build: Bump minimum supported GCC to g++-9 #27662

It is a bit frustrating to write valid C++ code only to realize that g++-8 fails to parse it later on.

The only non-EOL operating system still shipping with g++-8 is CentOS Stream 8. I think it is reasonable for users of affected Linux distributions to:

• Upgrade their operating system, or compiler to a supported version.
• Alternatively, stay with a previous release of Bitcoin Core as long as it is supported.

Fixes https://github.com/bitcoin/bitcoin/issues/27537

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Conflicts

No conflicts as of last run.

Does this comment hold:

Added 27.0 for now, which should be uncontroversial.

?

Concept ACK - seems fine for 26.0.

The only non-EOL operating system still shipping with g++-8 is CentOS 8.

seems fine for 26.0.

Considering https://www.centos.org/centos-linux-eol/:

CentOS Linux 8 will reach End Of Life (EOL) on December 31st, 2021.

I agree.

Concept ACK.

UPD. CentOS Stream 8 EOL is May 31st, 2024.

Looks like the only affected code is constexpr functions calling assert(...).

Failure can be tested with:

git show HEAD~|git apply --reverse  # First checkout this pull, then revert the centos 9 bump commit
podman kill ci_i686_centos && MAKEJOBS="-j$(nproc)" FILE_ENV="./ci/test/00_setup_env_i686_centos.sh" ./ci/test_run_all.sh  # Run the CI task

...
qt/addresstablemodel.cpp: In function ‘constexpr AddressTableEntry::Type translateTransactionType(wallet::AddressPurpose, bool)’:
qt/addresstablemodel.cpp:64:5: error: call to non-‘constexpr’ function ‘void __assert_fail(const char*, const char*, unsigned int, const char*)’
     assert(false);
     ^~~~~~

Concept ACK - this is logical and seems uncontentious.

Could adjust the comment here: https://github.com/bitcoin/bitcoin/blob/edd2a8644531e6fb8f64ad6acce86f8fdec9ab26/src/txrequest.cpp#L72 to remove mention of GCC 8.x.

Could pull in this commit, to remove the GCC 8 -lstdc++fs check: https://github.com/fanquake/bitcoin/commit/b2632e7c7b7dfc5487629d80e11a8cdeaa649ed9.

Could pull in this commit, to remove the GCC 8 -lstdc++fs check: https://github.com/fanquake/bitcoin/commit/b2632e7c7b7dfc5487629d80e11a8cdeaa649ed9.

My preference would be to also bump clang and then just remove the whole check instead of fiddling with it, when all supported distros ship clang-10 or higher already.

Are you going to pull a Clang bump into this PR? I don't see the point of leaving redundant code, especially when the diff is trivial to review.

Ok. Lets kill l_filesystem.m4.

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