Explicitly specify type parameter on FromResidual impls in stdlib.

To work around coherence issue. Also adds regression test.

library/core/src/ops/control_flow.rs

@@ -116,7 +116,9 @@ fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
 }
 
 #[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 {

library/core/src/option.rs

@@ -2495,7 +2495,9 @@ fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
 }
 
 #[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 {

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!()
+    }
+}

tests/ui/try-trait/bad-interconversion.stderr

@@ -45,7 +45,7 @@ LL |     Some(Err("hello")?)
    |                      ^ use `.ok()?` if you want to discard the `Result<Infallible, &str>` error information
    |
    = help: the trait `FromResidual<Result<Infallible, &str>>` is not implemented for `Option<u16>`
-   = help: the trait `FromResidual` is implemented for `Option<T>`
+   = help: the trait `FromResidual<Option<Infallible>>` is implemented for `Option<T>`
 
 error[E0277]: the `?` operator can only be used on `Option`s in a function that returns `Option`
   --> $DIR/bad-interconversion.rs:27:33
@@ -56,7 +56,7 @@ LL |     Some(ControlFlow::Break(123)?)
    |                                 ^ this `?` produces `ControlFlow<{integer}, Infallible>`, which is incompatible with `Option<u64>`
    |
    = help: the trait `FromResidual<ControlFlow<{integer}, Infallible>>` is not implemented for `Option<u64>`
-   = help: the trait `FromResidual` is implemented for `Option<T>`
+   = help: the trait `FromResidual<Option<Infallible>>` is implemented for `Option<T>`
 
 error[E0277]: the `?` operator can only be used on `ControlFlow`s in a function that returns `ControlFlow`
   --> $DIR/bad-interconversion.rs:32:39

tests/ui/try-trait/option-to-result.stderr

@@ -20,7 +20,7 @@ LL |     a?;
    |      ^ use `.ok()?` if you want to discard the `Result<Infallible, i32>` error information
    |
    = help: the trait `FromResidual<Result<Infallible, i32>>` is not implemented for `Option<i32>`
-   = help: the trait `FromResidual` is implemented for `Option<T>`
+   = help: the trait `FromResidual<Option<Infallible>>` is implemented for `Option<T>`
 
 error: aborting due to 2 previous errors