Configure script breakage with the new -Werror=implicit-function-declaration

[alxu]

@AaronBallman

There’s only so much we can do in these situations to communicate upcoming changes. I should have done more in this case, like posting an RFC. But even then, “I don’t follow the forums” is just as plausible as “I don’t follow the release notes”, so I expect folks would still be caught out. This change was in tree since April, so there’s been approximately five months for people to have brought this concern up for critical situations. Additionally, there were several months of release candidates with which folks were welcome to test the compiler. We rely on people who have strong opinions about the compiler to either do testing of the tree as it progresses or of one of the release candidates.

With respect, considering the scope of the impact, potentially every autoconf script, and the nature of the issue, silent misbehavior, I feel that a line in the middle of the (clang-only, not LLVM) release notes is not very good notice. Considering that you agree that this change could potentially have major impacts, it seems to me that it should at the least be at the top of the clang release notes. I can’t seem to find any notice of this change on the forums either: the RFC [RFC] Enabling -Wstrict-prototypes by default in C discussed autoconf issues, but from my reading, it was agreed not to use -Werror by default. It’s unclear to me how a distributor would have been aware of this change to test the impacts in a release candidate unless they carefully track every commit message or release note.

edit: A good comparison can be made to when gcc 10 set -fno-common by default, which was highly disruptive to Linux distros because of the sheer range of the breakage. -Werror=implicit-function-declaration likely affects much more packages, and worse, as has been beaten to death but I feel I must reiterate, unlike -fno-common which was almost always visible breakage, -Werror=implicit-function-declaration often causes hidden breakage. To be clear, I think these are good changes, but they are very significant changes that deserve far more consideration and announcement than a Phab patch and single release line note.

@MaskRay

• Configure sys-devel/clang with an appropriate CLANG_CONFIG_FILE_SYSTEM_DIR. Change clang to a shell script which invokes ${default_triple}-clang. This is because xxx-clang loads xxx.cfg while clang doesn’t.

This solution sounds effectively similar to simply patching the default back to -Wno-error=implicit-function-declaration, which would cause fragmentation, but I don’t see why it would be any more or less so.