add a test for subtyping in tail call signature check

Author: WaffleLapkin

tests/ui/explicit-tail-calls/subtyping.rs

@@ -0,0 +1,28 @@
+// Check that signature comparison for tail calls allows subtyping.
+//
+//@ check-pass
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn f<'short>(
+    a: &'static (),
+    b: &'short (),
+    c: fn(&'static ()),
+    d: for<'a> fn(&'a ()),
+) -> (&'short (), fn(&'static ()), for<'a> fn(&'a ())) {
+    become g(b, a, d, c);
+}
+
+fn g<'short>(
+    // swapped short/static
+    a: &'short (),
+    b: &'static (),
+    // swapped binder/non binder
+    c: for<'a> fn(&'a ()),
+    d: fn(&'static ()),
+    // 'short=>'static; non binder=>binder
+) -> (&'static (), for<'a> fn(&'a ()), for<'a> fn(&'a ())) {
+    (b, c, c)
+}
+
+fn main() {}