[clang] [C] Diagnose use of C++ keywords in C (PR #137234)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > > Similarly, ObjC selector components exist outside of the normal keyword rules, and the warning should never kick in on them.
> > 
> > 
> > I don't know about selectors all that much; can you give me a test case that you think I should handle?
> 
> In general: look at `ParseObjCSelectorPiece` and how it generally accepts keywords in the selector. Actually, the list of keywords there seems out of date and notably doesn't include post-C++98 keywords like `decltype`. The logic is absolutely meant to encompass every keyword; note that it even includes basic C keywords like `if` and `struct`. You probably don't want to fix that, but I can ping someone at Apple to do it.
> 
> Anyway, selectors show up in three places in the ObjC grammar:
> 
>     * `@selector` expressions, e.g. `@selector(virtual:typeid:bool:)`
> 
>     * message send expressions, e.g. `[obj virtual: 0 typeid: 0 bool: 0]`
> 
>     * method declarations, e.g. `
> 
>     * declarations, e.g.
> 
> 
> ```
> @interface Foo
> - (void) virtual: (int) x typeid: (int) y bool: (int) z;
> @end
> ```
> 
> But there are also some special-case parsing rules for `class` because it is an important property name for `NSObject`. For example, in Objective-C++, the member name following `-> ` or `.` is allowed to be `class` despite it being a keyword.

Ooof, it seems like there may not be a good way of handling this with the approach recommended by @zygoloid where we diagnose from `HandleIdentifier()` in the preprocessor. That requires far too much looking around to try to identify whether we're in the middle of a selector or not. The best I could think of would be to use some sort of "don't complain about keyword compatibility, we're doing <things>" RAII object to set a flag on the preprocessor so we can read it in `HandleIdentifier`. If we go that route, I could also stop looking to see if the preceding token is `@` from `HandleIdentifier()` and just use that mode. But is this too much of a hack?

https://github.com/llvm/llvm-project/pull/137234