diff --git a/include/string b/include/string
index 620e6f8..7c022dc 100644
--- a/include/string
+++ b/include/string
@@ -51,8 +51,8 @@
     typedef mbstate_t state_type;
 
     static void assign(char_type& c1, const char_type& c2) noexcept;
-    static bool eq(char_type c1, char_type c2) noexcept;
-    static bool lt(char_type c1, char_type c2) noexcept;
+    static constexpr bool eq(char_type c1, char_type c2) noexcept;
+    static constexpr bool lt(char_type c1, char_type c2) noexcept;
 
     static int              compare(const char_type* s1, const char_type* s2, size_t n);
     static size_t           length(const char_type* s);
@@ -61,11 +61,11 @@
     static char_type*       copy(char_type* s1, const char_type* s2, size_t n);
     static char_type*       assign(char_type* s, size_t n, char_type a);
 
-    static int_type  not_eof(int_type c) noexcept;
-    static char_type to_char_type(int_type c) noexcept;
-    static int_type  to_int_type(char_type c) noexcept;
-    static bool      eq_int_type(int_type c1, int_type c2) noexcept;
-    static int_type  eof() noexcept;
+    static constexpr int_type  not_eof(int_type c) noexcept;
+    static constexpr char_type to_char_type(int_type c) noexcept;
+    static constexpr int_type  to_int_type(char_type c) noexcept;
+    static constexpr bool      eq_int_type(int_type c1, int_type c2) noexcept;
+    static constexpr int_type  eof() noexcept;
 };
 
 template <> struct char_traits<char>;
@@ -506,10 +506,10 @@
     static void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT
         {__c1 = __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     static int              compare(const char_type* __s1, const char_type* __s2, size_t __n);
@@ -519,19 +519,20 @@
     static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n);
     static char_type*       assign(char_type* __s, size_t __n, char_type __a);
 
-    _LIBCPP_INLINE_VISIBILITY static int_type  not_eof(int_type __c) _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY
+    static _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     _LIBCPP_INLINE_VISIBILITY
-    static char_type to_char_type(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  to_int_type(char_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  eof() _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
         {return int_type(EOF);}
 };
 
@@ -631,10 +632,10 @@
     static void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT
         {__c1 = __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
             {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return (unsigned char)__c1 < (unsigned char)__c2;}
 
     _LIBCPP_INLINE_VISIBILITY
@@ -655,19 +656,20 @@
     static char_type* assign(char_type* __s, size_t __n, char_type __a)
         {return (char_type*)memset(__s, to_int_type(__a), __n);}
 
-    _LIBCPP_INLINE_VISIBILITY static int_type  not_eof(int_type __c) _NOEXCEPT
+    _LIBCPP_INLINE_VISIBILITY
+    static _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     _LIBCPP_INLINE_VISIBILITY
-    static char_type to_char_type(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type to_int_type(char_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type((unsigned char)__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  eof() _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
         {return int_type(EOF);}
 };
 
@@ -686,10 +688,10 @@
     static void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT
         {__c1 = __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     _LIBCPP_INLINE_VISIBILITY
@@ -712,19 +714,19 @@
         {return (char_type*)wmemset(__s, __a, __n);}
 
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  not_eof(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     _LIBCPP_INLINE_VISIBILITY
-    static char_type to_char_type(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type to_int_type(char_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type eof() _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
         {return int_type(WEOF);}
 };
 
@@ -743,10 +745,10 @@
     static void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT
         {__c1 = __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     static int              compare(const char_type* __s1, const char_type* __s2, size_t __n);
@@ -757,19 +759,19 @@
     static char_type*       assign(char_type* __s, size_t __n, char_type __a);
 
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  not_eof(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     _LIBCPP_INLINE_VISIBILITY
-    static char_type to_char_type(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type to_int_type(char_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type eof() _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
         {return int_type(0xDFFF);}
 };
 
@@ -863,10 +865,10 @@
     static void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT
         {__c1 = __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
         {return __c1 < __c2;}
 
     static int              compare(const char_type* __s1, const char_type* __s2, size_t __n);
@@ -877,19 +879,19 @@
     static char_type*       assign(char_type* __s, size_t __n, char_type __a);
 
     _LIBCPP_INLINE_VISIBILITY
-    static int_type  not_eof(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
         {return eq_int_type(__c, eof()) ? ~eof() : __c;}
     _LIBCPP_INLINE_VISIBILITY
-    static char_type to_char_type(int_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
         {return char_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type to_int_type(char_type __c) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
         {return int_type(__c);}
     _LIBCPP_INLINE_VISIBILITY
-    static bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+    static _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
         {return __c1 == __c2;}
     _LIBCPP_INLINE_VISIBILITY
-    static int_type eof() _NOEXCEPT
+    static _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
         {return int_type(0xFFFFFFFF);}
 };