Fix CI failures in from_chars locale handling (PR #5237)

Three issues in the extended-locale number parsing were breaking CI:

- On Windows, _create_locale() was called with LC_ALL_MASK, a POSIX-only
  constant that doesn't exist in the Windows CRT; it needs LC_ALL there.
- `::isspace_l`/`::_isspace_l` were called with a leading `::`, but on
  glibc, mingw-w64 and some other C libraries these are macros that expand
  to a parenthesized expression, so `::` followed by the expansion is a
  syntax error ("expected unqualified-id"). Dropping the `::` fixes this
  on all affected toolchains (gcc/clang on Linux, nvcc, mingw).
- The Windows implementation assumed the full `_<funcname>_l` family is
  always available, but some MinGW toolchains only provide a subset
  (e.g. missing `_strtoll_l`/`_strtoull_l`/`_strtof_l`/`_strtold_l`).
  The extended-locale fast path is now only used for genuine MSVC
  (including clang-cl, which mimics the MSVC ABI/CRT); other Windows
  toolchains fall back to the locale-dependent standard functions.

Also fixes a clang-tidy hicpp-named-parameter/readability-named-parameter
warning-as-error on two unnamed `init_val_t` test parameters.

Verified locally with gcc/clang on Linux, clang++ in C++20 mode, and
mingw-w64 (both g++ and clang targeting x86_64-w64-mingw32).

include/nlohmann/detail/conversions/from_chars.hpp

@@ -190,6 +190,58 @@ inline from_chars_result from_chars(const char* first, const char* last, T& valu
 
 #if defined(_WIN32)
 
+/*!
+@brief Extended locale functions on Windows
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the Windows implementation. The primary class template definition
+falls back to the standard locale-dependent functions, which is used on
+MinGW, whose C runtime only provides an incomplete subset of the
+`_<funcname>_l` family (e.g., missing `_strtof_l`/`_strtold_l` depending on
+the toolchain version). A specialization for genuine MSVC (including
+clang-cl, which mimics the MSVC ABI and CRT) is used instead, as the full
+`_<funcname>_l` family has reliably been available there since at least
+Visual Studio 2005.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+#if defined(_MSC_VER)
+
 /*!
 @brief Get a pointer to a globally shared instance of the "C" locale on Windows
 
@@ -208,58 +260,51 @@ inline _locale_t get_c_locale_t() noexcept
         }
     };
     using LocaleUPtr = std::unique_ptr<std::remove_pointer<_locale_t>::type, LocaleTDeleter>;
-    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL_MASK, "C"), LocaleTDeleter{}};
+    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL, "C"), LocaleTDeleter{}};
     return c_locale.get();
 }
 
-/*!
-@brief Extended locale functions on Windows
-
-A trait object which provides wrappers for isspace and the strto* family of
-functions, based on the platform's support for extended locale APIs.
-
-This is the Windows implementation where extended locale support is always
-available through the `_<funcname>_l` variants.
-*/
-template <typename = const char*, typename = float>
-struct extended_locale_traits
+template <typename T>
+struct extended_locale_traits<T, float>
 {
     static constexpr bool is_locale_independent = true;
     static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
 
     static int isspace(int c) noexcept
     {
-        return ::_isspace_l(c, get_c_locale_t());
+        return _isspace_l(c, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)
     static long long strtoll(const char* str, char** endptr) noexcept
     {
-        return ::_strtoll_l(str, endptr, 10, get_c_locale_t());
+        return _strtoll_l(str, endptr, 10, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)
     static unsigned long long strtoull(const char* str, char** endptr) noexcept
     {
-        return ::_strtoull_l(str, endptr, 10, get_c_locale_t());
+        return _strtoull_l(str, endptr, 10, get_c_locale_t());
     }
 
-    static float strtof(const char* str, char** str_end) noexcept
+    static float strtof(T str, char** str_end)
     {
-        return ::_strtof_l(str, str_end, get_c_locale_t());
+        return _strtof_l(str, str_end, get_c_locale_t());
     }
 
-    static double strtod(const char* str, char** str_end) noexcept
+    static double strtod(T str, char** str_end)
     {
-        return ::_strtod_l(str, str_end, get_c_locale_t());
+        return _strtod_l(str, str_end, get_c_locale_t());
     }
 
-    static long double strtold(const char* str, char** str_end) noexcept
+    static long double strtold(T str, char** str_end)
     {
-        return ::_strtold_l(str, str_end, get_c_locale_t());
+        return _strtold_l(str, str_end, get_c_locale_t());
     }
 };
 
+#endif
+
 #else
 
 /*!
@@ -360,7 +405,7 @@ struct extended_locale_traits<T, decltype(strtof_l(
 
     static int isspace(int c) noexcept
     {
-        return ::isspace_l(c, get_c_locale_t());
+        return isspace_l(c, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)

single_include/nlohmann/json.hpp

@@ -7906,6 +7906,58 @@ inline from_chars_result from_chars(const char* first, const char* last, T& valu
 
 #if defined(_WIN32)
 
+/*!
+@brief Extended locale functions on Windows
+
+A trait object which provides wrappers for isspace and the strto* family of
+functions, based on the platform's support for extended locale APIs.
+
+This is the Windows implementation. The primary class template definition
+falls back to the standard locale-dependent functions, which is used on
+MinGW, whose C runtime only provides an incomplete subset of the
+`_<funcname>_l` family (e.g., missing `_strtof_l`/`_strtold_l` depending on
+the toolchain version). A specialization for genuine MSVC (including
+clang-cl, which mimics the MSVC ABI and CRT) is used instead, as the full
+`_<funcname>_l` family has reliably been available there since at least
+Visual Studio 2005.
+*/
+template <typename = const char*, typename = float>
+struct extended_locale_traits
+{
+    static constexpr bool is_locale_independent = false;
+
+    JSON_HEDLEY_PURE
+    static char get_decimal_point() noexcept
+    {
+        const auto* loc = localeconv();
+        JSON_ASSERT(loc != nullptr);
+        return (loc->decimal_point == nullptr) ? '.' : *(loc->decimal_point);
+    }
+
+    static int isspace(int c) noexcept
+    {
+        return std::isspace(c);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static long long strtoll(const char* str, char** endptr) noexcept
+    {
+        return std::strtoll(str, endptr, 10);
+    }
+
+    // NOLINTNEXTLINE(runtime/int)
+    static unsigned long long strtoull(const char* str, char** endptr) noexcept
+    {
+        return std::strtoull(str, endptr, 10);
+    }
+
+    static constexpr float(&strtof)(const char*, char**) = std::strtof;
+    static constexpr double(&strtod)(const char*, char**) = std::strtod;
+    static constexpr long double(&strtold)(const char*, char**) = std::strtold;
+};
+
+#if defined(_MSC_VER)
+
 /*!
 @brief Get a pointer to a globally shared instance of the "C" locale on Windows
 
@@ -7924,58 +7976,51 @@ inline _locale_t get_c_locale_t() noexcept
         }
     };
     using LocaleUPtr = std::unique_ptr<std::remove_pointer<_locale_t>::type, LocaleTDeleter>;
-    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL_MASK, "C"), LocaleTDeleter{}};
+    static const LocaleUPtr c_locale = LocaleUPtr{::_create_locale(LC_ALL, "C"), LocaleTDeleter{}};
     return c_locale.get();
 }
 
-/*!
-@brief Extended locale functions on Windows
-
-A trait object which provides wrappers for isspace and the strto* family of
-functions, based on the platform's support for extended locale APIs.
-
-This is the Windows implementation where extended locale support is always
-available through the `_<funcname>_l` variants.
-*/
-template <typename = const char*, typename = float>
-struct extended_locale_traits
+template <typename T>
+struct extended_locale_traits<T, float>
 {
     static constexpr bool is_locale_independent = true;
     static constexpr char(&get_decimal_point)() = get_locale_independent_decimal_point;
 
     static int isspace(int c) noexcept
     {
-        return ::_isspace_l(c, get_c_locale_t());
+        return _isspace_l(c, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)
     static long long strtoll(const char* str, char** endptr) noexcept
     {
-        return ::_strtoll_l(str, endptr, 10, get_c_locale_t());
+        return _strtoll_l(str, endptr, 10, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)
     static unsigned long long strtoull(const char* str, char** endptr) noexcept
     {
-        return ::_strtoull_l(str, endptr, 10, get_c_locale_t());
+        return _strtoull_l(str, endptr, 10, get_c_locale_t());
     }
 
-    static float strtof(const char* str, char** str_end) noexcept
+    static float strtof(T str, char** str_end)
     {
-        return ::_strtof_l(str, str_end, get_c_locale_t());
+        return _strtof_l(str, str_end, get_c_locale_t());
     }
 
-    static double strtod(const char* str, char** str_end) noexcept
+    static double strtod(T str, char** str_end)
     {
-        return ::_strtod_l(str, str_end, get_c_locale_t());
+        return _strtod_l(str, str_end, get_c_locale_t());
     }
 
-    static long double strtold(const char* str, char** str_end) noexcept
+    static long double strtold(T str, char** str_end)
     {
-        return ::_strtold_l(str, str_end, get_c_locale_t());
+        return _strtold_l(str, str_end, get_c_locale_t());
     }
 };
 
+#endif
+
 #else
 
 /*!
@@ -8076,7 +8121,7 @@ struct extended_locale_traits<T, decltype(strtof_l(
 
     static int isspace(int c) noexcept
     {
-        return ::isspace_l(c, get_c_locale_t());
+        return isspace_l(c, get_c_locale_t());
     }
 
     // NOLINTNEXTLINE(runtime/int)

tests/src/unit-from_chars.cpp

@@ -79,7 +79,7 @@ check(std::string str, T expected_value, std::ptrdiff_t expected_ptr_offset, std
 
 template <typename T>
 typename std::enable_if<std::is_integral<T>::value, void>::type
-check(std::string str, init_val_t, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+check(std::string str, init_val_t /*unused*/, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
 {
     T const init_value = 42;
     T value = init_value;
@@ -89,7 +89,7 @@ check(std::string str, init_val_t, std::ptrdiff_t expected_ptr_offset, std::errc
 
 template <typename T>
 typename std::enable_if<std::is_floating_point<T>::value, void>::type
-check(std::string str, init_val_t, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
+check(std::string str, init_val_t /*unused*/, std::ptrdiff_t expected_ptr_offset, std::errc expected_ec)
 {
     {
         T const init_value = 42;