Normalize possibly unnormalized type in relate_type_and_user_type

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)?;
 

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, _, _>(|_| {});
+}