From 8c5c428622ee62dc9dc4243adee7df6c2ed385ca Mon Sep 17 00:00:00 2001
From: Roman Koshelev <roman.koshelev@bk.ru>
Date: Sun, 12 Sep 2021 18:07:16 +0300
Subject: [PATCH] Add copy2() constexpr

---
 include/fmt/format.h | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 5402c280..db0986a2 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -1049,11 +1049,19 @@ inline auto equal2(const char* lhs, const char* rhs) -> bool {
 }
 
 // Copies two characters from src to dst.
-template <typename Char> void copy2(Char* dst, const char* src) {
-  *dst++ = static_cast<Char>(*src++);
-  *dst = static_cast<Char>(*src);
+template <typename Char>
+FMT_CONSTEXPR20 FMT_INLINE void copy2(Char* dst, const char* src) { 
+  if (!is_constant_evaluated() && std::is_same<Char, char>::value) {
+    memcpy(dst, src, 2); 
+  } else {
+    // We read both bytes before writing so that the compiler can do it in
+    // one pair of read/write instructions (even if Char aliases char)
+    char dc0 = *src++;
+    char dc1 = *src;
+    *dst++ = static_cast<Char>(dc0);
+    *dst = static_cast<Char>(dc1);
+  }
 }
-FMT_INLINE void copy2(char* dst, const char* src) { memcpy(dst, src, 2); }
 
 template <typename Iterator> struct format_decimal_result {
   Iterator begin;