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libstdc++: Separate construct/convertibility tests for std::tuple

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P2321R2 adds additional conditionally explicit constructors to std::tuple
which we'll concisely implement in a subsequent patch using explicit(bool),
like in our C++20 std::pair implementation.  This prerequisite patch
adds member typedefs to _TupleConstraints for testing element-wise
constructibility and convertibility separately; we'll use the first in
the new constructors' constraints, and the second in their explicit
specifier.

In passing, this patch also redefines the existing member predicates
__is_ex/implicitly_constructible in terms of these new members.  This
seems to reduce compile time and memory usage by about 10% for large
tuples when using the converting constructors that're constrained by
_Explicit/_ImplicitCtor.

libstdc++-v3/ChangeLog:

	* include/std/tuple (_TupleConstraints::__convertible): Define.
	(_TupleConstraints::__constructible): Define.
	(_TupleConstraints::__is_explicitly_constructible): Redefine this
	in terms of __convertible and __constructible.
	(_TupleConstraints::__is_implicitly_constructible): Likewise.
This commit is contained in:
Patrick Palka 2022-08-23 13:35:49 -04:00
parent e85bb1881e
commit 02f6b405f0
1 changed files with 10 additions and 4 deletions
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14
libstdc++-v3/include/std/tuple
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@ -553,14 +553,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<bool, typename... _Types>
struct _TupleConstraints
{
template<typename... _UTypes>
using __constructible = __and_<is_constructible<_Types, _UTypes>...>;
template<typename... _UTypes>
using __convertible = __and_<is_convertible<_UTypes, _Types>...>;
// Constraint for a non-explicit constructor.
// True iff each Ti in _Types... can be constructed from Ui in _UTypes...
// and every Ui is implicitly convertible to Ti.
template<typename... _UTypes>
static constexpr bool __is_implicitly_constructible()
{
return __and_<is_constructible<_Types, _UTypes>...,
is_convertible<_UTypes, _Types>...
return __and_<__constructible<_UTypes...>,
__convertible<_UTypes...>
>::value;
}
@ -570,8 +576,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename... _UTypes>
static constexpr bool __is_explicitly_constructible()
{
return __and_<is_constructible<_Types, _UTypes>...,
__not_<__and_<is_convertible<_UTypes, _Types>...>>
return __and_<__constructible<_UTypes...>,
__not_<__convertible<_UTypes...>>
>::value;
}