LWG 3305 was approved earlier this year in Tokyo. We need to give an
error if using std::any_cast<void>, but std::any_cast<void()> is valid
(but always returns null).
libstdc++-v3/ChangeLog:
* include/std/any (any_cast(any*), any_cast(const any*)): Add
static assertion to reject void types, as per LWG 3305.
* testsuite/20_util/any/misc/lwg3305.cc: New test.
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