Add a regression test for an ICE when using an explicit tail call in a default trait method's implementation.

Author: jakubadamw

tests/ui/explicit-tail-calls/default-trait-method.rs

@@ -0,0 +1,36 @@
+// A regression test for <https://github.com/rust-lang/rust/issues/144985>.
+// Previously, using `become` in a default trait method would lead to an ICE
+// in a path determining whether the method in question is marked as `#[track_caller]`.
+//
+//@ run-pass
+
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+trait Trait {
+    fn bar(&self) -> usize {
+        123
+    }
+
+    fn foo(&self) -> usize {
+        become self.bar();
+    }
+}
+
+struct Struct;
+
+impl Trait for Struct {}
+
+struct OtherStruct;
+
+impl Trait for OtherStruct {
+    #[track_caller]
+    fn bar(&self) -> usize {
+        456
+    }
+}
+
+fn main() {
+    assert_eq!(Struct.foo(), 123);
+    assert_eq!(OtherStruct.foo(), 456);
+}