Add test for issue-70944

Author: JohnTitor

src/test/ui/traits/issue-70944.rs

@@ -0,0 +1,23 @@
+// check-pass
+// Regression test of #70944, should compile fine.
+
+use std::ops::Index;
+
+pub struct KeyA;
+pub struct KeyB;
+pub struct KeyC;
+
+pub trait Foo: Index<KeyA> + Index<KeyB> + Index<KeyC> {}
+pub trait FooBuilder {
+    type Inner: Foo;
+    fn inner(&self) -> &Self::Inner;
+}
+
+pub fn do_stuff(foo: &impl FooBuilder) {
+    let inner = foo.inner();
+    &inner[KeyA];
+    &inner[KeyB];
+    &inner[KeyC];
+}
+
+fn main() {}