Re: [PATCH] Simplify overflow checks in duration literals

[Jonathan Wakely <jwakely at redhat dot com>]

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).