful with potentially normalizeable
             // aliases here. See `generalize_alias_term` for more information.
             ty::ConstKind::Alias(ty::IsRigid::No, alias_const) => {
+                // a normalizable const in coherence mode misses overlaps; see #157937.
+                // expand anon consts with `expand_abstract_consts`.
+                let defer_in_coherence = self.infcx.typing_mode_raw().is_coherence()
+                    && matches!(
+                        tcx.expand_abstract_consts(c).kind(),
+                        ty::ConstKind::Alias(_, e)
+                            if !matches!(e.kind, ty::AliasConstKind::Anon { .. })
+                    );
+
                 match self.structurally_relate_aliases {
                     // Hack: Fall back to old behavior if GCE is enabled (it used to just be the Yes
                     // path), as doing this new No path breaks some GCE things. I expect GCE to be
                     // ripped out soon so this shouldn't matter soon.
-                    StructurallyRelateAliases::No if !tcx.features().generic_const_exprs() => {
+                    StructurallyRelateAliases::No
+                        if !tcx.features().generic_const_exprs() || defer_in_coherence =>
+                    {
                         self.generalize_alias_term(alias_const.into()).map(|v| v.expect_const())
                     }
                     _ => {
diff --git a/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.rs b/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.rs
new file mode 100644
index 0000000000000..7f7766dbf035e
--- /dev/null
+++ b/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.rs
@@ -0,0 +1,33 @@
+// Cited from [#157937](https://github.com/rust-lang/rust/issues/157937)
+
+#![feature(generic_const_exprs)]
+trait Trait {
+    const CONST: usize;
+}
+struct A<T> {
+    _marker: T,
+}
+impl<const N: usize> Trait for [i8; N] {
+    const CONST: usize = N;
+}
+impl<const N: usize> From<usize> for A<[i8; N]> {
+    fn from(_: usize) -> Self {
+        todo!()
+    }
+}
+impl<T: Trait> From<A<[i8; T::CONST]>> for A<T> {
+//~^ ERROR: conflicting implementations of trait `From<A<[i8; _]>>` for type `A<[i8; _]>` [E0119]
+    fn from(_: A<[i8; T::CONST]>) -> Self {
+        todo!()
+    }
+}
+fn f<T: Trait>() -> A<T>
+where
+    [(); T::CONST]:,
+{
+    let a = A::from(0);
+    A::from(a)
+}
+fn main() {
+    f::<[i8; 1]>();
+}
diff --git a/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.stderr b/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.stderr
new file mode 100644
index 0000000000000..8d72acec7da52
--- /dev/null
+++ b/tests/ui/const-generics/generic_const_exprs/coherence-overlap-const-projection-157937.stderr
@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `From<A<[i8; _]>>` for type `A<[i8; _]>`
+  --> $DIR/coherence-overlap-const-projection-157937.rs:18:1
+   |
+LL | impl<T: Trait> From<A<[i8; T::CONST]>> for A<T> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<T> From<T> for T;
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.