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Guard from_chars use on JSON_HAS_CPP_17, not just __cpp_lib_to_chars

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libstdc++ 15 defines __cpp_lib_to_chars even in C++14 mode (via
bits/version.h pulled in by other headers), but <charconv> is only included
under JSON_HAS_CPP_17. That made parse_float_from_chars() reference
std::from_chars without the header in C++14 builds, breaking gcc-latest,
icpx, and the offline-testdata jobs.

Gate the use on JSON_HAS_CPP_17 && __cpp_lib_to_chars so it matches the
include condition exactly; C++11/14 always take the scalar fallback.
Verified by forcing __cpp_lib_to_chars in a C++14 build: the guard
suppresses std::from_chars and it compiles. C++17 behavior is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXcDtEma2PjxgmPS9cQGzA
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-08-04 08:55:09 +02:00
co-authored by Claude Opus 4.8
parent 387d766f4e
commit 8d7828aff0
2 changed files with 8 additions and 2 deletions
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include/nlohmann/detail/input/number_parse.hpp
+4 -1
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@@ -263,7 +263,10 @@ expects (side-stepping the P4168 divergence between implementations).
template<typename FloatType> template<typename FloatType>
bool parse_float_from_chars(const char* first, const char* last, FloatType& out) noexcept bool parse_float_from_chars(const char* first, const char* last, FloatType& out) noexcept
{ {
#if defined(__cpp_lib_to_chars) // JSON_HAS_CPP_17 must gate the use as well as the <charconv> include above:
// some standard libraries (e.g. libstdc++ 15) define __cpp_lib_to_chars even
// in C++14 mode, where <charconv> is not included.
#if defined(JSON_HAS_CPP_17) && defined(__cpp_lib_to_chars)
const auto result = std::from_chars(first, last, out); const auto result = std::from_chars(first, last, out);
return result.ec == std::errc() && result.ptr == last; return result.ec == std::errc() && result.ptr == last;
#else #else
single_include/nlohmann/json.hpp
+4 -1
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@@ -8062,7 +8062,10 @@ expects (side-stepping the P4168 divergence between implementations).
template<typename FloatType> template<typename FloatType>
bool parse_float_from_chars(const char* first, const char* last, FloatType& out) noexcept bool parse_float_from_chars(const char* first, const char* last, FloatType& out) noexcept
{ {
#if defined(__cpp_lib_to_chars) // JSON_HAS_CPP_17 must gate the use as well as the <charconv> include above:
// some standard libraries (e.g. libstdc++ 15) define __cpp_lib_to_chars even
// in C++14 mode, where <charconv> is not included.
#if defined(JSON_HAS_CPP_17) && defined(__cpp_lib_to_chars)
const auto result = std::from_chars(first, last, out); const auto result = std::from_chars(first, last, out);
return result.ec == std::errc() && result.ptr == last; return result.ec == std::errc() && result.ptr == last;
#else #else