From ad7584d8b60119ca3717117a1eb6a16d753c5d74 Mon Sep 17 00:00:00 2001
From: Cory Fields <cory-nospam-@coryfields.com>
Date: Mon, 26 Feb 2024 19:52:14 +0000
Subject: 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.
---
 configure.ac    |  8 --------
 src/serialize.h | 20 ++++++++++----------
 2 files changed, 10 insertions(+), 18 deletions(-)

diff --git a/configure.ac b/configure.ac
index 50e6870dd9..45c114688b 100644
--- a/configure.ac
+++ b/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); ]])],
diff --git a/src/serialize.h b/src/serialize.h
index 7b336ce1af..ab57376a57 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -6,10 +6,6 @@
 #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>
@@ -17,6 +13,7 @@
 #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); }
-- 
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