clangd-indexer should use or document -extra-arg="-Wno-everything"

[smhc]

Projects that use g++ may not necessarily compile cleanly with clang due to several warnings treated as errors. The following option to clangd-indexer significantly helps in creating a useful index for these projects:

-extra-arg="-Wno-everything"

Given the purpose of the index, perhaps this should be the default option?
If not, maybe it could at least be promoted more in documentation/usage as it results in a much more useful index for the purposes of symbol lookups etc.

[smhc]

After more thought - does the background indexer for clangd have an equivalent option for:

-extra-arg="-Wno-everything"

I assume clangd would fail to index code that clang would fail to compile if it hits an -Werror type of warning. Bearing in mind that g++ wouldn't fail to compile the same code, hence the compiler options do not specify to ignore the problematic warnings.

[smhc]

For some more context:
We have a project compiled with g++ that uses "-Wall -Werror". This compiles fine with g++, but fails in clang++ due to the addition of other warnings (e.g -Wunused-function) that g++ doesn't have.

Effectively what we need is for clangd to honour the fact the compile_commands.json specifies 'g++' and turn off all clang++ specific warnings. This could be done automatically, or via a command line option to provide additional args to the compiler, similar to what clangd-indexer supports.

It may be necessary have two sets of 'extra-args'. One for the indexer, which would be safe to turn off all warnings, and another setting for the diagnostics where you'd likely want to leave on all the g++ warnings.

[sam-mccall]

Yeah there are a few different ways to address your problem, but i think the cleanest is to disable all warnings during indexing as you suggest.

This should happen for all the different types of indexing we do (static, dynamic, background, it's a mess...).

Futzing with compile commands is always fiddly (it can be a clang-cl style command line too!) but that's just a better reason to do this in one place.

[smhc]

I can't help but think clang should have a gcc version x compatible mode where it disables additional warnings that gcc version x doesn't have. Or at the very least doesn't convert them into errors even if asked to. I assume it hasn't been asked for often enough to warrant doing.

[sam-mccall]

I assume clangd would fail to index code that clang would fail to compile if it hits an -Werror type of warning. Bearing in mind that g++ wouldn't fail to compile the same code, hence the compiler options do not specify to ignore the problematic warnings.

Turns out this assumption doesn't hold :-) Non-fatal errors don't stop indexing, and all symbols are still collected.

However there are still reasons to do this:

• -ferror-limit (which has a default) can cause warnings with -Werror to create a fatal error, that does stop indexing
• the warning spam from clangd-indexer isn't helpful
• some warnings are expensive, we should avoid computing them at all

[sam-mccall]

Fixed in https://reviews.llvm.org/rL357186
(Though I suspect this wouldn't have prevented an index from building in most cases)

I can't help but think clang should have a gcc version x compatible mode where it disables additional warnings that gcc version x doesn't have. Or at the very least doesn't convert them into errors even if asked to. I assume it hasn't been asked for often enough to warrant doing.

I can see how this would be desirable, though it also sounds hard to keep in sync. In any case that's indeed a FR for clang.