Fix warning for uninitialized va_list variable.

[ktopley-apple]

rdar://problem/40626177

Initializes a va_list variable that is currently not initialized. This is purely to silence a compiler warning because the value will not be used.

[ktopley-apple]

@swift-ci please smoke test

[jrose-apple]

I'm not sure that's a valid initialization for va_list on all platforms, but I guess it's okay for Darwin. You should land this on master first, though, and then cherry-pick to 4.2.

[ktopley-apple]

I don't think there is a globally correct initialization. Fortunately, this only matters for Darwin.

[ktopley-apple]

@swift-ci please smoke test

[ktopley-apple]

@swift-ci please smoke test

[najacque]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 337699d7251d5192246ecd6cbc32542b0632977f

[najacque]

@swift-ci please smoke test macOS platform

[jrose-apple]

That error is actually relevant:

16:36:39 
/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch/swift/stdlib/public/SDK/os/os.m:164:15: error: scalar initializer cannot be empty
16:36:39   va_list x = {};
16:36:39               ^~

If this were a .mm file you'd be okay, but not as a .m.

[jrose-apple]

I guess this is a good reminder not to just do smoke tests if you're dealing with something that might be platform-specific.

[ktopley-apple]

@swift-ci please test

[ktopley-apple]

@swift-ci please test macOS platform

[ktopley-apple]

@swift-ci please test Linux platform

[ktopley-apple]

@swift-ci please smoke test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f5255fed188244f2d4740e61f0f00a1022ac4c34

[ktopley-apple]

@swift-ci please smoke test Linux platform

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 337699d7251d5192246ecd6cbc32542b0632977f

[ktopley-apple]

@swift-ci test Linux platform

[ktopley-apple]

It seems that this works:

static va_list x;

We can use a static whether or not it gets copied on function calls because in this case we know that the value is not going to be used (and definitely not mutated).

[jrose-apple]

Clever enough answer. I wish there was something more semantic, but this is probably fine. (Just silencing the warning on that particular line of code using pragmas probably would have been fine too.)

[ktopley-apple]

I was never able to get the warning when building outside of CI. I would like to understand why that is, when I have time to look more closely.