Is a class with deleted copy-constructor trivially copyable? - c++

Question

Is this class:
class A {
public:
A() = default;
A(const A&) = delete;
};
trivially copyable? (At least clang seems to think so (live))
In particular, would
A a,b;
std::memcpy(&a, &b, sizeof(A));
invoke undefined behavior?
Context: This answer [deleted because proven wrong] plus its comment tree.

Answer 1

Update: The proposed resolution of CWG 1734, currently in "ready" status, would modify [class]/p6 to read:
A trivially copyable class is a class:
where each copy constructor, move constructor, copy assignment operator, and move assignment operator (12.8 [class.copy], 13.5.3
[over.ass]) is either deleted or trivial,
that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator,
and
that has a trivial, non-deleted destructor (12.4 [class.dtor]).
This renders classes like
struct B {
B() = default;
B(const B&) = delete;
B& operator=(const B&) = delete;
};
no longer trivially copyable. (Classes of this sort include synchronization primitives like std::atomic<T> and std::mutex.)
However, the A in the OP has a implicitly declared, non-deleted copy assignment operator that is trivial, so it remains trivially copyable.
The original answer for the pre-CWG1734 situation is preserved below for reference.
Yes, somewhat counterintuitively, it is trivially copyable. [class]/p6:
A trivially copyable class is a class that:
has no non-trivial copy constructors (12.8),
has no non-trivial move constructors (12.8),
has no non-trivial copy assignment operators (13.5.3, 12.8),
has no non-trivial move assignment operators (13.5.3, 12.8), and
has a trivial destructor (12.4).
[class.copy]/p12:
A copy/move constructor for class X is trivial if it is not
user-provided, its parameter-type-list is equivalent to the
parameter-type-list of an implicit declaration, and if
class X has no virtual functions (10.3) and no virtual base classes (10.1), and
class X has no non-static data members of volatile-qualified type, and
the constructor selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is
trivial;
Similarly ([class.copy]/p25):
A copy/move assignment operator for class X is trivial if it is not
user-provided, its parameter-type-list is equivalent to the
parameter-type-list of an implicit declaration, and if
class X has no virtual functions (10.3) and no virtual base classes (10.1), and
class X has no non-static data members of volatile-qualified type, and
the assignment operator selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is
trivial;
[class.dtor]/p5:
A destructor is trivial if it is not user-provided and if:
the destructor is not virtual,
all of the direct base classes of its class have trivial destructors, and
for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial
destructor.
[dcl.fct.def.default]/p5:
A function is user-provided if it is user-declared and not explicitly
defaulted or deleted on its first declaration.
Indeed, this has been a source of problems for the committee itself, because under the current definition atomic<T> (along with mutexes and condition variables) would be trivially copyable. (And obviously, allowing someone to memcpy over an atomic or a mutex without invoking UB would be ... let's just say seriously problematic.) See also N4460.