Rollup merge of #47668 - nikomatsakis:issue-47511, r=eddyb

do not ICE when return type includes unconstrained anon region

It turns out that this *can* happen after all, if the region is only
used in projections from the input types.

Fixes https://github.com/rust-lang/rust/issues/47511

r? @eddyb

src/test/ui/issue-47511.rs

@@ -0,0 +1,35 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #47511: anonymous lifetimes can appear
+// unconstrained in a return type, but only if they appear just once
+// in the input, as the input to a projection.
+
+fn f(_: X) -> X {
+    //~^ ERROR return type references an anonymous lifetime
+    unimplemented!()
+}
+
+fn g<'a>(_: X<'a>) -> X<'a> {
+    //~^ ERROR return type references lifetime `'a`, which is not constrained
+    unimplemented!()
+}
+
+type X<'a> = <&'a () as Trait>::Value;
+
+trait Trait {
+    type Value;
+}
+
+impl<'a> Trait for &'a () {
+    type Value = ();
+}
+
+fn main() {}

src/test/ui/issue-47511.stderr

@@ -0,0 +1,16 @@
+error[E0581]: return type references an anonymous lifetime which is not constrained by the fn input types
+  --> $DIR/issue-47511.rs:15:15
+   |
+15 | fn f(_: X) -> X {
+   |               ^
+   |
+   = note: lifetimes appearing in an associated type are not considered constrained
+
+error[E0581]: return type references lifetime `'a`, which is not constrained by the fn input types
+  --> $DIR/issue-47511.rs:20:23
+   |
+20 | fn g<'a>(_: X<'a>) -> X<'a> {
+   |                       ^^^^^
+
+error: aborting due to 2 previous errors
+