Clang-CL 15.x reporting warning when using /clang:-xc

[dgehriger]

OS: Windows

Unlike previous versions, when running clang-cl.exe version 15.x as follows from a cmd.exe prompt:

clang-cl /WX /clang:-xc /clang:-E /clang:-dM nul

It outputs the following warning:

clang-cl: error: '-x c' after last input file has no effect [-Werror,-Wunused-command-line-argument]

Expected outcome is no error, and dumping of preprocessor variables.

Among other things, this prevents clang-cl 15.x from being used by Visual Studio Code's C++ extension for Intellisense.

[efriedma-quic]

As a workaround, the following should work on both old and new versions of clang-cl:

clang-cl /WX /TC /E /clang:-dM nul

[AaronBallman]

@Fznamznon -- I assigned this one to you because I figured it was one you might be able to knock out pretty quickly, but if you don't feel like it, feel free to unassign yourself. :-)

[Fznamznon]

For some reason -x c argument gets bigger index than input file argument in cl mode. Not sure why yet.

[Fznamznon]

Ah, I see. In cl mode /clang: arguments are parsed and appended at the end of the list. I would suggest just to not emit a warning in cl mode since it was implemented to match gcc behavior anyway (e2a1f8e).
Another thing comes in mind that if all clang: arguments go to the end of list, clang:-x doesn't make sense at all, which is actually we get warning about just not with the right words.

[AaronBallman]

@MaskRay -- is it expected that all /clang: arguments are appended? That seems likely to cause a fair amount of confusion in practice, but perhaps I'm missing something.

[Fznamznon]

In case silencing the warning in cl mode sounds okay-ish, I've prepared https://reviews.llvm.org/D142757 . Otherwise if we want to emit a correct warning in cl mode, we need to change how to /clang: arguments are processed and for that I'm not sure I have enough driver experience to do quickly.

[MaskRay]

The error is emitted as intended. /clang:-xc nul is handled like nul -xc and the -xc does not take effect.
This likely has been the case since https://reviews.llvm.org/D53457 added /clang: in 2018-11. therefore I don't think the new feature has a legitimate use case.
It's difficult to use interleaved semantics instead of appending semantics.

So simply ignoring the error in CL mode (https://reviews.llvm.org/D142757) looks wrong to me.
I think we should just report an error for /clang:-xc. Left a comment on the patch about the change.

[huangqinjin]

clang-cl -help shows that -x is supported directly. But clang-cl -x c++-module report an error

unsupported option '-x c++-module'; did you mean '/TC' or '/TP'?

Unfortunately there is no coresponding flag to specify C++20 Modules.