diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
index f1519e5c7b6..26b69a38521 100644
--- a/libstdc++-v3/src/c++17/floating_from_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -300,12 +300,16 @@ namespace
 	errno = 0;
 	char* endptr;
 	T tmpval;
+#if _GLIBCXX_USE_C99_STDLIB
 	if constexpr (is_same_v<T, float>)
 	  tmpval = std::strtof(str, &endptr);
-	if constexpr (is_same_v<T, double>)
+	else if constexpr (is_same_v<T, double>)
 	  tmpval = std::strtod(str, &endptr);
 	else if constexpr (is_same_v<T, long double>)
 	  tmpval = std::strtold(str, &endptr);
+#else
+	tmpval = std::strtod(str, &endptr);
+#endif
 	const int conv_errno = std::__exchange(errno, save_errno);
 
 #if _GLIBCXX_USE_C99_FENV_TR1
@@ -319,7 +323,7 @@ namespace
 	const ptrdiff_t n = endptr - str;
 	if (conv_errno == ERANGE) [[unlikely]]
 	  {
-	    if (std::isinf(tmpval)) // overflow
+	    if (isinf(tmpval)) // overflow
 	      ec = errc::result_out_of_range;
 	    else // underflow (LWG 3081 wants to set value = tmpval here)
 	      ec = errc::result_out_of_range;