[swift-corelibs-dev] libdispatch import order / nondeterminism

[Daniel Dunbar]

I don't have the full context here, but...

For C and regular header include style, it works roughly the same on linux, except that we don't have all the modules declared in one level. However:

  #include <dispatch/dispatch.h>

will cause the compiler to look inside the dispatch subdirectory, at which point it should seem the module map file and then load dispatch as a module.

However, this doesn't work for Swift because Swift doesn't go through the header search path, it goes through the "module search" path (like @import foo in Obj-C). For that to work, Clang will need to know to preload the module map file in order to understand how to map dispatch. Clang has `-fmodule-map-file=<path>` to cause Clang to scan (not necessarily load) a module map in advance.

 - Daniel

> On Feb 10, 2016, at 8:31 PM, Daniel A. Steffen via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
> 
> having -I /usr/local/include/dispatch doesn’t seem right to me, the C header include convention is all <dispatch/*.h> so the dispatch/ directory itself should not be part of the search path. 
> 
> maybe that means that something different needs to happen to pick up the module files (AFAIK on Darwin this works differently by having a toplevel modulefile in /usr/include that references the submodules, I don’t know how modules are intended to work on other platforms)
> 
> Daniel
> 
>> On Feb 10, 2016, at 18:09, Drew Crawford via swift-corelibs-dev <swift-corelibs-dev at swift.org <mailto:swift-corelibs-dev at swift.org>> wrote:
>> 
>> 
>>> On Feb 10, 2016, at 8:08 PM, Philippe Hausler <phausler at apple.com <mailto:phausler at apple.com>> wrote:
>>> 
>>> The -I/usr/local/include/dispatch probably sees time.h in there and thinks that is part of a system root header? Just a speculation.
>> 
>> I agree 100%, but I don't know how to fix it.
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