[cfe-dev] Command line compiler options

[Neumann, Adrian]

Couldn't this problem be avoided by linking to the appropriate spots in the 
GCC/MSVC documentation whenever clang provides a features that is supposed to 
be exactly the same as the corresponding feature in the other compilers?

I can't find the bugzilla where I described this, but there's actually another 
problem, which is potentially more serious. Even if we did magically have a 
large body of documentation that covered a lot of these gaps, it's not a sure 
thing that we would want to incorporate this content into our docs [1]. The 
issue is that many of our options are specifically there for GCC or MSVC 
compatibility, so providing an independent (duplicated) description of the 
option opens the door to divergence. Our current rough "policy" of documenting 
features of clang that are not compatibility features (or where they are 
incompatible in some way) is actually pretty economical and in line with 
SPOT/DRY principles. It also means that all of our bug reports are "bug in 
compatibility with GCC/MSVC" and we can asymptotically approach compatibility 
(i.e. fixable with patches). Compare this to "I, a user of your compiler, 
relied on a feature that you documented as behaving in a particular way, and 
now you are changing it" which leaves us between a rock and a hard place: stay 
incompatible with the other compilers or break our own compatibility promises 
(not fixable with patches; we are now in the realm of social/political 
issues). It's an open question whether the open-source project wants to risk 
that support problem, or whether there is a safe set of options we can 
document without worrying about this; but arbitrarily thorough documentation 
essentially turns any existing compatibility bugs (or misunderstandings of a 
GCC/MSVC option on the part of the documenter) into potentially "forever" 
support costs.
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