diff --git a/include/fast_float/ascii_number.h b/include/fast_float/ascii_number.h
index 4b298b3..a17e035 100644
--- a/include/fast_float/ascii_number.h
+++ b/include/fast_float/ascii_number.h
@@ -515,6 +515,7 @@ parse_int_string(UC const *p, UC const *pend, T &value,
 
   UC const *const start_num = p;
 
+  // use SIMD if possible
   while (p != pend && *p == UC('0')) {
     ++p;
   }
diff --git a/include/fast_float/float_common.h b/include/fast_float/float_common.h
index daf770c..9ef2e87 100644
--- a/include/fast_float/float_common.h
+++ b/include/fast_float/float_common.h
@@ -68,7 +68,7 @@ using from_chars_result = from_chars_result_t<char>;
 
 template <typename UC> struct parse_options_t {
   FASTFLOAT_CONSTEXPR20 explicit parse_options_t(chars_format fmt = chars_format::general,
-                                     UC dot = UC('.'), unsigned char b = 10) noexcept
+                                     UC dot = UC('.'), uint8_t b = 10) noexcept
       : format(fmt), decimal_point(dot), base(b) {}
 
   /** Which number formats are accepted */
@@ -84,7 +84,7 @@ template <typename UC> struct parse_options_t {
   /** The character used as decimal point */
   const UC decimal_point;
   /** The base used for integers */
-  const unsigned char base;
+  const uint8_t base; /* only allowed from 2 to 36 */
 };
 
 using parse_options = parse_options_t<char>;
@@ -224,7 +224,7 @@ using parse_options = parse_options_t<char>;
 
 namespace fast_float {
 
-fastfloat_really_inline constexpr bool cpp20_and_in_constexpr() noexcept {
+fastfloat_really_inline FASTFLOAT_CONSTEVAL20 bool cpp20_and_in_constexpr() noexcept {
 #if FASTFLOAT_HAS_IS_CONSTANT_EVALUATED
   return std::is_constant_evaluated();
 #else
@@ -1024,6 +1024,8 @@ to_float(
 #endif
 }
 
+#ifndef FASTFLOAT_ONLY_POSITIVE_C_NUMBER_WO_INF_NAN
+
 template <typename = void> struct space_lut {
   static constexpr bool value[] = {
       0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -1049,6 +1051,8 @@ template <typename UC> constexpr bool is_space(UC c) {
   return c < 256 && space_lut<>::value[uint8_t(c)];
 }
 
+#endif
+
 template <typename UC> static constexpr uint64_t int_cmp_zeros() {
   static_assert((sizeof(UC) == 1) || (sizeof(UC) == 2) || (sizeof(UC) == 4),
                 "Unsupported character size");
diff --git a/include/fast_float/parse_number.h b/include/fast_float/parse_number.h
index 0863d00..47f04e8 100644
--- a/include/fast_float/parse_number.h
+++ b/include/fast_float/parse_number.h
@@ -314,12 +314,15 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
       first++;
     }
   }
-#endif
   if (first == last) {
     answer.ec = std::errc::invalid_argument;
     answer.ptr = first;
     return answer;
   }
+#else
+  // We are in parser code with external loop that checks bounds.
+  [[assume((first < last))]];
+#endif
   parsed_number_string_t<UC> const pns =
       parse_number_string<UC>(first, last, options);
   if (!pns.valid) {
@@ -369,8 +372,15 @@ from_chars_int_advanced(UC const *first, UC const *last, T &value,
       first++;
     }
   }
+#else
+  // We are in parser code with external loop that checks bounds.
+  [[assume((first < last))]];
 #endif
-  if (first == last || options.base < 2 || options.base > 36) {
+  if (
+#ifndef FASTFLOAT_ONLY_POSITIVE_C_NUMBER_WO_INF_NAN
+      first == last
+#endif
+      options.base < 2 || options.base > 36) {
     from_chars_result_t<UC> answer;
     answer.ec = std::errc::invalid_argument;
     answer.ptr = first;