c++: get_nsdmi in template context [PR108116]

c++: get_nsdmi in template context [PR108116]

Here during ahead of time checking of C{}, we indirectly call get_nsdmi
for C::m from finish_compound_literal, which in turn calls
break_out_target_exprs for C::m's (non-templated) initializer, during
which we build a call to A::~A and check expr_noexcept_p for it (from
build_vec_delete_1).  But this is all done with processing_template_decl
set, so the built A::~A call is templated (whose form was recently
changed by r12-6897-gdec8d0e5fa00ceb2) which expr_noexcept_p doesn't
expect, and we crash.

This patch fixes this by clearing processing_template_decl before
the call to break_out_target_exprs from get_nsdmi.  And since it more
generally seems we shouldn't be seeing (or producing) non-templated
trees in break_out_target_exprs, this patch also adds an assert to
that effect.

	PR c++/108116

gcc/cp/ChangeLog:

	* constexpr.cc (maybe_constant_value): Clear
	processing_template_decl before calling break_out_target_exprs.
	* init.cc (get_nsdmi): Likewise.
	* tree.cc (break_out_target_exprs): Assert processing_template_decl
	is cleared.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-template24.C: New test.

(cherry picked from commit cf59c8983ef6590f0d69014f8dc8778b5b7691c6)