[cfe-dev] Module macro semantics with #undef's

[Tim Prince via cfe-dev]

Hi everyone,

The modules documentation (http://clang.llvm.org/docs/Modules.html#macros) says this about importing multiple definitions of the same macro:

> For example, suppose:
>   - <stdio.h> defines a macro getc (and exports its #define)
>   - <cstdio> imports the <stdio.h> module and undefines the macro (and exports its #undef)
> The #undef overrides the #define, and a source file that imports both modules in any order will not see getc defined as a macro.

I've been writing some test cases, and I've confirmed this behavior in a trivial test case. However, when I import two modules that both define a macro, that macro still seems to be defined even in the presence of another macro that #undef's it. Here are two test cases that demonstrate the divergent behavior:

> $ cat undef.c
> #include "Module/UndefFoo.h"
> #include "Module/DefineFoo.h"
> 
> #ifdef FOO
> #error FOO was defined
> #endif
> $ cat redef-undef.c
> #include "Module/UndefFoo.h"
> #include "Module/DefineFoo.h"
> #include "Module/RedefineFoo.h"
> 
> #ifdef FOO
> #error FOO was defined
> #endif
> $

And here's the contents of the module itself:

> $ ls Module/
> DefineFoo.h  module.modulemap  RedefineFoo.h  UndefFoo.h
> $ cat Module/module.modulemap
> module Module {
>   module DefineFoo {
>     header "DefineFoo.h"
>   }
>   module UndefFoo {
>     header "UndefFoo.h"
>   }
>   module RedefineFoo {
>     header "RedefineFoo.h"
>   }
> }
> $ cat Module/DefineFoo.h
> #define FOO 1
> $ cat Module/RedefineFoo.h
> #define FOO 1
> $ cat Module/UndefFoo.h
> #undef FOO
> $

Now, when I run these test cases:

> $ /opt/pkg/clang-3.9.0/bin/clang -fmodules -c undef.c
> $ /opt/pkg/clang-3.9.0/bin/clang -fmodules -c redef-undef.c
> redef-undef.c:6:2: error: FOO was defined
> #error FOO was defined
>  ^
> 1 error generated.
> $

The documentation seems to suggest that the latter case should also pass without errors. Is this inconsistency a bug or is there some subtlety I'm missing here?

Thanks!
- Tim