Re: c++20: require gcc12?

[Greg Troxel <gdt%lexort.com@localhost>]

Martin Husemann <martin%duskware.de@localhost> writes:

> On Tue, Feb 11, 2025 at 07:30:12AM -0500, Greg Troxel wrote:
>> Martin Husemann <martin%duskware.de@localhost> writes:
>> 
>> > This is very strange and probably points at a serious bug in the gcc pkg.
>> > Any chance to investigate details of the configure failure?
>> >
>> > AFAICT there is no c++20 specific test code to compile and the c++
>> > frontend does understand "-std=c++20".

Sorry, I misread does as does not!

> Yes, that is what I meant. The AC_... test used in the pkg does not
> seem to compile anything special for C++20, it checks things up to C++17
> and whether the compiler accepts -std=c++20 (because newer C++ standards
> came with proper feature test macros so there is no need to compile test
> code).
>
> I am trying to reproduce the failure locally on a NetBSD 9 machine.

I think I understand what's going on.

configure.ac in bitcoin passes --std=c++20.  And then it looks at
__cplusplus.

gcc10 takes c++20 but does not declare conformance.  gcc12 does.  (This
argues even more strongly for fixing gcc.mk to require 12.)  Is this a
bug in our gcc?  I would say it's an upstream issue -- but gcc says that
you need 12 to be compliant with c++20, so not really a bug.

A minimal test program:

----------------------------------------
#include <stdio.h>

int
main()
{
  printf("__cplusplus %ld\n", __cplusplus);
  return 0;
}
----------------------------------------
#!/bin/sh

for std in 11 14 17 20 23; do

    echo "STD ${std}:"
    g++ --std=c++${std} -o foo${std} foo.cc
    ./foo${std}

    echo ""
done
----------------------------------------

gcc 12 base, NetBSD current, matches gcc 12 pkgsrc on NetBSD current

STD 11:
__cplusplus 201103

STD 14:
__cplusplus 201402

STD 17:
__cplusplus 201703

STD 20:
__cplusplus 202002

STD 23:
__cplusplus 202100

----------------------------------------

gcc 10 pkgsrc on NetBSD current

./build-foo 
STD 11:
__cplusplus 201103

STD 14:
__cplusplus 201402

STD 17:
__cplusplus 201703

STD 20:
__cplusplus 201709

STD 23:
g++: error: unrecognized command-line option ‘--std=c++23’; did you mean ‘--std=c++03’?
__cplusplus 202100