c++: some missing-SFINAE fixes
It looks like we aren't respecting SFINAE for:
* an invalid/non-constant conditional explicit-specifier
* a non-constant conditional noexcept-specifier
* a non-constant argument to __integer_pack
This patch fixes these in the usual way, by passing complain and
propagating error_mark_node appropriately.
gcc/cp/ChangeLog:
* decl.cc (build_explicit_specifier): Pass complain to
cxx_constant_value.
* except.cc (build_noexcept_spec): Likewise.
* pt.cc (expand_integer_pack): Likewise.
(tsubst_function_decl): Propagate error_mark_node returned
from build_explicit_specifier.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/noexcept-type26.C: New test.
* g++.dg/cpp2a/explicit19.C: New test.
* g++.dg/ext/integer-pack6.C: New test.