[PATCH] [modules] PR20507: Avoid silent textual inclusion.

[Sean Silva]

================
Comment at: lib/Lex/ModuleMap.cpp:346-347
@@ -345,4 +345,1 @@
-      // Cannot use a module if it is unavailable.
-      if (!H.getModule()->isAvailable())
-        continue;
       if (!Result || isBetterKnownHeader(H, Result))
----------------
rsmith wrote:
> Should we also teach `isBetterKnownHeader` to consider an available module to be better than an unavailable one? (If I have two modules covering a header file, and one of them requires "blah" and the other requires "!blah", we should presumably pick the available module.)
I'm having a hard time thinking of a legitimate use for that, or in general to ever have two modules referencing the same file. Do you have something in particular in mind?

But that does raise the larger question is how to make this insensitive to whatever order we are iterating here (which I assume is determined in some relatively fragile way by the order of command line arguments or order in the module map file). From a user's perspective, it is extremely jarring to have the compiler's behavior depend in a fragile way on such things (like the unix linker order-dependence fiasco). I would prefer to just have a hard error if two modules reference the same header, thus we wouldn't even be iterating here at all, and users will always be immediately alerted to errors.

http://reviews.llvm.org/D10423

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