diff --git a/include/fast_float/ascii_number.h b/include/fast_float/ascii_number.h
index 81c986a..dea1efb 100644
--- a/include/fast_float/ascii_number.h
+++ b/include/fast_float/ascii_number.h
@@ -299,7 +299,7 @@ parse_number_string(UC const *p, UC const *pend,
       return report_parse_error<UC>(
           p, parse_error::missing_integer_or_dot_after_sign);
     }
-    if (basic_json_fmt) {
+    FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
       if (!is_integer(*p)) { // a sign must be followed by an integer
         return report_parse_error<UC>(p,
                                       parse_error::missing_integer_after_sign);
@@ -328,7 +328,7 @@ parse_number_string(UC const *p, UC const *pend,
   UC const *const end_of_integer_part = p;
   int64_t digit_count = int64_t(end_of_integer_part - start_digits);
   answer.integer = span<UC const>(start_digits, size_t(digit_count));
-  if (basic_json_fmt) {
+  FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
     // at least 1 digit in integer part, without leading zeros
     if (digit_count == 0) {
       return report_parse_error<UC>(p, parse_error::no_digits_in_integer_part);
@@ -357,7 +357,7 @@ parse_number_string(UC const *p, UC const *pend,
     answer.fraction = span<UC const>(before, size_t(p - before));
     digit_count -= exponent;
   }
-  if (basic_json_fmt) {
+  FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
     // at least 1 digit in fractional part
     if (has_decimal_point && exponent == 0) {
       return report_parse_error<UC>(p,
diff --git a/include/fast_float/constexpr_feature_detect.h b/include/fast_float/constexpr_feature_detect.h
index 648b55d..6751afe 100644
--- a/include/fast_float/constexpr_feature_detect.h
+++ b/include/fast_float/constexpr_feature_detect.h
@@ -27,6 +27,12 @@
 #define FASTFLOAT_HAS_IS_CONSTANT_EVALUATED 0
 #endif
 
+#if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606L
+#define FASTFLOAT_IF_CONSTEXPR17(x) if constexpr (x)
+#else
+#define FASTFLOAT_IF_CONSTEXPR17(x) if (x)
+#endif
+
 // Testing for relevant C++20 constexpr library features
 #if FASTFLOAT_HAS_IS_CONSTANT_EVALUATED && FASTFLOAT_HAS_BIT_CAST &&           \
     defined(__cpp_lib_constexpr_algorithms) &&                                 \