Deprecate nested types in functional

[JBludau]

The functions in Kokkos_Functional.hpp mimic parts of <functional> but are device portable. See greater as example.
In the std lib the nested typenames result_type, first_argument_type, and second_argument_type are deprecated in c++17 and removed in 20. The respective functions in Kokkos should be marked deprecated when bumping to c++17

Unfortunately, it looks like using declarations can not be attributed with [[deprecated]] but typedef can.

[masterleinad]

Unfortunately, it looks like using declarations can not be attributed with [[deprecated]] but typedef can.

https://godbolt.org/z/9rYv9xxa8 shows that putting [[deprecated]] last seems to work.

[JBludau]

Unfortunately, it looks like using declarations can not be attributed with [[deprecated]] but typedef can.

https://godbolt.org/z/9rYv9xxa8 shows that putting [[deprecated]] last seems to work.

Interesting

[masterleinad]

Do we want to remove it with Kokkos 4 or when using C++20 support? If the former one, we should adjust the comments and wrap it in #ifdef KOKKOS_ENABLE_DEPRECATED_CODE_3. In the latter case with #if defined(KOKKOS_ENABLE_CXX14) || defined(KOKKOS_ENABLE_CXX17).

[JBludau]

Do we want to remove it with Kokkos 4 or when using C++20 support? If the former one, we should adjust the comments and wrap it in #ifdef KOKKOS_ENABLE_DEPRECATED_CODE_3. In the latter case with #if defined(KOKKOS_ENABLE_CXX14) || defined(KOKKOS_ENABLE_CXX17).

I think we should stay in line with what the std does here

[masterleinad]

Fine with me.

[JBludau]

CI failure seems unrelated
1/37 Test #1: KokkosCore_UnitTest_OpenMP ...................***Timeout 1500.69 sec

[dalg24]

I don't get the deprecation message

[JBludau]

After some discussion with Damien I propose to drop the message completely. Hinting towards the cause (deprecation of binary_function in the std) is kind of misleading and this indicates that compilers are not rushing to remove it.

[dalg24]

I would have gone for

#ifdef KOKKOS_ENABLE_DEPRECATED_CODE_3
   using first_argument_type KOKKOS_DEPRECATED  = T;
   using second_argument_type KOKKOS_DEPRECATED = T;
   using result_type KOKKOS_DEPRECATED          = bool;
#endif

I would prefer to plan for simple removal in Kokkos 4.

[JBludau]

ci failure unrelated

[ajpowelsnl]

FYI - cppreference details deprecation of argument_type, first_argument_type, second_argument_type.

What will the usage be after deprecation?

[masterleinad]

What will the usage be after deprecation?

They are not necessary anymore as explained, e.g., in https://stackoverflow.com/a/35907663, because of decltype and perfect forwarding.