--query-driver does not work with fallback flags for .h file if driver is gcc and objective-c++ support is not installed

[HighCommander4]

One of the main use cases of the Compiler: config option is to allow specifying the compiler to query for built-in include paths in projects that don't have a compile_commands.json file.

While the Compiler: config option itself was added in clangd 14, this use case was broken until clangd 16 due to #1089 (the edit to the compile command to replace argv[0] with Compiler: was performed after the system include extractor had already run).

Today, I discovered that this use case is still partially broken even in clangd 16.

If the project has no compile_commands.json file, the initial compile command is the fallback command returned by GlobalCompilationDatabase::getFallbackCommand(). If the file being opened is a .h file, the fallback command contains -xobjective-c++-header, since we don't know the language of the header and objective-c++ is the most permissive of the supported languages. This -x flag is preserved in the command used to extract system includes from the driver.

If the driver is a gcc, we run into an unfortunate snag: gcc's objective-c++ support is often contained in a separate package, e.g. gobjc++, which is not necessarily installed if the user isn't actually writing objective-c++. In such cases, the system include extraction command fails with:

g++: error trying to exec 'cc1objplus': execvp: No such file or directory

[HighCommander4]

I should add that the issue also arises in cases where the project itself has a compile_commands.json file, but the user is trying to open an external header (and not using --compile-commands-dir).

[HighCommander4]

I installed gobjc++ and looked at the built-in include paths with -xobjective-c++-header, and they're the same as with -xc++-header.

Maybe it's safe to just unconditionally downgrade -xobjective-c++-header to -xc++-header in the system include extractor?

[HighCommander4]

Linking to proposed fix for this: https://reviews.llvm.org/D147905

[HighCommander4]

Fixed on trunk, which is now targeting clangd 18.

It would be nice to have the fix in clangd 17 as well (too late for clangd 16); I will request a cherry-pick.