Add test for revealing auto traits in the defining scope

Author: oli-obk

src/test/ui/type-alias-impl-trait/reveal_local.rs

@@ -0,0 +1,26 @@
+#![feature(type_alias_impl_trait)]
+
+use std::fmt::Debug;
+
+type Foo = impl Debug;
+//~^ ERROR cycle detected
+//~| ERROR cycle detected
+
+fn is_send<T: Send>() { }
+
+fn not_good() {
+    // Error: this function does not constrain `Foo` to any particular
+    // hidden type, so it cannot rely on `Send` being true.
+    is_send::<Foo>();
+}
+
+fn not_gooder() {
+    // Constrain `Foo = u32`
+    let x: Foo = 22_u32;
+
+    // while we could know this from the hidden type, it would
+    // need extra roundabout logic to support it.
+    is_send::<Foo>();
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/reveal_local.stderr

@@ -0,0 +1,51 @@
+error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
+  --> $DIR/reveal_local.rs:5:12
+   |
+LL | type Foo = impl Debug;
+   |            ^^^^^^^^^^
+   |
+note: ...which requires type-checking `not_good`...
+  --> $DIR/reveal_local.rs:11:1
+   |
+LL | fn not_good() {
+   | ^^^^^^^^^^^^^
+   = note: ...which again requires computing type of `Foo::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/reveal_local.rs:1:1
+   |
+LL | / #![feature(type_alias_impl_trait)]
+LL | |
+LL | | use std::fmt::Debug;
+LL | |
+...  |
+LL | |
+LL | | fn main() {}
+   | |____________^
+
+error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
+  --> $DIR/reveal_local.rs:5:12
+   |
+LL | type Foo = impl Debug;
+   |            ^^^^^^^^^^
+   |
+note: ...which requires type-checking `not_gooder`...
+  --> $DIR/reveal_local.rs:17:1
+   |
+LL | fn not_gooder() {
+   | ^^^^^^^^^^^^^^^
+   = note: ...which again requires computing type of `Foo::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/reveal_local.rs:1:1
+   |
+LL | / #![feature(type_alias_impl_trait)]
+LL | |
+LL | | use std::fmt::Debug;
+LL | |
+...  |
+LL | |
+LL | | fn main() {}
+   | |____________^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0391`.