[committed] libstdc++: Add std::from_chars for floating-point types

[Jonathan Wakely]

On 27/07/20 11:41 +0200, Rainer Orth wrote:
>Hi Jonathan,
>
>> This adds the missing std::from_chars overloads for floating-point
>> types, as required for C++17 conformance.
>>
>> The implementation is a hack and not intended to be used in the long
>> term. Rather than parsing the string directly, this determines the
>> initial portion of the string that matches the pattern determined by the
>> chars_format parameter, then creates a NTBS to be parsed by strtod (or
>> strtold or strtof).
>>
>> Because creating a NTBS requires allocating memory, but std::from_chars
>> is noexcept, we need to be careful to minimise allocation. Even after
>> being careful, allocation failure is still possible, and so a
>> non-conforming std::no_more_memory error code might be returned.
>>
>> Because strtod et al depend on the current locale, but std::from_chars
>> does not, we change the current thread's locale to "C" using newlocale
>> and uselocale before calling strtod, and restore it afterwards.
>>
>> Because strtod doesn't have the equivalent of a std::chars_format
>> parameter, it has to examine the input to determine the format in use,
>> even though the std::from_chars code has already parsed it once (or
>> twice for large input strings!)
>>
>> By replacing the use of strtod we could avoid allocation, avoid changing
>> locale, and use optimised code paths specific to each std::chars_format
>> case. We would also get more portable behaviour, rather than depending
>> on the presence of uselocale, and on any bugs or quirks of the target
>> libc's strtod. Replacing strtod is a project for a later date.
>
>two of the new tests FAIL on Solaris 11.3 only:
>
>+FAIL: 20_util/from_chars/4.cc (test for excess errors)
>+UNRESOLVED: 20_util/from_chars/4.cc compilation failed to produce executable
>+FAIL: 20_util/from_chars/5.cc (test for excess errors)
>+UNRESOLVED: 20_util/from_chars/5.cc compilation failed to produce executable
>
>Excess errors:
>/vol/gcc/src/hg/master/local/libstdc++-v3/testsuite/20_util/from_chars/4.cc:41: error: no matching function for call to 'from_chars(char*, char*, double&, std::chars_format&)'
>[...]
>
>AFAICT that's due to the fact that Solaris 11.3 (unlike 11.4) lacks
>uselocale.

Ah yes, the tests should not run unconditionally. I'll fix that today,
thanks.