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From: "b.stanimirov at abv dot bg" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/98473] New: std::vector<T>::insert(pos, first, last)
 doesn't compile for T which has a deleted assignment operator
Date: Tue, 29 Dec 2020 18:05:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98473

            Bug ID: 98473
           Summary: std::vector<T>::insert(pos, first, last) doesn't
                    compile for T which has a deleted assignment operator
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: b.stanimirov at abv dot bg
  Target Milestone: ---

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

```
struct X {
    X();
    X(const X&);
    X& operator=3D(const X&) =3D delete; // !!
    X(X&&) noexcept;
    X& operator=3D(X&&) noexcept;
    int data =3D 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, sti=
ll
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
workarounds *with worse performance*:
https://stackoverflow.com/questions/65489039/how-to-efficiently-insert-mult=
iple-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.

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

This looks like a bug in libstdc++ (as opposed to a omission by the standar=
d)=