[Bug c++/121068] Placement new of array element is rejected at compile-time

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121068

--- Comment #10 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
If the object pointed by __location is not transparently replaceable, for
example if I would create an object inside std::byte array that is an member,
then does std::launder(__location) produce pointer to new object? __location
does not automatically point there per (https://eel.is/c++draft/basic.life#10),
by we meet precondition for launder (https://eel.is/c++draft/ptr.launder#2).

This cannot happen for compile-time evaluation, as calling construct_at such
way would require equivalent of reinterpret_cast, so we could do, but I am not
sure if that is necessary.
+#if __cpp_lib_launder
+  if consteval 
+    {
+      return std::launder(__location);
+    }
+  else
+    {
+      return std::launder(static_cast<_Tp*>(__loc));
+    }
+#else
+  return static_cast<_Tp*>(__loc);
+#endif