Summary:
VS 2017 RC brings with it fixes for some things and unfortunately, a few known regressions in expression SFINAE. In this case, alias templates with dependent `decltype()` calls are broken, so switch to a templated struct instead.
It's the very last issue mentioned in: https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/
Reviewed By: yfeldblum
Differential Revision:
D4199676
fbshipit-source-id:
9ce157cc891182509a30b000de1b509063387244
/// possible to construct a value of this type, but it is always the same value
/// every time, so it is uninteresting.
struct Unit {
/// possible to construct a value of this type, but it is always the same value
/// every time, so it is uninteresting.
struct Unit {
+ // These are structs rather than type aliases because MSVC 2017 RC has
+ // trouble correctly resolving dependent expressions in type aliases
+ // in certain very specific contexts, including a couple where this is
+ // used. See the known issues section here for more info:
+ // https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/
+
- using Lift = std::conditional<std::is_same<T, void>::value, Unit, T>;
+ struct Lift : std::conditional<std::is_same<T, void>::value, Unit, T> {};
- using Drop = std::conditional<std::is_same<T, Unit>::value, void, T>;
+ struct Drop : std::conditional<std::is_same<T, Unit>::value, void, T> {};
bool operator==(const Unit& /*other*/) const { return true; }
bool operator!=(const Unit& /*other*/) const { return false; }
bool operator==(const Unit& /*other*/) const { return true; }
bool operator!=(const Unit& /*other*/) const { return false; }