fix: Correct assoc ty bound var starting index

Author: ShoyuVanilla

src/tools/rust-analyzer/crates/hir-ty/src/chalk_db.rs

@@ -637,7 +637,10 @@ pub(crate) fn associated_ty_data_query(
         .fill_with_bound_vars(crate::DebruijnIndex::INNERMOST, 0)
         .build();
     let pro_ty = TyBuilder::assoc_type_projection(db, type_alias, Some(trait_subst))
-        .fill_with_bound_vars(crate::DebruijnIndex::INNERMOST, generic_params.len_self())
+        .fill_with_bound_vars(
+            crate::DebruijnIndex::INNERMOST,
+            generic_params.parent_generics().map_or(0, |it| it.len()),
+        )
         .build();
     let self_ty = TyKind::Alias(AliasTy::Projection(pro_ty)).intern(Interner);
 

src/tools/rust-analyzer/crates/hir-ty/src/tests/regression.rs

@@ -2278,3 +2278,26 @@ fn test(x: bool) {
         "#]],
     );
 }
+
+#[test]
+fn issue_19730() {
+    check_infer(
+        r#"
+trait Trait<T = Self> {}
+
+trait Foo {
+    type Bar<A, B>: Trait;
+
+    fn foo<A, B>(bar: Self::Bar<A, B>) {
+        let _ = bar;
+    }
+}
+"#,
+        expect![[r#"
+            83..86 'bar': Foo::Bar<Self, A, B>
+            105..133 '{     ...     }': ()
+            119..120 '_': Foo::Bar<Self, A, B>
+            123..126 'bar': Foo::Bar<Self, A, B>
+        "#]],
+    );
+}