Does not compile on Ubuntu Linux 20.04 g++ 9.4.0

[ifrguy]

OS:
Distributor ID: Ubuntu
Description: Ubuntu 20.04.4 LTS
Release: 20.04
Codename: focal

Arch:
x86_64

g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Compile command: g++ -o jtc -Wall -std=c++17 -Ofast jtc.cpp

Compiler output see attached file
jtc.err.gz

[ldn-softdev]

Interesting. It looks like the version of gcc you use pre-processes the concatenation token ## in a different way than prior versions (I'm talking of the version of GCC and not of C++ standard).

Namely, considering this macro definition (where the error is seen most of the time):

#define __DBG_0_ARG__() __dbg__     /* DBG(): access to debug object */
#define __DBG_1_ARG__(X)            /* DBG(N) {..}: debug operator, prints debug indention */\
    if( __dbg__(X) ) \
     TLOCK(__dbg__.mutex()) \
      if( __dbg__(X, __func__) )                                // now print the prompt
#define __DBG_2_ARG__(O, X)         /* DBG(F, N) {..}: using debug operator from F */\
    if( O.__dbg__(X) ) \
     TLOCK(O.__dbg__.mutex()) \
      if( O.__dbg__(X, __func__) )                              // now print the prompt
#define __DBG_4TH_ARG__(arg1, arg2, arg3, arg4, ...) arg4
#define __DBG_CHOOSER__(ARGS...) \
    __DBG_4TH_ARG__(dummy, ##ARGS, __DBG_2_ARG__, __DBG_1_ARG__, __DBG_0_ARG__)
#define DBG(ARGS...) __DBG_CHOOSER__(ARGS)(ARGS)

the line :
454 | template<class X> Debug(X &x) { x.DBG().severity(x); };
must result in resolving DBG() into the token __DBG_0_ARG__, while the error indicates that it resolved into the token __DBG_1_ARG__ instead.

This means, that the argument substitution in the line
__DBG_4TH_ARG__(dummy, ##ARGS, __DBG_2_ARG__, __DBG_1_ARG__, __DBG_0_ARG__)
resulted in 5 arguments, whereas it's expected to be expanded into 4 arguments (given token ## and the empty argument ARGS).

It could be a change in the macro behavior in the newer c++ compilers or a peculiar behavior of this gcc version. I'd need to take more time to experiment with different compiler versions, and perhaps rewrite some macros, given c++20 introduced new __VA_OPT__ macro to handle similar expansions I do in DBG macro.