Add inference cycle TAIT test

src/test/ui/type-alias-impl-trait/inference-cycle.rs

@@ -0,0 +1,26 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+mod m {
+    type Foo = impl std::fmt::Debug;
+    //~^ ERROR: cycle detected when computing type of `m::Foo::{opaque#0}` [E0391]
+
+    // Cycle: error today, but it'd be nice if it eventually worked
+
+    pub fn foo() -> Foo {
+        is_send(bar())
+    }
+
+    pub fn bar() {
+        is_send(foo()); // Today: error
+    }
+
+    fn baz() {
+        let f: Foo = 22_u32;
+        //~^ ERROR: mismatched types [E0308]
+    }
+
+    fn is_send<T: Send>(_: T) {}
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/inference-cycle.stderr

@@ -0,0 +1,37 @@
+error[E0391]: cycle detected when computing type of `m::Foo::{opaque#0}`
+  --> $DIR/inference-cycle.rs:5:16
+   |
+LL |     type Foo = impl std::fmt::Debug;
+   |                ^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires type-checking `m::bar`...
+  --> $DIR/inference-cycle.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/inference-cycle.rs:4:1
+   |
+LL | mod m {
+   | ^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/inference-cycle.rs:19:22
+   |
+LL |     type Foo = impl std::fmt::Debug;
+   |                -------------------- the expected opaque type
+...
+LL |         let f: Foo = 22_u32;
+   |                ---   ^^^^^^ expected opaque type, found `u32`
+   |                |
+   |                expected due to this
+   |
+   = note: expected opaque type `impl Debug`
+                     found type `u32`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0391.
+For more information about an error, try `rustc --explain E0308`.