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Guard <charconv> include with __has_include for GCC 7

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GCC 7 sets __cplusplus to the C++17 value under -std=gnu++1z, so
JSON_HAS_CPP_17 is defined, but its libstdc++ ships no <charconv> header
(added in GCC 8; floating-point from_chars in GCC 11). The unconditional
"#if defined(JSON_HAS_CPP_17) #include <charconv>" therefore failed to
compile there: "fatal error: charconv: No such file or directory" in the
ci_test_compilers_gcc (7) job.

Wrap the include in __has_include(<charconv>), mirroring the library's
existing handling of <version> and <filesystem> in macro_scope.hpp. When
the header is absent, __cpp_lib_to_chars stays undefined and
parse_float_from_chars() takes its scalar fallback, so the from_chars use
site (already gated on __cpp_lib_to_chars) is never reached. GCC 8-10,
which have <charconv> but no floating-point from_chars, are unaffected:
they include the header but still take the fallback. GCC 11+ 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
2026-07-20 22:51:03 +00:00
parent 0d5b70a95a
commit 41833047cc
2 changed files with 18 additions and 6 deletions
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include/nlohmann/detail/input/number_parse.hpp
+9 -3
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@@ -16,9 +16,15 @@
#include <nlohmann/detail/macro_scope.hpp>
#if defined(JSON_HAS_CPP_17)
#include <charconv> // from_chars (only used when __cpp_lib_to_chars is defined)
#include <system_error> // errc
// std::from_chars lives in <charconv>, but being in C++17 mode does not
// guarantee the header exists: GCC 7 sets __cplusplus to C++17 yet ships no
// <charconv> (added in GCC 8; floating-point support in GCC 11). Guard the
// include with __has_include so such toolchains fall back to the scalar path.
#if defined(JSON_HAS_CPP_17) && defined(__has_include)
#if __has_include(<charconv>)
#include <charconv> // from_chars (only used when __cpp_lib_to_chars is defined)
#include <system_error> // errc
#endif
#endif
// This file contains the value-conversion helpers used by the lexer to turn an
single_include/nlohmann/json.hpp
+9 -3
View File
@@ -7799,9 +7799,15 @@ NLOHMANN_JSON_NAMESPACE_END
// #include <nlohmann/detail/macro_scope.hpp>
#if defined(JSON_HAS_CPP_17)
#include <charconv> // from_chars (only used when __cpp_lib_to_chars is defined)
#include <system_error> // errc
// std::from_chars lives in <charconv>, but being in C++17 mode does not
// guarantee the header exists: GCC 7 sets __cplusplus to C++17 yet ships no
// <charconv> (added in GCC 8; floating-point support in GCC 11). Guard the
// include with __has_include so such toolchains fall back to the scalar path.
#if defined(JSON_HAS_CPP_17) && defined(__has_include)
#if __has_include(<charconv>)
#include <charconv> // from_chars (only used when __cpp_lib_to_chars is defined)
#include <system_error> // errc
#endif
#endif
// This file contains the value-conversion helpers used by the lexer to turn an