Modernize Codebase Using C++20 Features

INSTALL.md

@@ -57,7 +57,7 @@ To install Postgresql, follow instructions from the [Postgresql download page](h
 
 ## Build Dependencies
 
-- c++ toolchain and headers that supports c++17
+- c++ toolchain and headers that supports c++20
     - `clang` >= 20.0
     - `g++` >= 14.0
 - `pkg-config`

README.md

@@ -10,7 +10,7 @@
 
 Stellar-core is a replicated state machine that maintains a local copy of a cryptographic ledger and processes transactions against it, in consensus with a set of peers.
 It implements the [Stellar Consensus Protocol](https://github.com/stellar/stellar-core/blob/master/src/scp/readme.md), a _federated_ consensus protocol.
-It is written in C++17 and runs on Linux, OSX and Windows.
+It is written in C++20 and runs on Linux, OSX and Windows.
 Learn more by reading the [overview document](https://github.com/stellar/stellar-core/blob/master/docs/readme.md).
 
 # Documentation

configure.ac

@@ -49,7 +49,7 @@ AC_SUBST(LIBTOOL_DEPS)
 AC_LANG(C++)
 # if modifying the following macro for a future C++ version, please update CXX
 # for enable-afl in the fuzzer configuration block below
-AX_CXX_COMPILE_STDCXX(17, noext,mandatory)
+AX_CXX_COMPILE_STDCXX(20, noext,mandatory)
 AX_FRESH_COMPILER
 # -pthread seems to be required by -std=c++14 on some hosts
 AX_APPEND_COMPILE_FLAGS([-pthread])
@@ -249,17 +249,17 @@ AS_IF([test "x$enable_afl" = "xyes"], [
           [clang*], [AC_CHECK_PROGS([AFL_CLANG], [afl-clang-fast])
                      AC_CHECK_PROGS([AFL_CLANGPP], [afl-clang-fast++])
                      CC="afl-clang-fast"
-                     # below we hard code -std=c++17 since updates to AX_CXX_COMPILE_STDCXX append it to
+                     # below we hard code -std=c++20 since updates to AX_CXX_COMPILE_STDCXX append it to
                      # CXX, not to CXXFLAGS and thus when setting CXX we override this. For a more detailed explanation
                      # see: https://github.com/stellar/docker-stellar-core/pull/66#issuecomment-521886881
-                     CXX="afl-clang-fast++ -std=c++17 -DAFL_LLVM_MODE=1"],
+                     CXX="afl-clang-fast++ -std=c++20 -DAFL_LLVM_MODE=1"],
           [gcc*],   [AC_CHECK_PROGS([AFL_GCC], [afl-gcc])
                      AC_CHECK_PROGS([AFL_GPP], [afl-g++])
                      CC="afl-gcc"
-                     # below we hard code -std=c++17 since updates to AX_CXX_COMPILE_STDCXX append it to
+                     # below we hard code -std=c++20 since updates to AX_CXX_COMPILE_STDCXX append it to
                      # CXX, not to CXXFLAGS and thus when setting CXX we override this. For a more detailed explanation
                      # see: https://github.com/stellar/docker-stellar-core/pull/66#issuecomment-521886881
-                     CXX="afl-g++ -std=c++17"],
+                     CXX="afl-g++ -std=c++20"],
           [AC_MSG_ERROR([Don't know how to instrument CC=$CC with AFL])])
 ])
 AM_CONDITIONAL([USE_AFL_FUZZ], [test "x$enable_afl" == "xyes"])

lib/autocheck/include/autocheck/classifier.hpp

@@ -38,7 +38,7 @@ namespace autocheck {
         typename Func,
         typename Enable = typename std::enable_if<
           !std::is_convertible<
-            typename std::result_of<Func(const Args&...)>::type,
+            std::invoke_result_t<Func, const Args&...>,
             std::string
           >::value
         >::type