1 files changed, 40 insertions, 0 deletions
diff --git a/src/test/ui/coercion/coerce-unsize-subtype.rs b/src/test/ui/coercion/coerce-unsize-subtype.rs
new file mode 100644
index 000000000..45b53300c
--- /dev/null
+++ b/src/test/ui/coercion/coerce-unsize-subtype.rs
@@ -0,0 +1,40 @@
+// run-pass
+#![allow(dead_code)]
+// pretty-expanded FIXME #23616
+
+use std::rc::Rc;
+
+fn lub_short<'a, T>(_: &[&'a T], _: &[&'a T]) {}
+
+// The two arguments are a subtype of their LUB, after coercion.
+fn long_and_short<'a, T>(xs: &[&'static T; 1], ys: &[&'a T; 1]) {
+    lub_short(xs, ys);
+}
+
+// The argument coerces to a subtype of the return type.
+fn long_to_short<'a, 'b, T>(xs: &'b [&'static T; 1]) -> &'b [&'a T] {
+    xs
+}
+
+// Rc<T> is covariant over T just like &T.
+fn long_to_short_rc<'a, T>(xs: Rc<[&'static T; 1]>) -> Rc<[&'a T]> {
+    xs
+}
+
+// LUB-coercion (if-else/match/array) coerces `xs: &'b [&'static T: N]`
+// to a subtype of the LUB of `xs` and `ys` (i.e., `&'b [&'a T]`),
+// regardless of the order they appear (in if-else/match/array).
+fn long_and_short_lub1<'a, 'b, T>(xs: &'b [&'static T; 1], ys: &'b [&'a T]) {
+    let _order1 = [xs, ys];
+    let _order2 = [ys, xs];
+}
+
+// LUB-coercion should also have the exact same effect when `&'b [&'a T; N]`
+// needs to be coerced, i.e., the resulting type is not &'b [&'static T], but
+// rather the `&'b [&'a T]` LUB.
+fn long_and_short_lub2<'a, 'b, T>(xs: &'b [&'static T], ys: &'b [&'a T; 1]) {
+    let _order1 = [xs, ys];
+    let _order2 = [ys, xs];
+}
+
+fn main() {}