1 files changed, 39 insertions, 0 deletions

diff --git a/tests/ui/issues/issue-20831-debruijn.rs b/tests/ui/issues/issue-20831-debruijn.rs
new file mode 100644
index 000000000..20d980763
--- /dev/null
+++ b/tests/ui/issues/issue-20831-debruijn.rs
@@ -0,0 +1,39 @@
+// Regression test for #20831: debruijn index account was thrown off
+// by the (anonymous) lifetime in `<Self as Publisher>::Output`
+// below. Note that changing to a named lifetime made the problem go
+// away.
+
+use std::cell::RefCell;
+use std::ops::{Shl, Shr};
+
+pub trait Subscriber {
+    type Input;
+}
+
+pub trait Publisher<'a> {
+    type Output;
+    fn subscribe(&mut self, _: Box<dyn Subscriber<Input=Self::Output> + 'a>);
+}
+
+pub trait Processor<'a> : Subscriber + Publisher<'a> { }
+
+impl<'a, P> Processor<'a> for P where P : Subscriber + Publisher<'a> { }
+
+struct MyStruct<'a> {
+    sub: Box<dyn Subscriber<Input=u64> + 'a>
+}
+
+impl<'a> Publisher<'a> for MyStruct<'a> {
+    type Output = u64;
+    fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
+        // Not obvious, but there is an implicit lifetime here -------^
+        //~^^ ERROR cannot infer
+        //
+        // The fact that `Publisher` is using an implicit lifetime is
+        // what was causing the debruijn accounting to be off, so
+        // leave it that way!
+        self.sub = t;
+    }
+}
+
+fn main() {}