At the Bloomington C++ meeting we discussed some issues with the constexpr
specification that the clang team encountered while trying to implement it.
Among the issues was a problem that also came up recently for us as BZ
50248: if the constexpr-ness of a template instantiation depends on its
body, we need to instantiate it in order to decide whether or not an
implicitly-declared function that uses it is constexpr. The resolution of
DR 1358 is that an instantiation of a constexpr template is constexpr even
if it can never produce a constant expression.