Proposed resolution:

This wording is relative to N5014.

1. Modify [new.syn] as indicated:

   constexpr void* operator new  (std::size_t size, void* ptr) noexcept;
   constexpr void* operator new[](std::size_t size, void* ptr) noexcept;
   constexpr void operator delete  (void* ptr, void*) noexcept;
   constexpr void operator delete[](void* ptr, void*) noexcept;
   

2. Modify [new.delete.placement] as indicated:

   constexpr void* operator new(std::size_t size, void* ptr) noexcept;
   

   […]

   constexpr void* operator new[](std::size_t size, void* ptr) noexcept;
   

   […]

   constexpr void operator delete(void* ptr, void*) noexcept;
   

   -7- Effects: Intentionally performs no action.

   -8- Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's non-array placement operator new terminates by throwing an exception ([expr.new]).

   constexpr void operator delete[](void* ptr, void*) noexcept;
   

   -9- Effects: Intentionally performs no action.

   -10- Remarks: Default function called when any part of the initialization in a placement new-expression that invokes the library's array placement operator new terminates by throwing an exception ([expr.new]).

The P2747R2 paper made placement operator new constexpr. At that time constexpr exceptions weren't in C++26, so that was all that was needed. But later on when P3068R5 was voted in, the P2747R2 changes look insufficient. The problem is that when you throw from a constructor during operator new, it invokes placement operator delete. And P2747R2 didn't touch that.

This makes it impossible to handle an exception thrown by a placement new-expression during constant evaluation.