[clang-tools-extra] [clangd] [C++ Modules] Skip PCH when TU imports modules (PR #187432)

[Aleksandr Platonov via cfe-commits]

https://github.com/ArcsinX commented:

Hello.
If I understand correctly, with this patch we have the following logic:
- we build preamble
- we build AST without preamble usage if the TU uses C++20 modules

This means that we loose preamble optimization approach and at every key stroke (file modification from an editor) in the file body we will rebuild the whole AST (including `#include`s section).

Maybe one can expect that C++20 modules usage means that preamble mostly consists of C++20 modules, thus AST rebuild can be fast, but in general it looks like performance degradation.

If we consider this as a trade-off between performance loss and diagnostics (which do not affect functionality? We can still do go-to-definition), personally I more lean towards having this diagnostics, because:
- We can suppress some diagnostics using configuration https://clangd.llvm.org/config#suppress
- As you said, the problem is not initially in clangd, but in clang. And it seems we should not fix this in clangd at any cost.

https://github.com/llvm/llvm-project/pull/187432