[vcpkg] Fix build on older compilers.

[huahang]

Describe the pull request

• What does your PR fix? Fixes #

  • std::is_const_v and std::is_reference_v are introduced in c++17, not available when using older compilers. This change replaces them with std::is_const<T>::value and std::is_reference<T>::value.
  • https://en.cppreference.com/w/cpp/types/is_const
  • https://en.cppreference.com/w/cpp/types/is_reference

• Which triplets are supported/not supported? Have you updated the CI baseline?

  • No CI baseline updated.

• Does your PR follow the maintainer guide?

  • Yes.

Signed-off-by: Hans hans@dkmt.io

[ghost]

All CLA requirements met.

[ras0219]

Hi @huahang; could you specifically mention which compiler versions this change enables? We also require the C++ Filesystem TS which imposes an additional minimum requirement.

[huahang]

Hi @huahang; could you specifically mention which compiler versions this change enables? We also require the C++ Filesystem TS which imposes an additional minimum requirement.

The specific compiler I am using is clang-7 in debian 9. C++ Filesystem TS works fine. It just lacks support of std::is_const_v and std::is_reference_v.

The command I use to build vcpkg is:
CC=clang-7 CXX=clang++-7 ./bootstrap-vcpkg.sh

[huahang]

Hi @huahang; could you specifically mention which compiler versions this change enables? We also require the C++ Filesystem TS which imposes an additional minimum requirement.

The specific compiler I am using is clang-7 in debian 9. C++ Filesystem TS works fine. It just lacks support of std::is_const_v and std::is_reference_v.

The command I use to build vcpkg is:
CC=clang-7 CXX=clang++-7 ./bootstrap-vcpkg.sh

@ras0219 @JackBoosY
Hi, the author has responded. Is there any update?
:-)

[BillyONeal]

Thanks for your contribution!