[PATCH] Simplify overflow checks in duration literals

[Jonathan Wakely]

On 14/08/18 09:59 -0400, Marek Polacek wrote:
>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?

Yes, thanks for that! That was what made me revisit the TODO. But
using integral_constant there doesn't produce the required error,
because it's in a system header. The standard requires it to be
ill-formed, so I can't rely on the narrowing diagnostic :-(

But I realised it could be simplified anyway (and that could have been
done years ago, without waiting for the fixes to narrowing in template
arguments).