[PR c++/87814] undefer deferred noexcept on tsubst if request

[PR c++/87814] undefer deferred noexcept on tsubst if request

tsubst_expr and tsubst_copy_and_build are not expected to handle
DEFERRED_NOEXCEPT exprs, but if tsubst_exception_specification takes a
DEFERRED_NOEXCEPT expr with !defer_ok, it just passes the expr on for
tsubst_copy_and_build to barf.

This patch arranges for tsubst_exception_specification to combine the
incoming args with those already stored in a DEFERRED_NOEXCEPT, and
then substitute them into the pattern, when retaining a deferred
noexcept is unacceptable.

for  gcc/cp/ChangeLog

	PR c++/87814
	* pt.c (tsubst_exception_specification): Handle
	DEFERRED_NOEXCEPT with !defer_ok.

for  gcc/testsuite/ChangeLog

	PR c++/87814
	* g++.dg/cpp1z/pr87814.C: New.

From-SVN: r267155