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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114817] Wrong codegen for std::copy of "trivially
 copyable but not trivially assignable" type
Date: Fri, 26 Apr 2024 18:43:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114817

--- Comment #3 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
https://github.com/boostorg/container/issues/153 , from 2020, is another
similar issue. There, boost::container::vector had assumed that if
__has_trivial_copy(T) and is_copy_constructible_v<T>, then T could be reloc=
ated
via memcpy; but in fact __has_trivial_copy(T) is also true for move-only ty=
pes.
So it was possible to create a type such that (__has_trivial_copy(T) &&
__is_constructible(T, T&)) and yet not __is_trivially_constructible(T, T&).

// https://godbolt.org/z/x5Wda1M6E
struct T {
    template<class U> T(U&&);
    T(T&&);
    ~T();
};
static_assert(__has_trivial_copy(T) && __is_constructible(T, T&));
  // ...and yet...
static_assert(not __is_trivially_constructible(T, T&));=