From 9871e160eae4376dea2b9fa3aa1ccca3607fc1c3 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 28 May 2025 21:13:58 +0000
Subject: [PATCH] Normalize possibly unnormalized type in
 relate_type_and_user_type

---
 compiler/rustc_borrowck/src/type_check/mod.rs |  3 ++
 .../normalizing-user-annotation.rs            | 31 +++++++++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 tests/ui/nll/user-annotations/normalizing-user-annotation.rs

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 4d57e7aac584..3c95ebf9ffab 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -482,6 +482,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         }
         trace!(?curr_projected_ty);
 
+        // Need to renormalize `a` as typecheck may have failed to normalize
+        // higher-ranked aliases if normalization was ambiguous due to inference.
+        let a = self.normalize(a, locations);
         let ty = self.normalize(curr_projected_ty.ty, locations);
         self.relate_types(ty, v.xform(ty::Contravariant), a, locations, category)?;
 
diff --git a/tests/ui/nll/user-annotations/normalizing-user-annotation.rs b/tests/ui/nll/user-annotations/normalizing-user-annotation.rs
new file mode 100644
index 000000000000..fa8b3bfd577d
--- /dev/null
+++ b/tests/ui/nll/user-annotations/normalizing-user-annotation.rs
@@ -0,0 +1,31 @@
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// Regression test for <https://github.com/rust-lang/rust/issues/141708>.
+
+// See description in there; this has to do with fundamental limitations
+// to the old trait solver surrounding higher-ranked aliases with infer
+// vars. This always worked in the new trait solver, but I added a revision
+// just for good measure.
+
+trait Foo<'a> {
+    type Assoc;
+}
+
+impl Foo<'_> for i32 {
+    type Assoc = u32;
+}
+
+impl Foo<'_> for u32 {
+    type Assoc = u32;
+}
+
+fn foo<'b: 'b, T: for<'a> Foo<'a>, F: for<'a> Fn(<T as Foo<'a>>::Assoc)>(_: F) -> (T, F) {
+    todo!()
+}
+
+fn main() {
+    let (x, c): (i32, _) = foo::<'static, _, _>(|_| {});
+}