https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71187
Bug ID: 71187
Summary: declval() can be implemented without requiring a
template instantiation
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: eric.niebler at gmail dot com
Target Milestone: ---
declval gets used *everywhere*. It doesn't need to instantiate a template.
Rather than:
template<typename _Tp>
inline typename add_rvalue_reference<_Tp>::type
declval() noexcept
{
static_assert(__declval_protector<_Tp>::__stop,
"declval() must not be used!");
return __declval_protector<_Tp>::__delegate();
}
declval can be defined as:
template<typename _Tp, typename _Up = _Tp&&>
_Up
__declval(int);
template<typename _Tp>
_Tp
__declval(long);
template<typename _Tp>
auto declval() noexcept -> decltype(__declval<_Tp>(0))
{
static_assert(__declval_protector<_Tp>::__stop,
"declval() must not be used!");
return __declval_protector<_Tp>::__delegate();
}
In a large code base that makes heavy use of templates, this small change is a
measured 4% compile-time win (g++ (GCC) 5.x 20160302).
I think the as-if rule gives implementers enough latitude to make this change.
