[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> To clarify a little bit
> 
> > [...] The "already included" state is global across all modules (which is necessary so that non-modular headers don't get compiled into multiple translation units and cause redeclaration errors).
> 
> The necessity isn't actually true. The same definition in multiple modules shouldn't confuse clang as long as these definitions are identical. But this is a side issue.
Sometimes it does confuse clang, at least I saw problems with a `typedef enum` when I made an include-once header `textual`.

> To re-iterate what this change is about:
> 
> 1. `#import` implies a file is a single-include
> 2. Textual header in a module map implies a file is a multi-include (aka re-entrant)
> 3. When we have both `#import` and the header marked as textual, `#import` "wins", i.e. the file is single-include
> 4. You want to make that when we have both `#import` and a textual header, textual should "win", i.e. the file should be multi-include
> 
> Is it correct?

That's correct. `#import` is an external source - often it comes from the users of the header and not the author, and the users might not be consistent with each other. `textual` comes from the author and a much stronger indicator of intent.

https://github.com/llvm/llvm-project/pull/83660