https://llvm.org/bugs/show_bug.cgi?id=27798
Bug ID: 27798
Summary: declval() can be implemented without requiring a
template instantiation.
Product: libc++
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: All Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: enieb...@boost.org
CC: llvm-bugs@lists.llvm.org, mclow.li...@gmail.com
Classification: Unclassified
declval gets used *everywhere*. It doesn't need to instantiate a template.
Rather than:
template <class _Tp>
typename add_rvalue_reference<_Tp>::type
declval() noexcept;
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));
In a large code base that makes heavy use of templates, this small change is a
measured 2% compile-time win.
I think the as-if rule gives implementers enough latitude to make this change.
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