Fall back to constexpr append for floating point

Use macros to check if the standard library version supports std::to_char
for floating point, otherwise fall back to the constexpr append.

We do need to specifically check the library version, NOT the compiler
version. At least on compilers that will let you use a different library
like GCC and Clang. clang-cl is a possibility, but it will pretend to be
MSVC. This is why we check for libc++ before checking _MSC_VER.

src/snitch_append.cpp

@@ -9,6 +9,13 @@ namespace snitch::impl {
 namespace {
 using snitch::small_string_span;
 
+// libstdc++11 or greater
+// libc++14 or greater
+// MSVC 19.24 or greater (AKA 2019 16.4)
+// Apple clang has no support to date
+#if (defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE > 11) || \
+    (defined(_LIBCPP_VERSION ) && _LIBCPP_VERSION > 14000) || \
+    (defined(_MSC_VER) && _MSC_VER > 1924)
 template<std::floating_point T>
 bool append_to_chars(small_string_span ss, T value) noexcept {
     constexpr auto fmt = std::chars_format::scientific;
@@ -31,6 +38,12 @@ bool append_to_chars(small_string_span ss, T value) noexcept {
     ss.grow(end - ss.end());
     return true;
 }
+#else
+        template<std::floating_point T>
+        bool append_to_chars(small_string_span ss, T value) noexcept {
+            return append_constexpr(ss, value);
+        }
+#endif
 
 template<std::integral T>
 bool append_to_chars(small_string_span ss, T value, int base = 10) noexcept {

tests/runtime_tests/string_utility.cpp

@@ -403,7 +403,7 @@ TEST_CASE("append floats", "[utility]") {
 #else
         // Without std::bit_cast (or C++23), we are unable to tell the difference between -0.0f and
         // +0.0f in constexpr expressions. Therefore -0.0f in constexpr gets displayed as +0.0f.
-        CONSTEXPR_CHECK(a(-0.0f) == aed{{"0.000000e+00"sv, true}, {"-0.000000e+00"sv, true}});
+        CONSTEXPR_CHECK(a(-0.0f) == ae{"0.000000e+00"sv, true});
 #endif
         CONSTEXPR_CHECK(a(1.0f) == ae{"1.000000e+00"sv, true});
         CONSTEXPR_CHECK(a(1.5f) == ae{"1.500000e+00"sv, true});
@@ -514,7 +514,7 @@ TEST_CASE("append doubles", "[utility]") {
         // Without std::bit_cast (or C++23), we are unable to tell the difference between -0.0f and
         // +0.0f in constexpr expressions. Therefore -0.0f in constexpr gets displayed as +0.0f.
         CONSTEXPR_CHECK(
-            a(-0.0) == aed{{"0.000000000000000e+00"sv, true}, {"-0.000000000000000e+00"sv, true}});
+            a(-0.0) == ae{"0.000000000000000e+00"sv, true});
 #endif
         CONSTEXPR_CHECK(a(1.0) == ae{"1.000000000000000e+00"sv, true});
         CONSTEXPR_CHECK(a(1.5) == ae{"1.500000000000000e+00"sv, true});