Add regression test

Author: oli-obk

tests/ui/impl-trait/bind-hidden-type-in-projection-matching.rs

@@ -0,0 +1,25 @@
+#![feature(type_alias_impl_trait)]
+trait Trait<'a> {
+    type Out<U>;
+}
+
+impl<'a, T> Trait<'a> for T {
+    type Out<U> = T;
+}
+
+type Foo = impl Sized;
+//~^ ERROR: unconstrained opaque type
+
+fn weird_bound<X>(x: &<X as Trait<'static>>::Out<Foo>) -> X
+//~^ ERROR: item does not constrain
+where
+    for<'a> X: Trait<'a>,
+    for<'a> <X as Trait<'a>>::Out<()>: Copy,
+{
+    let x = *x; //~ ERROR: cannot move out of `*x`
+    todo!();
+}
+
+fn main() {
+    let _: () = weird_bound(&());
+}

tests/ui/impl-trait/bind-hidden-type-in-projection-matching.stderr

@@ -0,0 +1,36 @@
+error: item does not constrain `Foo::{opaque#0}`, but has it in its signature
+  --> $DIR/bind-hidden-type-in-projection-matching.rs:13:4
+   |
+LL | fn weird_bound<X>(x: &<X as Trait<'static>>::Out<Foo>) -> X
+   |    ^^^^^^^^^^^
+   |
+   = note: consider moving the opaque type's declaration and defining uses into a separate module
+note: this opaque type is in the signature
+  --> $DIR/bind-hidden-type-in-projection-matching.rs:10:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+
+error: unconstrained opaque type
+  --> $DIR/bind-hidden-type-in-projection-matching.rs:10:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same module
+
+error[E0507]: cannot move out of `*x` which is behind a shared reference
+  --> $DIR/bind-hidden-type-in-projection-matching.rs:19:13
+   |
+LL |     let x = *x;
+   |             ^^ move occurs because `*x` has type `<X as Trait<'_>>::Out<Foo>`, which does not implement the `Copy` trait
+   |
+help: consider removing the dereference here
+   |
+LL -     let x = *x;
+LL +     let x = x;
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0507`.