[PATCH] Implement LWG 3062, Unnecessary decay_t in is_execution_policy_v

[Thomas Rodgers]

The revised attached patch has been ested x86_64-linux, committed to trunk.
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Thomas Rodgers writes:

> Revised patch.
>
> From 074685cf74b48604244c0c6f1d8cba63ff8915e5 Mon Sep 17 00:00:00 2001
> From: Thomas Rodgers <trodgers@redhat.com>
> Date: Wed, 24 Apr 2019 15:53:45 -0700
> Subject: [PATCH] Implement LWG 3062, Unnecessary decay_t in
>  is_execution_policy_v
>
> 	should be remove_cvref_t
> 	* include/pstl/execution_defs.h (__enable_if_execution_policy):
>         Use std::__remove_cv_ref_t when building with GCC.
> ---
>  libstdc++-v3/include/pstl/execution_defs.h | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/libstdc++-v3/include/pstl/execution_defs.h b/libstdc++-v3/include/pstl/execution_defs.h
> index 86c7a5a770d..0ed4cc30914 100644
> --- a/libstdc++-v3/include/pstl/execution_defs.h
> +++ b/libstdc++-v3/include/pstl/execution_defs.h
> @@ -152,9 +152,15 @@ constexpr bool is_execution_policy_v = __pstl::execution::is_execution_policy<_T
>  namespace __internal
>  {
>  template <class _ExecPolicy, class _T>
> +#if _GLIBCXX_RELEASE >= 9
> +using __enable_if_execution_policy =
> +    typename std::enable_if<__pstl::execution::is_execution_policy<std::__remove_cvref_t<_ExecPolicy>>::value,
> +                            _T>::type;
> +#else
>  using __enable_if_execution_policy =
>      typename std::enable_if<__pstl::execution::is_execution_policy<typename std::decay<_ExecPolicy>::type>::value,
>                              _T>::type;
> +#endif
>  } // namespace __internal
>  
>  } // namespace __pstl