Add regression tests for 123303

Author: compiler-errors

tests/ui/traits/stack-error-order-dependence-2.rs

@@ -0,0 +1,24 @@
+//@ check-pass
+// Regression test for <https://github.com/rust-lang/rust/issues/123303>.
+// This time EXCEPT without `dyn` builtin bounds :^)
+
+pub trait Trait: Supertrait {}
+
+trait Impossible {}
+impl<F: ?Sized + Impossible> Trait for F {}
+
+pub trait Supertrait {}
+
+impl<T: ?Sized + Trait + Impossible> Supertrait for T {}
+
+fn needs_supertrait<T: ?Sized + Supertrait>() {}
+fn needs_trait<T: ?Sized + Trait>() {}
+
+struct A;
+impl Trait for A where A: Supertrait {}
+impl Supertrait for A {}
+
+fn main() {
+    needs_supertrait::<A>();
+    needs_trait::<A>();
+}

tests/ui/traits/stack-error-order-dependence.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+// Regression test for <https://github.com/rust-lang/rust/issues/123303>.
+
+pub trait Trait: Supertrait {}
+
+trait Impossible {}
+impl<F: ?Sized + Impossible> Trait for F {}
+
+pub trait Supertrait {}
+
+impl<T: ?Sized + Trait + Impossible> Supertrait for T {}
+
+fn needs_supertrait<T: ?Sized + Supertrait>() {}
+fn needs_trait<T: ?Sized + Trait>() {}
+
+fn main() {
+    needs_supertrait::<dyn Trait>();
+    needs_trait::<dyn Trait>();
+}