[libcxx-commits] [PATCH] D145376: [libc++] add declval failure assertion for instantiation

[A. Jiang via Phabricator via libcxx-commits]

frederick-vs-ja added inline comments.


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Comment at: libcxx/include/__utility/declval.h:29
 
-template <class _Tp>
-decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT;
+template <class _Tp, bool _False = false>
+_LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
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I think it's better not to modify the template head.
Doesn't libc++ have a concentrated `__dependent_false`? If so, I think we can still use `!__is_same(_Tp, _Tp)` in `static_assert`.


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Comment at: libcxx/include/__utility/declval.h:30
+template <class _Tp, bool _False = false>
+_LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
+  static_assert(_False, "Calling declval is ill-formed, see [declval]/2.");
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The return type is... cursed for //cv// `void`.

It seems that `decltype(std::declval<const void>)` should be `const void()` (until C++17) / `const void() noexcept` (since C++17) (same for `volatile void` and `const volatile void`), but the cv-qualifiers are dropped in `decltype(std::__declval<_Tp>(0))`.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D145376/new/

https://reviews.llvm.org/D145376