Fix projection substitution order considering GATs

Author: lowr

chalk-solve/src/clauses.rs

@@ -820,13 +820,14 @@ fn push_alias_alias_eq_clause<I: Interner>(
         //      <<X as Y>::A as Z>::B == U :- <T as Z>::B == U, <X as Y>::A == T
         // }
         builder.push_binders(binders, |builder, bound_var| {
+            let (_, trait_args, assoc_args) = builder.db.split_projection(&projection_ty);
             let fresh_self_subst = Substitution::from_iter(
                 interner,
-                std::iter::once(bound_var.clone().cast(interner)).chain(
-                    projection_ty.substitution.as_slice(interner)[1..]
-                        .iter()
-                        .cloned(),
-                ),
+                assoc_args
+                    .iter()
+                    .cloned()
+                    .chain(std::iter::once(bound_var.clone().cast(interner)))
+                    .chain(trait_args[1..].iter().cloned()),
             );
             let fresh_alias = AliasTy::Projection(ProjectionTy {
                 associated_ty_id: projection_ty.associated_ty_id,

tests/test/projection.rs

@@ -609,6 +609,30 @@ fn normalize_gat_with_higher_ranked_trait_bound() {
     }
 }
 
+#[test]
+fn gat_in_alias_in_alias_eq() {
+    test! {
+        program {
+            trait Foo {
+                type Rebind<U>: Foo;
+            }
+
+            struct S<T> { }
+            impl<T> Foo for S<T> {
+                type Rebind<U> = S<U>;
+            }
+        }
+
+        goal {
+            exists<T> {
+                <<S<u32> as Foo>::Rebind<i32> as Foo>::Rebind<usize>: Foo
+            }
+        } yields {
+            expect![[r#"Unique"#]]
+        }
+    }
+}
+
 #[test]
 fn forall_projection() {
     test! {