enable C++20 three_way_operator when supported

Author: Arthapz

include/frozen/bits/defines.h

@@ -29,13 +29,23 @@
 
 // Code taken from https://stackoverflow.com/questions/43639122/which-values-can-msvc-lang-have
 #if defined(FROZEN_LETITGO_IS_MSVC)
-  #if _MSVC_LANG > 201402
+  #if _MSVC_LANG > 201703L
+    #define FROZEN_LETITGO_HAS_CXX20  1
+  #else
+    #define FROZEN_LETITGO_HAS_CXX20  0
+  #endif
+  #if _MSVC_LANG > 201402L
     #define FROZEN_LETITGO_HAS_CXX17  1
   #else /* _MSVC_LANG > 201402 */
     #define FROZEN_LETITGO_HAS_CXX17  0
   #endif /* _MSVC_LANG > 201402 */
 #else /* _MSVC_LANG etc. */
-  #if __cplusplus > 201402
+  #if __cplusplus > 201703L
+    #define FROZEN_LETITGO_HAS_CXX20  1
+  #else /* __cplusplus > 201402 */
+    #define FROZEN_LETITGO_HAS_CXX20  0
+  #endif /* __cplusplus > 201402 */
+  #if __cplusplus > 201402L
     #define FROZEN_LETITGO_HAS_CXX17  1
   #else /* __cplusplus > 201402 */
     #define FROZEN_LETITGO_HAS_CXX17  0
@@ -51,6 +61,12 @@
   #endif
 #endif
 
+#if FROZEN_LETITGO_HAS_CXX20 && __has_include(<compare>)
+  #if defined(_cpp_lib_three_way_comparison) && __cpp_lib_three_way_comparison >= 201907
+    #define FROZEN_LETITGO_HAS_THREE_WAY_COMPARISON
+  #endif
+#endif
+
 #ifdef __cpp_char8_t
   #define FROZEN_LETITGO_HAS_CHAR8T
 #endif

include/frozen/string.h

@@ -35,6 +35,10 @@
 #ifdef FROZEN_LETITGO_HAS_STRING_VIEW
 #include <string_view>
 #endif
+
+#ifdef FROZEN_LETITGO_HAS_THREE_WAY_COMPARISON
+#include <compare>
+#endif
 #endif
 
 namespace frozen {
@@ -57,7 +61,7 @@ class basic_string {
   constexpr basic_string(std::basic_string_view<chr_t> data)
       : data_(data.data()), size_(data.size()) {}
 
-  explicit constexpr operator std::basic_string_view<chr_t>() const {
+  constexpr explicit operator std::basic_string_view<chr_t>() const {
     return std::basic_string_view<chr_t>(data_, size_);
   }
 #endif
@@ -70,7 +74,32 @@ class basic_string {
 
   constexpr chr_t operator[](std::size_t i) const { return data_[i]; }
 
-  constexpr bool operator==(basic_string other) const {
+#ifdef FROZEN_LETITGO_HAS_THREE_WAY_COMPARISON
+  constexpr std::weak_ordering operator<=>(const basic_string &other) const {
+    bool equal = true;
+    if(size_ == other.size_)
+      for (std::size_t i = 0; i < size_; ++i)
+        if (data_[i] != other.data_[i]) {
+          equal = false;
+          break;
+        }
+
+    if(equal) return std::weak_ordering::equivalent;
+
+    unsigned i = 0;
+    while (i < size() && i < other.size()) {
+      if ((*this)[i] < other[i]) {
+        return std::weak_ordering::less;
+      }
+      if ((*this)[i] > other[i]) {
+        return std::weak_ordering::greater;
+      }
+      ++i;
+    }
+    return size() < other.size() ? std::weak_ordering::less : std::weak_ordering::greater;
+  }
+#else
+  constexpr bool operator==(const basic_string &other) const {
     if (size_ != other.size_)
       return false;
     for (std::size_t i = 0; i < size_; ++i)
@@ -79,6 +108,10 @@ class basic_string {
     return true;
   }
 
+  friend constexpr bool operator!=(const basic_string &lhs, const basic_string &rhs) {
+      return !(lhs == rhs);
+  }
+
   constexpr bool operator<(const basic_string &other) const {
     unsigned i = 0;
     while (i < size() && i < other.size()) {
@@ -102,6 +135,7 @@ class basic_string {
   friend constexpr bool operator<=(const basic_string& lhs, const basic_string& rhs) {
     return !(lhs > rhs);
   }
+#endif
 
   constexpr const chr_t *data() const { return data_; }
   constexpr const chr_t *begin() const { return data(); }