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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/68350] std::uninitialized_copy overly restrictive for
 trivially_copyable types
Date: Thu, 24 Feb 2022 14:45:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D68350

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #9)
> - If the output type is trivially default constructible and is assignable
> from
>   the input type, we can use std::copy (but it's unclear whether that giv=
es
> any
>   benefit if we can't turn it into memcpy/memmove).

It's also not clear that it would be allowed.

struct A { };
struct B {
  B() =3D default;
  B(A) { do_stuff(); }
  B& operator=3D(A) { do_other_stuff(); }
};

void (A* f, A* l, B* out) {
  std::uninitialized_copy(f, l, out);
}

It's observable whether we construct B from A, or default construct B and t=
hen
assign to it from A.

My patch just removes all uses of std::copy/fill/fill_n in the
std::uninitialized_copy/fill/fill_n algos.=