From c8ce14018f082a747307a5bbc5b46ec24c2642dd Mon Sep 17 00:00:00 2001
From: Takayuki Maeda <takoyaki0316@gmail.com>
Date: Sun, 1 Feb 2026 05:27:13 +0900
Subject: [PATCH] check if ProjectionTy or ProjectionConst

fmt
---
 .../src/error_reporting/traits/fulfillment_errors.rs   |  2 ++
 .../opaque-alias-relate-issue-151331.rs                | 10 ++++++++++
 .../opaque-alias-relate-issue-151331.stderr            |  9 +++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.rs
 create mode 100644 tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.stderr

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index 6872d038fb7f..45e4f5fe23a6 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -1607,6 +1607,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         };
 
                     if let Some(lhs) = lhs.to_alias_term()
+                        && let ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst = lhs.kind(self.tcx)
                         && let Some((better_type_err, expected_term)) =
                             derive_better_type_error(lhs, rhs)
                     {
@@ -1615,6 +1616,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             better_type_err,
                         )
                     } else if let Some(rhs) = rhs.to_alias_term()
+                        && let ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst = rhs.kind(self.tcx)
                         && let Some((better_type_err, expected_term)) =
                             derive_better_type_error(rhs, lhs)
                     {
diff --git a/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.rs b/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.rs
new file mode 100644
index 000000000000..684f2498d584
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.rs
@@ -0,0 +1,10 @@
+//@ compile-flags: -Znext-solver=globally
+#![feature(type_alias_impl_trait)]
+
+type Foo = Vec<impl Send>;
+
+#[define_opaque(Foo)]
+fn make_foo() -> Foo {}
+//~^ ERROR type mismatch resolving
+
+fn main() {}
diff --git a/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.stderr b/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.stderr
new file mode 100644
index 000000000000..dd73ed1a247c
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.stderr
@@ -0,0 +1,9 @@
+error[E0271]: type mismatch resolving `Foo == ()`
+  --> $DIR/opaque-alias-relate-issue-151331.rs:7:18
+   |
+LL | fn make_foo() -> Foo {}
+   |                  ^^^ types differ
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0271`.