library/core/tests/ops.rs

@@ -1,4 +1,5 @@
 mod control_flow;
+mod from_residual;
 
 use core::ops::{
     Bound, Deref, DerefMut, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive,

library/core/tests/ops/from_residual.rs

@@ -0,0 +1,26 @@
+//! Regression test that Option and ControlFlow can have downstream FromResidual impls.
+//! cc https://github.com/rust-lang/rust/issues/99940,
+//! This does NOT test that issue in general; Option and ControlFlow's FromResidual
+//! impls in core were changed to not be affected by that issue.
+
+use core::ops::{ControlFlow, FromResidual};
+
+struct Local;
+
+impl<T> FromResidual<Local> for Option<T> {
+    fn from_residual(_: Local) -> Option<T> {
+        unimplemented!()
+    }
+}
+
+impl<B, C> FromResidual<Local> for ControlFlow<B, C> {
+    fn from_residual(_: Local) -> ControlFlow<B, C> {
+        unimplemented!()
+    }
+}
+
+impl<T, E> FromResidual<Local> for Result<T, E> {
+    fn from_residual(_: Local) -> Result<T, E> {
+        unimplemented!()
+    }
+}