[libc++][P0980] Marked member functions move/copy/assign of char_traits constexpr.

[ldionne]

libcxx/include/__string

227

I think it'd be better to have if (__libcpp_is_constant_evaluated()) { ... } else { ... } than just the early return. I normally don't care about this stuff, however in the case of is_constant_evaluated, I feel like having a clear if-else makes it clearer that we're doing the same thing in both branches, one in the compile-time world and one in the runtime world.

Also, it might be a good idea to define a __memmove_constexpr function that we can call as-is from the else branch?

[mpark]

Added helper functions __move_constexpr, __copy_constexpr, and __assign_constexpr.

[ldionne]

This is looking good. I especially like how the tests require only minimal change between the runtime and the compile-time version. I think this is what we should strive for pretty much every time we constexpr-ify something going forward.

libcxx/include/__string

199

This would be clearer (or something along those lines):

// constexpr version of the C functions of the same name

202

__memmove_constexpr?

215

__memcpy_constexpr?

223

__memset_constexpr?

664

Is that necessary?

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp

20

Did you forget to include the change to test_macros.h (or whatever file) where you add TEST_CONSTEXPR_CXX20? I can't find it.

Also, I would suggest TEST_CONSTEXPR_AFTER_CXX17, to mirror the checks we usually make (which are always of the form #if STD > 17.

[ldionne]

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp

20

Did you forget to include the change to test_macros.h (or whatever file) where you add TEST_CONSTEXPR_CXX20? I can't find it.

Nevermind this, I just saw the other review. My bad.

Also, I would suggest TEST_CONSTEXPR_AFTER_CXX17, to mirror the checks we usually make (which are always of the form #if STD > 17.

Forget this, I'll comment on D68837 instead.

[mpark]

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp

20

Perhaps you decided to drop this issue since I don't see a follow-up comment on D68837.
Just as a note, I went with TEST_CONSTEXPR_CXX20 to be consistent with TEST_CONSTEXPR_CXX14.

[mpark]

Elaborated on the constexpr comment.

libcxx/include/__string

202

I decided against this name here because this is more refactoring of the move algorithm for constexpr.
Specifically, we use this as the constexpr version of wmemmove as well as memmmove.

Same line of reasoning for the below requests.

664

Yes, this is necessary due to this test:

assert(std::char_traits<char16_t>::move(NULL, s1, 0) == NULL);

where, if n == 0, we should not perform the __s1 < __s2 check since that is UB.

[mpark]

Minor formatting fixes.

[mpark]

Rebased on master.

[mpark]

Rebased on top of the new is_constant_evaluated aware macro.

[mpark]

Rebased

[mpark]

Updated char_traits synopsis.

[mpark]

Rebased on master.

libcxx/include/__string

static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int compare(const char_type* __s1, const char_type* __s2, size_t __n);
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     size_t length(const char_type* __s);
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a);
-    static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n);
+    static _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n);
     _LIBCPP_INLINE_VISIBILITY
-    static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n);
+    static _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n);
     _LIBCPP_INLINE_VISIBILITY
-    static char_type*       assign(char_type* __s, size_t __n, char_type __a);
+    static _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type*       assign(char_type* __s, size_t __n, char_type __a);
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     static inline _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     static inline _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
for (; __n; --__n)
         if (eq(*__s, __a))
             return __s;
         ++__s;
     }
     return 0;
 }
 
 template <class _CharT>
-_CharT*
+_LIBCPP_CONSTEXPR_AFTER_CXX17 _CharT*
 char_traits<_CharT>::move(char_type* __s1, const char_type* __s2, size_t __n)
 {
     char_type* __r = __s1;
     if (__s1 < __s2)
     {
         for (; __n; --__n, ++__s1, ++__s2)
             assign(*__s1, *__s2);
     }
     else if (__s2 < __s1)
     {
         __s1 += __n;
         __s2 += __n;
         for (; __n; --__n)
             assign(*--__s1, *--__s2);
     }
     return __r;
 }
 
 template <class _CharT>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 _CharT*
 char_traits<_CharT>::copy(char_type* __s1, const char_type* __s2, size_t __n)
 {
     _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
     char_type* __r = __s1;
     for (; __n; --__n, ++__s1, ++__s2)
         assign(*__s1, *__s2);
     return __r;
 }
 
 template <class _CharT>
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 _CharT*
 char_traits<_CharT>::assign(char_type* __s, size_t __n, char_type __a)
 {
     char_type* __r = __s;
     for (; __n; --__n, ++__s)
         assign(*__s, __a);
     return __r;
 }
 
 // char_traits<char>
 
 template <>
 struct _LIBCPP_TEMPLATE_VIS char_traits<char>
 {
     typedef char      char_type;
     typedef int       int_type;
     typedef streamoff off_type;
     typedef streampos pos_type;
     typedef mbstate_t state_type;
 
     static inline _LIBCPP_CONSTEXPR_AFTER_CXX14
     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
     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
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type* move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+        {
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memmove(__s1, __s2, __n);
+            }
+            if (__s1 < __s2) {
+              _VSTD::copy(__s2, __s2 + __n, __s1);
+            } else {
+              _VSTD::copy_backward(__s2, __s2 + __n, __s1 + __n);
+            }
+            return __s1;
+        }
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    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);
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memcpy(__s1, __s2, __n);
+            }
+            _VSTD::copy(__s2, __s2 + __n, __s1);
+            return __s1;
+        }
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
+        {
+            if (__n == 0) return __s;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memset(__s, to_int_type(__a), __n);
+            }
+            _VSTD::fill(__s, __s + __n, __a);
+            return __s;
         }
-    static inline char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
-        {return __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);}
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type((unsigned char)__c);}
     static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
     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*)wmemmove(__s1, __s2, __n);}
-    static inline char_type* copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type* move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+        {
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)wmemmove(__s1, __s2, __n);
+            }
+            if (__s1 < __s2) {
+              _VSTD::copy(__s2, __s2 + __n, __s1);
+            } else {
+              _VSTD::copy_backward(__s2, __s2 + __n, __s1 + __n);
+            }
+            return __s1;
+        }
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    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*)wmemcpy(__s1, __s2, __n);
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)wmemcpy(__s1, __s2, __n);
+            }
+            _VSTD::copy(__s2, __s2 + __n, __s1);
+            return __s1;
+        }
+    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17
+    char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
+        {
+            if (__n == 0) return __s;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)wmemset(__s, __a, __n);
+            }
+            _VSTD::fill(__s, __s + __n, __a);
+            return __s;
         }
-    static inline char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
-        {return __n == 0 ? __s : (char_type*)wmemset(__s, __a, __n);}
-
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t>
     static inline constexpr bool lt(char_type __c1, char_type __c2) noexcept
         {return __c1 < __c2;}
 
     static constexpr
     int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
 
     static constexpr
     size_t           length(const char_type* __s) _NOEXCEPT;
- 
     _LIBCPP_INLINE_VISIBILITY static constexpr
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
- 
-    static 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 char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+    static constexpr
+    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
+        {
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memmove(__s1, __s2, __n);
+            }
+            if (__s1 < __s2) {
+              _VSTD::copy(__s2, __s2 + __n, __s1);
+            } else {
+              _VSTD::copy_backward(__s2, __s2 + __n, __s1 + __n);
+            }
+            return __s1;
+        }
+    static constexpr
+    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);
+            if (__n == 0) return __s1;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memcpy(__s1, __s2, __n);
+            }
+            _VSTD::copy(__s2, __s2 + __n, __s1);
+            return __s1;
         }
- 
-    static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
-        {return __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);}
+    static constexpr
+    char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
+        {
+            if (__n == 0) return __s;
+            if (!__libcpp_is_constant_evaluated()) {
+              return (char_type*)memset(__s, to_int_type(__a), __n);
+            }
+            _VSTD::fill(__s, __s + __n, __a);
+            return __s;
+        }
 
     static inline constexpr int_type  not_eof(int_type __c) noexcept
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline constexpr char_type to_char_type(int_type __c) noexcept
         {return char_type(__c);}
     static inline constexpr int_type to_int_type(char_type __c) noexcept
         {return int_type(__c);}
     static inline constexpr bool eq_int_type(int_type __c1, int_type __c2) noexcept
static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     size_t           length(const char_type* __s) _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT;
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
char_traits<char16_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
     {
         if (eq(*__s, __a))
             return __s;
         ++__s;
     }
     return 0;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char16_t*
 char_traits<char16_t>::move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
 {
+    if (__n == 0) return __s1;
     char_type* __r = __s1;
     if (__s1 < __s2)
     {
         for (; __n; --__n, ++__s1, ++__s2)
             assign(*__s1, *__s2);
     }
     else if (__s2 < __s1)
     {
         __s1 += __n;
         __s2 += __n;
         for (; __n; --__n)
             assign(*--__s1, *--__s2);
     }
     return __r;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char16_t*
 char_traits<char16_t>::copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
+    if (__n == 0) return __s1;
     char_type* __r = __s1;
     for (; __n; --__n, ++__s1, ++__s2)
         assign(*__s1, *__s2);
     return __r;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char16_t*
 char_traits<char16_t>::assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
 {
+    if (__n == 0) return __s;
     char_type* __r = __s;
     for (; __n; --__n, ++__s)
         assign(*__s, __a);
     return __r;
 }
 
 template <>
 struct _LIBCPP_TEMPLATE_VIS char_traits<char32_t>
static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     size_t           length(const char_type* __s) _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT;
 
     static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
char_traits<char32_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
     {
         if (eq(*__s, __a))
             return __s;
         ++__s;
     }
     return 0;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char32_t*
 char_traits<char32_t>::move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
 {
+    if (__n == 0) return __s1;
     char_type* __r = __s1;
     if (__s1 < __s2)
     {
         for (; __n; --__n, ++__s1, ++__s2)
             assign(*__s1, *__s2);
     }
     else if (__s2 < __s1)
     {
         __s1 += __n;
         __s2 += __n;
         for (; __n; --__n)
             assign(*--__s1, *--__s2);
     }
     return __r;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char32_t*
 char_traits<char32_t>::copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
 {
     _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
+    if (__n == 0) return __s1;
     char_type* __r = __s1;
     for (; __n; --__n, ++__s1, ++__s2)
         assign(*__s1, *__s2);
     return __r;
 }
 
-inline
+inline _LIBCPP_CONSTEXPR_AFTER_CXX17
 char32_t*
 char_traits<char32_t>::assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
 {
+    if (__n == 0) return __s;
     char_type* __r = __s;
     for (; __n; --__n, ++__s)
         assign(*__s, __a);
     return __r;
 }
 
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/assign3.pass.cpp

 
 // static char_type* assign(char_type* s, size_t n, char_type a);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     char s2[3] = {0};
     assert(std::char_traits<char>::assign(s2, 3, char(5)) == s2);
     assert(s2[0] == char(5));
     assert(s2[1] == char(5));
     assert(s2[2] == char(5));
     assert(std::char_traits<char>::assign(NULL, 0, char(5)) == NULL);
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/copy.pass.cpp

 
 // static char_type* copy(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     char s1[] = {1, 2, 3};
     char s2[3] = {0};
     assert(std::char_traits<char>::copy(s2, s1, 3) == s2);
     assert(s2[0] == char(1));
     assert(s2[1] == char(2));
     assert(s2[2] == char(3));
     assert(std::char_traits<char>::copy(NULL, s1, 0) == NULL);
     assert(std::char_traits<char>::copy(s1, NULL, 0) == s1);
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char/move.pass.cpp

 
 // static char_type* move(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     char s1[] = {1, 2, 3};
     assert(std::char_traits<char>::move(s1, s1+1, 2) == s1);
     assert(s1[0] == char(2));
     assert(s1[1] == char(3));
     assert(s1[2] == char(3));
     s1[2] = char(0);
     assert(std::char_traits<char>::move(s1+1, s1, 2) == s1+1);
     assert(s1[0] == char(2));
     assert(s1[1] == char(2));
     assert(s1[2] == char(3));
     assert(std::char_traits<char>::move(NULL, s1, 0) == NULL);
     assert(std::char_traits<char>::move(s1, NULL, 0) == s1);
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/assign3.pass.cpp

 
 // static char_type* assign(char_type* s, size_t n, char_type a);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char16_t s2[3] = {0};
     assert(std::char_traits<char16_t>::assign(s2, 3, char16_t(5)) == s2);
     assert(s2[0] == char16_t(5));
     assert(s2[1] == char16_t(5));
     assert(s2[2] == char16_t(5));
     assert(std::char_traits<char16_t>::assign(NULL, 0, char16_t(5)) == NULL);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/copy.pass.cpp

 
 // static char_type* copy(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char16_t s1[] = {1, 2, 3};
     char16_t s2[3] = {0};
     assert(std::char_traits<char16_t>::copy(s2, s1, 3) == s2);
     assert(s2[0] == char16_t(1));
     assert(s2[1] == char16_t(2));
     assert(s2[2] == char16_t(3));
     assert(std::char_traits<char16_t>::copy(NULL, s1, 0) == NULL);
     assert(std::char_traits<char16_t>::copy(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char16_t/move.pass.cpp

 
 // static char_type* move(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char16_t s1[] = {1, 2, 3};
     assert(std::char_traits<char16_t>::move(s1, s1+1, 2) == s1);
     assert(s1[0] == char16_t(2));
     assert(s1[1] == char16_t(3));
     assert(s1[2] == char16_t(3));
     s1[2] = char16_t(0);
     assert(std::char_traits<char16_t>::move(s1+1, s1, 2) == s1+1);
     assert(s1[0] == char16_t(2));
     assert(s1[1] == char16_t(2));
     assert(s1[2] == char16_t(3));
     assert(std::char_traits<char16_t>::move(NULL, s1, 0) == NULL);
     assert(std::char_traits<char16_t>::move(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/assign3.pass.cpp

 
 // static char_type* assign(char_type* s, size_t n, char_type a);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char32_t s2[3] = {0};
     assert(std::char_traits<char32_t>::assign(s2, 3, char32_t(5)) == s2);
     assert(s2[0] == char32_t(5));
     assert(s2[1] == char32_t(5));
     assert(s2[2] == char32_t(5));
     assert(std::char_traits<char32_t>::assign(NULL, 0, char32_t(5)) == NULL);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/copy.pass.cpp

 
 // static char_type* copy(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char32_t s1[] = {1, 2, 3};
     char32_t s2[3] = {0};
     assert(std::char_traits<char32_t>::copy(s2, s1, 3) == s2);
     assert(s2[0] == char32_t(1));
     assert(s2[1] == char32_t(2));
     assert(s2[2] == char32_t(3));
     assert(std::char_traits<char32_t>::copy(NULL, s1, 0) == NULL);
     assert(std::char_traits<char32_t>::copy(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char32_t/move.pass.cpp

 
 // static char_type* move(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     char32_t s1[] = {1, 2, 3};
     assert(std::char_traits<char32_t>::move(s1, s1+1, 2) == s1);
     assert(s1[0] == char32_t(2));
     assert(s1[1] == char32_t(3));
     assert(s1[2] == char32_t(3));
     s1[2] = char32_t(0);
     assert(std::char_traits<char32_t>::move(s1+1, s1, 2) == s1+1);
     assert(s1[0] == char32_t(2));
     assert(s1[1] == char32_t(2));
     assert(s1[2] == char32_t(3));
     assert(std::char_traits<char32_t>::move(NULL, s1, 0) == NULL);
     assert(std::char_traits<char32_t>::move(s1, NULL, 0) == s1);
 #endif  // _LIBCPP_HAS_NO_UNICODE_CHARS
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char8_t/assign3.pass.cpp

 
 // static char_type* assign(char_type* s, size_t n, char_type a);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
     char8_t s2[3] = {0};
     assert(std::char_traits<char8_t>::assign(s2, 3, char8_t(5)) == s2);
     assert(s2[0] == char8_t(5));
     assert(s2[1] == char8_t(5));
     assert(s2[2] == char8_t(5));
     assert(std::char_traits<char8_t>::assign(NULL, 0, char8_t(5)) == NULL);
 #endif
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char8_t/copy.pass.cpp

 
 // static char_type* copy(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
     char8_t s1[] = {1, 2, 3};
     char8_t s2[3] = {0};
     assert(std::char_traits<char8_t>::copy(s2, s1, 3) == s2);
     assert(s2[0] == char8_t(1));
     assert(s2[1] == char8_t(2));
     assert(s2[2] == char8_t(3));
     assert(std::char_traits<char8_t>::copy(NULL, s1, 0) == NULL);
     assert(std::char_traits<char8_t>::copy(s1, NULL, 0) == s1);
 #endif
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.char8_t/move.pass.cpp

 
 // static char_type* move(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
 #if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
     char8_t s1[] = {1, 2, 3};
     assert(std::char_traits<char8_t>::move(s1, s1+1, 2) == s1);
     assert(s1[0] == char8_t(2));
     assert(s1[1] == char8_t(3));
     assert(s1[2] == char8_t(3));
     s1[2] = char8_t(0);
     assert(std::char_traits<char8_t>::move(s1+1, s1, 2) == s1+1);
     assert(s1[0] == char8_t(2));
     assert(s1[1] == char8_t(2));
     assert(s1[2] == char8_t(3));
     assert(std::char_traits<char8_t>::move(NULL, s1, 0) == NULL);
     assert(std::char_traits<char8_t>::move(s1, NULL, 0) == s1);
 #endif
 
+  return true;
+}
+
+int main(int, char**)
+{
+    test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
+
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/assign3.pass.cpp

 
 // static char_type* assign(char_type* s, size_t n, char_type a);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     wchar_t s2[3] = {0};
     assert(std::char_traits<wchar_t>::assign(s2, 3, wchar_t(5)) == s2);
     assert(s2[0] == wchar_t(5));
     assert(s2[1] == wchar_t(5));
     assert(s2[2] == wchar_t(5));
     assert(std::char_traits<wchar_t>::assign(NULL, 0, wchar_t(5)) == NULL);
+    return true;
+}
+
+int main(int, char**)
+{
+  test();
+
+#if TEST_STD_VER > 17
+    static_assert(test(), "");
+#endif
 
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/copy.pass.cpp

 
 // static char_type* copy(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     wchar_t s1[] = {1, 2, 3};
     wchar_t s2[3] = {0};
     assert(std::char_traits<wchar_t>::copy(s2, s1, 3) == s2);
     assert(s2[0] == wchar_t(1));
     assert(s2[1] == wchar_t(2));
     assert(s2[2] == wchar_t(3));
     assert(std::char_traits<wchar_t>::copy(NULL, s1, 0) == NULL);
     assert(std::char_traits<wchar_t>::copy(s1, NULL, 0) == s1);
+    return true;
+}
+
+int main(int, char**)
+{
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test(), "");
+#endif
 
   return 0;
 }

libcxx/test/std/strings/char.traits/char.traits.specializations/char.traits.specializations.wchar.t/move.pass.cpp

 
 // static char_type* move(char_type* s1, const char_type* s2, size_t n);
 
 #include <string>
 #include <cassert>
 
 #include "test_macros.h"
 
-int main(int, char**)
+TEST_CONSTEXPR_CXX20 bool test()
 {
     wchar_t s1[] = {1, 2, 3};
     assert(std::char_traits<wchar_t>::move(s1, s1+1, 2) == s1);
     assert(s1[0] == wchar_t(2));
     assert(s1[1] == wchar_t(3));
     assert(s1[2] == wchar_t(3));
     s1[2] = wchar_t(0);
     assert(std::char_traits<wchar_t>::move(s1+1, s1, 2) == s1+1);
     assert(s1[0] == wchar_t(2));
     assert(s1[1] == wchar_t(2));
     assert(s1[2] == wchar_t(3));
     assert(std::char_traits<wchar_t>::move(NULL, s1, 0) == NULL);
     assert(std::char_traits<wchar_t>::move(s1, NULL, 0) == s1);
+    return true;
+}
+
+int main(int, char**)
+{
+  test();
+
+#if TEST_STD_VER > 17
+  static_assert(test(), "");
+#endif
 
   return 0;
 }

libcxx/test/support/test_macros.h

 # else
 #   define TEST_CONSTEXPR_CXX14
 # endif
 # if TEST_STD_VER > 14
 #   define TEST_THROW_SPEC(...)
 # else
 #   define TEST_THROW_SPEC(...) throw(__VA_ARGS__)
 # endif
+# if TEST_STD_VER > 17
+#   define TEST_CONSTEXPR_CXX20 constexpr
+# else
+#   define TEST_CONSTEXPR_CXX20
+# endif
 #else
 #if defined(TEST_COMPILER_CLANG)
 # define TEST_ALIGNOF(...) _Alignof(__VA_ARGS__)
 #else
 # define TEST_ALIGNOF(...) __alignof(__VA_ARGS__)
 #endif
 #define TEST_ALIGNAS(...) __attribute__((__aligned__(__VA_ARGS__)))
 #define TEST_CONSTEXPR