[cfe-dev] RFC: We should take a more conservative approach to libstdc++ compatibility...

[Chandler Carruth]

Greetings all! This is the result of a chat between Richard and myself just
now.

These days Clang does a great job of detecting GCC and libstdc++
installations and using them... such a great job that Clang installed today
will work with GCC 4.9 if such a thing existed and were installed.

As we've noticed recently, this can cause problems, and now that we have
clang releases and GCC releases with decent compatibility out in the wild
(3.2 and 4.{6,7} resp.), I think we should take a different approach:

1) Set a max GCC version we use, and on trunk have it be silly (v99.99.99).
2) On the release branch, lower this to the highest GCC version we test
that release against. This protects a released clang from using newer GCC
libstdc++ which is both good and bad -- no improvements from updates, but
no breakage from updates.
3) Teach the driver about incompatible versions of GCC and libstdc++, and
have it try to find a different version when available. (Mostly relevant to
4.4 and 4.5 and C++11 headers...)
4) Teach Clang itself to warn on a libstdc++ version macro which is one of
the versions that has incompatibilities so users understand what is
happening.


Thoughts?
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