Make the default constructors of tuple and pair conditionally explicit.

Make the default constructors of tuple and pair conditionally explicit.

2015-11-03  Ville Voutilainen  <ville.voutilainen@gmail.com>

	Make the default constructors of tuple and pair conditionally explicit.
	* include/std/type_traits (is_unsigned, __is_array_unknown_bounds,
	__is_default_constructible_atom, __is_default_constructible_safe,
	__is_direct_constructible_new_safe, __is_direct_constructible_ref_cast,
	__is_nt_default_constructible_impl, is_nothrow_default_constructible,
	is_nothrow_constructible, is_nothrow_assignable,
	is_trivially_constructible, is_trivially_copy_constructible,
	is_trivially_move_constructible, is_trivially_assignable,
	is_trivially_copy_assignable, is_trivially_move_assignable,
	is_trivially_destructible): Simplify.
	* include/std/type_traits (
	__do_is_implicitly_default_constructible_impl,
	__is_implicitly_default_constructible_impl,
	__is_implicitly_default_constructible_safe,
	__is_implicitly_default_constructible): New.
	* include/bits/stl_pair.h (pair::pair()): Use it.
	* include/std/tuple (tuple<_T1, _T2>::tuple): Use it.
	* include/std/tuple (_ImplicitlyDefaultConstructibleTuple): New.
	* include/std/tuple (tuple<_Types...>::tuple()): Use it.
	* testsuite/20_util/declval/requirements/1_neg.cc: Adjust.
	* testsuite/20_util/is_implicitly_default_constructible/requirements/explicit_instantiation.cc: New.
	* testsuite/20_util/is_implicitly_default_constructible/requirements/typedefs.cc: Likewise.
	* testsuite/20_util/is_implicitly_default_constructible/value.cc: Likewise.
	* testsuite/20_util/make_signed/requirements/typedefs_neg.cc: Adjust.
	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc: Likewise.
	* testsuite/20_util/pair/cons/explicit_construct.cc: Likewise.
	* testsuite/20_util/tuple/cons/explicit_construct.cc: Likewise.

From-SVN: r229699