[PATCH, libstdc++/61166] overflow when parse number in std::duration operator""

[Jonathan Wakely]

On 15/05/14 22:54 -0400, Ed Smith-Rowland wrote:
>On 05/15/2014 03:03 PM, Jonathan Wakely wrote:
>>Here's a finished patch to simplify <bits/parse_numbers.h>
>>
>>Tested x86_64-linux. Ed, any objection to this version?
>>
>This looks great, thanks!

I committed that to trunk, I'll put it on the 4.9 branch too.

>Having done that should we actually stop using it as suggested in the 
>bug trail? ;-)

I was going to do that, then realised that there's a defect in the
standard where it requires overflow in duration integer literals to be
diagnosed. That's only possible with literal operator templates, so I
think we should keep your _Parse_int code, but apply the attached
change to detect overflow.

As the TODO comment says, it should be sufficient to simply
instantiate integral_constant<_Rep, _Val::value> to give a diagnostic
when _Rep{_Value::value} is narrowing, but GCC only gives a warning
for it, and that's suppressed in a system header, so I do an explicit
static_assert.  That could be replaced with ...

#pragma GCC diagnostic push
#pragma GCC diagnostic error "-Woverflow"
#pragma GCC diagnostic error "-Wsystem-headers"
    template<typename _Dur, char... _Digits>
      constexpr _Dur __check_overflow()
      {
        using _Val = __parse_int::_Parse_int<_Digits...>;
        using _Rep = typename _Dur::rep;
        return _Dur{integral_constant<_Rep, _Val::value>::value};
      }
#pragma GCC diagnostic pop

... but I have plans to do that sort of thing more widely, which I'll
deal with another time as part of https://gcc.gnu.org/PR50871 and/or
https://gcc.gnu.org/PR58876 (what do other people think about using
diagnostic pragmas to locally re-enable diagnostics in our headers?)

Tested x86_64-linux, committed to trunk.
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