c++: ahead of time variable template-id coercion [PR89442]

c++: ahead of time variable template-id coercion [PR89442]

This patch makes us coerce the arguments of a variable template-id ahead
of time, as we do for class template-ids, which causes us to immediately
diagnose template parm/arg kind mismatches and arity mismatches.

Unfortunately this causes a regression in cpp1z/constexpr-if20.C: coercing
the variable template-id m<ar, as> ahead of time means we strip it of
typedefs, yielding m<typename C<i>::q, typename C<j>::q>, but in this
stripped form we're directly using 'i' and so we expect to have captured
it.  This is a variable template version of PR107437.

	PR c++/89442
	PR c++/107437

gcc/cp/ChangeLog:

	* cp-tree.h (lookup_template_variable): Add complain parameter.
	* parser.cc (cp_parser_template_id): Pass tf_warning_or_error
	to lookup_template_variable.
	* pt.cc (lookup_template_variable): Add complain parameter.
	Coerce template arguments here ...
	(finish_template_variable): ... instead of here.
	(lookup_and_finish_template_variable): Check for error_mark_node
	result from lookup_template_variable.
	(tsubst_copy) <case TEMPLATE_ID_EXPR>: Pass complain to
	lookup_template_variable.
	(instantiate_template): Use build2 instead of
	lookup_template_variable to build a TEMPLATE_ID_EXPR
	for most_specialized_partial_spec.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp/pr64127.C: Expect "expected unqualified-id at end
	of input" error.
	* g++.dg/cpp0x/alias-decl-ttp1.C: Fix template parameter/argument
	kind mismatch for variable template has_P_match_V.
	* g++.dg/cpp1y/pr72759.C: Expect "template argument 1 is invalid"
	error.
	* g++.dg/cpp1z/constexpr-if20.C: XFAIL test due to bogus "'i' is
	not captured" error.
	* g++.dg/cpp1z/noexcept-type21.C: Fix arity of variable template d.
	* g++.dg/diagnostic/not-a-function-template-1.C: Add default
	template argument to variable template A so that A<> is valid.
	* g++.dg/parse/error56.C: Don't expect "ISO C++ forbids
	declaration with no type" error.
	* g++.dg/parse/template30.C: Don't expect "parse error in
	template argument list" error.
	* g++.dg/cpp1y/var-templ82.C: New test.