new concept checks and the 3.0 ABI

[Joe Buck]

Hi,

Phil's new concept checks implementation looks cool, but there's an
important issue.  We (the SC) promised that we'd make the 3.0 C++ ABI
stand up for some time.  However, the new concept checking is on
the mainline, but not in the 3.0 release branch.  What should we do
about this?

Should we release it?  But it is very new.  If we don't release it,
it seems that we may have binary compatibility problems.