Add an ATB test

tests/ui/associated-type-bounds/no-gat-position.rs

@@ -0,0 +1,18 @@
+#![feature(associated_type_bounds)]
+
+// Test for <https://github.com/rust-lang/rust/issues/119857>.
+
+pub trait Iter {
+    type Item<'a>: 'a where Self: 'a;
+
+    fn next<'a>(&'a mut self) -> Option<Self::Item<'a, As1: Copy>>;
+    //~^ ERROR associated type bindings are not allowed here
+}
+
+impl Iter for () {
+    type Item<'a> = &'a mut [()];
+
+    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>> { None }
+}
+
+fn main() {}

tests/ui/associated-type-bounds/no-gat-position.stderr

@@ -0,0 +1,9 @@
+error[E0229]: associated type bindings are not allowed here
+  --> $DIR/no-gat-position.rs:8:56
+   |
+LL |     fn next<'a>(&'a mut self) -> Option<Self::Item<'a, As1: Copy>>;
+   |                                                        ^^^^^^^^^ associated type not allowed here
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0229`.