merge bitcoin#29484: replace char-is-int8_t autoconf detection with c++20 concept

Author: kwvg

configure.ac

@@ -1215,14 +1215,6 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>
  [ AC_MSG_RESULT([no])]
 )
 
-AC_MSG_CHECKING([for if type char equals int8_t])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <stdint.h>
-  #include <type_traits>]],
- [[ static_assert(std::is_same<int8_t, char>::value, ""); ]])],
- [ AC_MSG_RESULT([yes]); AC_DEFINE([CHAR_EQUALS_INT8], [1], [Define this symbol if type char equals int8_t]) ],
- [ AC_MSG_RESULT([no])]
-)
-
 dnl ensure backtrace() is found, check -lexecinfo if necessary
 if test "$TARGET_OS" != "windows"; then
   if test "$enable_stacktraces" != "no"; then

src/serialize.h

@@ -10,6 +10,7 @@
 
 #include <algorithm>
 #include <atomic>
+#include <concepts>
 #include <cstdint>
 #include <cstring>
 #include <ios>
@@ -193,9 +194,14 @@ template<typename X> const X& ReadWriteAsHelper(const X& x) { return x; }
     FORMATTER_METHODS(cls, obj)
 
 // clang-format off
-#ifndef CHAR_EQUALS_INT8
-template <typename Stream> void Serialize(Stream&, char) = delete; // char serialization forbidden. Use uint8_t or int8_t
-#endif
+
+// Typically int8_t and char are distinct types, but some systems may define int8_t
+// in terms of char. Forbid serialization of char in the typical case, but allow it if
+// it's the only way to describe an int8_t.
+template<class T>
+concept CharNotInt8 = std::same_as<T, char> && !std::same_as<T, int8_t>;
+
+template <typename Stream, CharNotInt8 V> void Serialize(Stream&, V) = delete; // char serialization forbidden. Use uint8_t or int8_t
 template <typename Stream> void Serialize(Stream& s, std::byte a) { ser_writedata8(s, uint8_t(a)); }
 template<typename Stream> inline void Serialize(Stream& s, int8_t a  ) { ser_writedata8(s, a); }
 template<typename Stream> inline void Serialize(Stream& s, uint8_t a ) { ser_writedata8(s, a); }
@@ -209,9 +215,7 @@ template<typename Stream, int N> inline void Serialize(Stream& s, const char (&a
 template<typename Stream, int N> inline void Serialize(Stream& s, const unsigned char (&a)[N]) { s.write(MakeByteSpan(a)); }
 template <typename Stream, typename B> void Serialize(Stream& s, Span<B> span) { (void)/* force byte-type */UCharCast(span.data()); s.write(AsBytes(span)); }
 
-#ifndef CHAR_EQUALS_INT8
-template <typename Stream> void Unserialize(Stream&, char) = delete; // char serialization forbidden. Use uint8_t or int8_t
-#endif
+template <typename Stream, CharNotInt8 V> void Unserialize(Stream&, V) = delete; // char serialization forbidden. Use uint8_t or int8_t
 template <typename Stream> void Unserialize(Stream& s, std::byte& a) { a = std::byte{ser_readdata8(s)}; }
 template<typename Stream> inline void Unserialize(Stream& s, int8_t& a  ) { a = ser_readdata8(s); }
 template<typename Stream> inline void Unserialize(Stream& s, uint8_t& a ) { a = ser_readdata8(s); }