[cfe-dev] Adding a -W flag for "using extended field designator is an extension"

[Jeff Walden]

"offsetof(T, field,subfield)" and "offsetof(T, arr[3])" are C/C++ extensions; only "offsetof(T, field)" is standard.  Clang warns about these with -pedantic, but there's no -W flag for this warning, so I can't disable it.  This patch adds -Wno-offsetof-extended-field-designator for this.

I don't know if this should be a new -W or simply be grouped under -Winvalid-offsetof.  The purposes of the two seem different, and finer-grained warnings seem preferable, so I've made a new warning.  This is my first patch, so I have no idea what the norm is for this.  Would using invalid-offsetof be better?

Also, I've cargo-culted this from another revision[0], so if the patch is totally off-base, please say so.  :-)

Jeff

0. http://llvm.org/viewvc/llvm-project?view=rev&revision=142284
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