Show that the opaque type works in the new solver and is actually instantiatable

Author: oli-obk

tests/ui/type-alias-impl-trait/self-referential-in-fn-trait.stderr renamed to tests/ui/type-alias-impl-trait/self-referential-in-fn-trait.next.stderr

@@ -1,5 +1,5 @@
 error: unconstrained opaque type
-  --> $DIR/self-referential-in-fn-trait.rs:3:16
+  --> $DIR/self-referential-in-fn-trait.rs:13:16
    |
 LL | type Foo<'a> = impl Fn() -> Foo<'a>;
    |                ^^^^^^^^^^^^^^^^^^^^

tests/ui/type-alias-impl-trait/self-referential-in-fn-trait.old.stderr

@@ -0,0 +1,10 @@
+error: unconstrained opaque type
+  --> $DIR/self-referential-in-fn-trait.rs:13:16
+   |
+LL | type Foo<'a> = impl Fn() -> Foo<'a>;
+   |                ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error: aborting due to previous error
+

tests/ui/type-alias-impl-trait/self-referential-in-fn-trait.rs

@@ -1,10 +1,25 @@
+//! This test checks that we do not
+//! end up with an infinite recursion,
+//! a cycle error or an overflow when
+//! encountering an opaque type that has
+//! an associated type that is just itself
+//! again.
+
 #![feature(type_alias_impl_trait)]
+// revisions: next old working
+//[next] compile-flags: -Ztrait-solver=next
+//[working] check-pass
 
 type Foo<'a> = impl Fn() -> Foo<'a>;
-//~^ ERROR: unconstrained opaque type
+//[old,next]~^ ERROR: unconstrained opaque type
 
 fn crash<'a>(_: &'a (), x: Foo<'a>) -> Foo<'a> {
     x
 }
 
+#[cfg(working)]
+fn foo<'a>() -> Foo<'a> {
+    foo
+}
+
 fn main() {}