[Bug libstdc++/68350] std::uninitialized_copy overly restrictive for trivially_copyable types

["arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68350

--- Comment #12 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
jwakely wrote:
> Correction: they need to be the same type. We can't memcpy here:
>
> struct A { };
> struct B { B() = default; B(A) { do_stuff(); } };
>
> void (A* f, A* l, B* out) {
>   std::uninitialized_copy(f, l, out);
> }

Right, in your case, the ctor `B(A)` runs user-defined code (it's not
"trivial") so obviously we can't do the optimization. But even in general, see
my 2018 blog post "Trivially-constructible-from."
https://quuxplusone.github.io/blog/2018/07/03/trivially-constructible-from/

  using A2 = long long;
  using B2 = int64_t;  // for the sake of argument, this is "long int"
  void (A* f, A* l, B* out) {
    std::uninitialized_copy(f, l, out);
  }

The library can't, by itself, determine that memcpy would be safe here. The
library needs help from the compiler, e.g. via a new compiler builtin
__is_trivially_constructible_from(T, U). (This is *not* the same as the
existing `is_trivially_constructible<T,U>` type trait, because blog post.)