[libcxx-commits] [PATCH] D115658: [libc++] Fix the noexceptness of __decay_copy.

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone added inline comments.


================
Comment at: libcxx/include/__utility/decay_copy.h:27
 #if _LIBCPP_STD_VER > 17
-    noexcept(is_nothrow_convertible_v<_Tp, remove_reference_t<_Tp>>)
+    noexcept(is_nothrow_convertible_v<_Tp, decay_t<_Tp>>)
 #endif
----------------
Actually, it occurs to me that several things are probably wrong here and we should just fix them all at once.
- We don't care about //convertible// but rather //constructible//.
- We should be returning `decay_t<_Tp>(static_cast<_Tp&&>(__t))`, not simply `static_cast<_Tp&&>(__t)`.
- We should be noexcept'ing on the same expression.
- We should just go ahead and make this a full "write it three times" function, so that `__decay_copy(mymutex)` SFINAEs away instead of pretending it exists and then hard-erroring during instantiation.
- In C++23, //DECAY_COPY// is changing over to `auto(x)`, which doesn't call move constructors when you pass it a prvalue, so we'll need something else anyway. How about we just make a macro like this?

```
#if PRESENT_DAY
 #define _LIBCPP_DECAY_COPY(x) static_cast<typename decay<decltype((x))>::type>(x)
#else
 #define _LIBCPP_DECAY_COPY(x) auto(x)
#endif
```
and replace all our usages throughout the codebase right now?

I have a vested interest because D115607 adds a //ton// of `__decay_copy` calls. :)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D115658/new/

https://reviews.llvm.org/D115658