From 41833047ccbf9a51566eb363a4333e5685029194 Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Mon, 20 Jul 2026 22:51:03 +0000
Subject: [PATCH] Guard <charconv> include with __has_include for GCC 7

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>
---
 include/nlohmann/detail/input/number_parse.hpp | 12 +++++++++---
 single_include/nlohmann/json.hpp               | 12 +++++++++---
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/nlohmann/detail/input/number_parse.hpp b/include/nlohmann/detail/input/number_parse.hpp
index 29be3a67e..e50c3f67f 100644
--- a/include/nlohmann/detail/input/number_parse.hpp
+++ b/include/nlohmann/detail/input/number_parse.hpp
@@ -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
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index f7dd2b705..70ed6f75d 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -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