serialization: replace char-is-int8_t autoconf detection with c++20 concept

This removes the only remaining autoconf macro in our serialization code,
so it can now be used trivially and safely out-of-tree.

Author: theuni

configure.ac

@@ -1174,14 +1174,6 @@ 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])]
-)
-
 AC_MSG_CHECKING([for fdatasync])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>]],
  [[ fdatasync(0); ]])],

src/serialize.h

@@ -6,17 +6,14 @@
 #ifndef BITCOIN_SERIALIZE_H
 #define BITCOIN_SERIALIZE_H
 
-#if defined(HAVE_CONFIG_H)
-#include <config/bitcoin-config.h>
-#endif
-
 #include <attributes.h>
 #include <compat/assumptions.h> // IWYU pragma: keep
 #include <compat/endian.h>
 #include <prevector.h>
 #include <span.h>
 
 #include <algorithm>
+#include <concepts>
 #include <cstdint>
 #include <cstring>
 #include <ios>
@@ -263,9 +260,14 @@ const Out& AsBase(const In& x)
 // i.e. anything that supports .read(Span<std::byte>) and .write(Span<const std::byte>)
 //
 // 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); }
@@ -279,9 +281,7 @@ template <typename Stream, BasicByte B, int N> void Serialize(Stream& s, const B
 template <typename Stream, BasicByte B, std::size_t N> void Serialize(Stream& s, const std::array<B, N>& a) { s.write(MakeByteSpan(a)); }
 template <typename Stream, BasicByte B> void Serialize(Stream& s, Span<B> span) { 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); }