__builtin_strlen is not supported for MSVC e.g and it is not correct to use it without knowledge of this "builtin" in advance

[danlark]

Summary

There is no analogue of constexpr __builtin_strlen for MSVC, so make it through defines as it was done for char_traits<wchar_t>

[EricWF]

What version of MSVC are you trying to use?

Last I checked we don't support MSVC.

__string

268

This doesn't work for GCC.

[danlark]

In D58022#1392267, @EricWF wrote:

What version of MSVC are you trying to use?

Last I checked we don't support MSVC.

We are using Microsoft Visual C++ 2017 14.16.27012 -- almost the last version

Marshall asked us to send all the patches that we did to make libcxx work for Windows

[EricWF]

__string

269

How about

#ifndef _LIBCPP_COMPILER_MSVC
return __builtin_strlen(__s);
#else
...
#endif

[danlark]

__string

269

done

[EricWF]

include/__string

216 ↗

(On Diff #186183)

Please keep the declaration the same.

[EricWF]

MSVC on godbolt provides a constexpr __builtin_strlen. https://godbolt.org/z/n_3vRY

[danlark]

In D58022#1392318, @EricWF wrote:

MSVC on godbolt provides a constexpr __builtin_strlen. https://godbolt.org/z/n_3vRY

I believe it is supported from C++17 --
https://godbolt.org/z/8xsDxu -- C++17
https://godbolt.org/z/SNUPsj -- C++14

__string

struct _LIBCPP_TEMPLATE_VIS char_traits<char>
     void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
     static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
             {return __c1 == __c2;}
     static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return (unsigned char)__c1 < (unsigned char)__c2;}
 
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    static inline size_t _LIBCPP_CONSTEXPR_AFTER_CXX14
-    length(const char_type* __s)  _NOEXCEPT {return __builtin_strlen(__s);}
+    static _LIBCPP_CONSTEXPR_AFTER_CXX14
+    size_t length(const char_type* __s) _NOEXCEPT;
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
     static inline char_type* move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
         {return __n == 0 ? __s1 : (char_type*) memmove(__s1, __s2, __n);}
     static inline char_type* copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
         {
             _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
             return __n == 0 ? __s1 : (char_type*)memcpy(__s1, __s2, __n);
for (; __n; --__n, ++__s1, ++__s2)
         if (lt(*__s2, *__s1))
             return 1;
     }
     return 0;
 #endif
 }
 
 inline _LIBCPP_CONSTEXPR_AFTER_CXX14
+size_t
+char_traits<char>::length(const char_type* __s) _NOEXCEPT
+{
+#if __has_feature(cxx_constexpr_string_builtins)
+    return __builtin_strlen(__s);
+#elif _LIBCPP_STD_VER <= 14
+    return strlen(__s);
+#else
+    size_t __len = 0;
+    for (; !eq(*__s, char_type(0)); ++__s)
+        ++__len;
+    return __len;
+#endif
+}
+
+inline _LIBCPP_CONSTEXPR_AFTER_CXX14
 const char*
 char_traits<char>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
 {
     if (__n == 0)
         return nullptr;
 #if __has_feature(cxx_constexpr_string_builtins)
     return __builtin_char_memchr(__s, to_int_type(__a), __n);
 #elif _LIBCPP_STD_VER <= 14