r258128 - Add -Wexpansion-to-undefined: warn when using `defined` in a macro definition.

[Diego Novillo via cfe-commits]

On Tue, Jan 19, 2016 at 3:43 PM, Nico Weber <thakis at google.com> wrote:

> I mean you could pass a -Wno flag. It's undefined behavior that's actually
> causing bugs in practice; it should probably be on by default.
>

But then you need to fix all the warnings it's producing in an llvm build.
That is currently blocking several folks (us included).
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