Allow std::vector::insert to compile when elements have a deleted copy assignment operator

[iboB]

Bugzilla Link	48619
Version	unspecified
OS	All
CC	@iboB,@dwblaikie,@mclow

Extended Description

Create a class X which is copy-constructible but not copy-assignable

struct X {
    X();
    X(const X&);
    X& operator=(const X&) = delete; // !!
    X(X&&) noexcept;
    X& operator=(X&&) noexcept;
    int data = 54;
};

Have vectors of X a and b. Try to add all elements of b at the front of a:

void add_to_front(std::vector<X>& a, const std::vector<X>& b) {
    a.insert(a.begin(), b.begin(), b.end());
}

Live demo: https://godbolt.org/z/K1WT8n

It doesn't compile. My guess is that code which will never get invoked, still needs to compile (or, worse yet, copy assignment does get invoked?!)

The only way to achieve the desired behavior efficiently is to use a custom vector implementation. There is a stackoverflow question which has some worse workarounds: https://stackoverflow.com/questions/65489039/how-to-efficiently-insert-multiple-copy-constructible-but-not-copy-assignable-el

This does compile and work on msvc, so a compile-time check for the copy-assignment code is possible.

[iboB]

As pointed out in the stackoverflow thread, copy-assignability is not listed here: https://en.cppreference.com/w/cpp/container/vector/insert#Parameters

Perhaps this is bug in libc++ instead of a omission by the standard.

[dwblaikie]

Yeah, sounds like a valid bug.

[philnik777]

I'm not really convinced that this is a bug worth fixing in the library. The code expects vector::insert to copy construct the elements in place, but not use the copy assignment operator. This means that we have to explicitly destroy and copy-construct instead of just doing the assignment that it was always meant to be. This seems to me more like a bug in the standard than anything else.

[frederick-vs-ja]

This means that we have to explicitly destroy and copy-construct instead of just doing the assignment that it was always meant to be.

I guess the expected operations are construction and rotate.

Edit: It seems that rotation can be reduced or avoided by using __temp_value and move assignment.