c++: static_assert (false) in template [DR2518]

c++: static_assert (false) in template [DR2518]

For a long time, people have expected to be able to write
static_assert (false) in a template and only have it diagnosed if the
template is instantiated, but we (and other implementations) gave an error
about the uninstantiated template because the standard says that if no valid
instantiation of the template is possible, the program is ill-formed, no
diagnostic required, and we try to diagnose IFNDR things when feasible.

At the meeting last week we were looking at CWG2518, which wanted to specify
that an implementation must not accept a program containing a failing #error
or static_assert.  We also looked at P2593, which proposed allowing
static_assert in an uninstantiated template.  We ended up combining these
two in order to avoid requiring implementations to reject programs with
static_assert (false) in uninstantiated templates.

The committee accepted this as a DR, so I'm making the change to all
standard modes.  This behavior was also conformant previously, since no
diagnostic was required in this case.

We continue to diagnose non-constant or otherwise ill-formed conditions, so
no changes to existing tests were needed.

	DR 2518
	PR c++/52809
	PR c++/53638
	PR c++/87389
	PR c++/89741
	PR c++/92099
	PR c++/104041
	PR c++/104691

gcc/cp/ChangeLog:

	* semantics.cc (finish_static_assert): Don't diagnose in
	template context.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2518.C: New test.