From aa854485cea4da95606b9b61fe5a6a0961e97402 Mon Sep 17 00:00:00 2001
From: Zachary S <zasample18+github@gmail.com>
Date: Fri, 9 Aug 2024 23:28:40 -0500
Subject: [PATCH] Explicitly specify type parameter on FromResidual impls in
 stdlib.

To work around coherence issue. Also adds regression test.
---
 core/src/ops/control_flow.rs    |  4 +++-
 core/src/option.rs              |  4 +++-
 core/tests/ops.rs               |  1 +
 core/tests/ops/from_residual.rs | 26 ++++++++++++++++++++++++++
 4 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 core/tests/ops/from_residual.rs

diff --git a/core/src/ops/control_flow.rs b/core/src/ops/control_flow.rs
index a2709c66b06ad..ab73dc19fcc73 100644
--- a/core/src/ops/control_flow.rs
+++ b/core/src/ops/control_flow.rs
@@ -116,7 +116,9 @@ impl<B, C> ops::Try for ControlFlow<B, C> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<B, C> ops::FromResidual for ControlFlow<B, C> {
+// Note: manually specifying the residual type instead of using the default to work around
+// https://github.com/rust-lang/rust/issues/99940
+impl<B, C> ops::FromResidual<ControlFlow<B, convert::Infallible>> for ControlFlow<B, C> {
     #[inline]
     fn from_residual(residual: ControlFlow<B, convert::Infallible>) -> Self {
         match residual {
diff --git a/core/src/option.rs b/core/src/option.rs
index 6c89c81018038..9cec79c17ca73 100644
--- a/core/src/option.rs
+++ b/core/src/option.rs
@@ -2495,7 +2495,9 @@ impl<T> ops::Try for Option<T> {
 }
 
 #[unstable(feature = "try_trait_v2", issue = "84277")]
-impl<T> ops::FromResidual for Option<T> {
+// Note: manually specifying the residual type instead of using the default to work around
+// https://github.com/rust-lang/rust/issues/99940
+impl<T> ops::FromResidual<Option<convert::Infallible>> for Option<T> {
     #[inline]
     fn from_residual(residual: Option<convert::Infallible>) -> Self {
         match residual {
diff --git a/core/tests/ops.rs b/core/tests/ops.rs
index 2ee0abd399bb6..501e0f33fe4cc 100644
--- a/core/tests/ops.rs
+++ b/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,
diff --git a/core/tests/ops/from_residual.rs b/core/tests/ops/from_residual.rs
new file mode 100644
index 0000000000000..d5c86ccbcd317
--- /dev/null
+++ b/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!()
+    }
+}