[llvm-bugs] [Bug 161928] UB in std::vector::operator=

[LLVM Bugs via llvm-bugs]

Issue	161928
Summary	UB in std::vector::operator=
Labels	new issue
Assignees
Reporter	rogeeff

    I believe the following reproducer demonstrate an UB in std::vector implementation of operator= incase if we are using a custom allocator:

`#include <vector>
#include <iostream>
#include <cstdlib>

// Simple custom allocator that checks it's not default-constructed
template <typename T>
class CustomAllocator {
public:
    using value_type = T;
    using propagate_on_container_copy_assignment = std::true_type;
    using propagate_on_container_move_assignment = std::true_type;
    using propagate_on_container_swap = std::true_type;
 
    CustomAllocator() : valid_(false) {}
    CustomAllocator(bool valid) : valid_(valid) {}

 template <typename U>
    CustomAllocator(const CustomAllocator<U>& other) : valid_(other.valid_) {}

    T* allocate(std::size_t n) {
        if (!valid_) {
            std::cerr << "Allocate called on invalid allocator\n";
            std::abort();
        }
        return static_cast<T*>(::operator new(n * sizeof(T)));
    }

    void deallocate(T* p, std::size_t n) {
        if (!valid_) {
 std::cerr << "Deallocate called on invalid allocator\n";
 std::abort();
        }
        ::operator delete(p);
    }

    bool operator==(const CustomAllocator&) const = default;

private:
    bool valid_;
};

int main() {
    CustomAllocator<int> alloc(true);
 std::vector<int, CustomAllocator<int>> v1(alloc);
    std::vector<int, CustomAllocator<int>> v2;

    v1.push_back(1);

    v2 = v1;

 return 0;
}
`

The problem is observed at runtime. when v2 is constructor we do not call allocator, but when v2 is assigned we invoke deallocate with (nullptr, 0). This is UB I believe and should be adressed in the library.

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