[PATCH] Simplify overflow checks in duration literals

[Marek Polacek]

On Tue, Aug 14, 2018 at 02:55:17PM +0100, Jonathan Wakely wrote:
> 	* include/std/chrono (__check_overflow): Simplify definition.
> 	(_Checked_integral_constant): Remove.
> 
> Tested x86_64-linux, committed to trunk.
> 
> 

> commit c0dec9d05de4695136694f78f2f76e3c9f15b3f1
> Author: Jonathan Wakely <jwakely@redhat.com>
> Date:   Tue Aug 14 14:33:38 2018 +0100
> 
>     Simplify overflow checks in duration literals
>     
>             * include/std/chrono (__check_overflow): Simplify definition.
>             (_Checked_integral_constant): Remove.
> 
> diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
> index da03fdccce4..871c896144a 100644
> --- a/libstdc++-v3/include/std/chrono
> +++ b/libstdc++-v3/include/std/chrono
> @@ -900,24 +900,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    {
>  #pragma GCC diagnostic push
>  #pragma GCC diagnostic ignored "-Wliteral-suffix"
> -    template<typename _Rep, unsigned long long _Val>
> -      struct _Checked_integral_constant
> -      : integral_constant<_Rep, static_cast<_Rep>(_Val)>
> -      {
> -	static_assert(_Checked_integral_constant::value >= 0
> -		      && _Checked_integral_constant::value == _Val,
> -		      "literal value cannot be represented by duration type");
> -      };
> -
>      template<typename _Dur, char... _Digits>
>        constexpr _Dur __check_overflow()
>        {
>  	using _Val = __parse_int::_Parse_int<_Digits...>;
> -	using _Rep = typename _Dur::rep;
> -	// TODO: should be simply integral_constant<_Rep, _Val::value>
> -	// but GCC doesn't reject narrowing conversions to _Rep.

Note that I fixed 57891 yesterday.  Or is this either 65043 or 78244?

> -	using _CheckedVal = _Checked_integral_constant<_Rep, _Val::value>;
> -	return _Dur{_CheckedVal::value};
> +	constexpr typename _Dur::rep __repval = _Val::value;
> +	static_assert(__repval >= 0 && __repval == _Val::value,
> +		      "literal value cannot be represented by duration type");
> +	return _Dur(__repval);
>        }
>  
>      constexpr chrono::duration<long double, ratio<3600,1>>


Marek