Remap RPITIT substs properly

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2260,6 +2260,13 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
     // since `data.substs` are the impl substs.
     let impl_fn_substs =
         obligation.predicate.substs.rebase_onto(tcx, tcx.parent(trait_fn_def_id), data.substs);
+    let impl_fn_substs = translate_substs(
+        selcx.infcx(),
+        obligation.param_env,
+        data.impl_def_id,
+        impl_fn_substs,
+        leaf_def.defining_node,
+    );
 
     if !check_substs_compatible(tcx, &leaf_def.item, impl_fn_substs) {
         let err = tcx.ty_error_with_message(

src/test/ui/impl-trait/in-trait/specialization-substs-remap.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+#![feature(specialization)]
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    fn bar(&self) -> impl Sized;
+}
+
+impl<U> Foo for U
+where
+    U: Copy,
+{
+    fn bar(&self) -> U {
+        *self
+    }
+}
+
+impl Foo for i32 {}
+
+fn main() {
+    let _: i32 = 1i32.bar();
+}