Add auto trait leakage TAIT test that should work but currently doesn't

Author: spastorino

src/test/ui/type-alias-impl-trait/auto-trait-leakage3.rs

@@ -0,0 +1,21 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+// FIXME This should compile, but it currently doesn't
+
+mod m {
+    type Foo = impl std::fmt::Debug;
+    //~^ ERROR: cycle detected when computing type of `m::Foo::{opaque#0}` [E0391]
+
+    pub fn foo() -> Foo {
+        22_u32
+    }
+
+    pub fn bar() {
+        is_send(foo());
+    }
+
+    fn is_send<T: Send>(_: T) {}
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/auto-trait-leakage3.stderr

@@ -0,0 +1,22 @@
+error[E0391]: cycle detected when computing type of `m::Foo::{opaque#0}`
+  --> $DIR/auto-trait-leakage3.rs:7:16
+   |
+LL |     type Foo = impl std::fmt::Debug;
+   |                ^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires type-checking `m::bar`...
+  --> $DIR/auto-trait-leakage3.rs:14:5
+   |
+LL |     pub fn bar() {
+   |     ^^^^^^^^^^^^
+   = note: ...which requires evaluating trait selection obligation `impl std::fmt::Debug: std::marker::Send`...
+   = note: ...which again requires computing type of `m::Foo::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in module `m`
+  --> $DIR/auto-trait-leakage3.rs:6:1
+   |
+LL | mod m {
+   | ^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.