Re: [PATCH] assert that deleting by pointer to base in unique_ptr does not cause UB

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 22 Sept 2021 at 18:09, Antony Polukhin wrote:
>
> std::unique_ptr allows construction from std::unique_ptr of derived
> type as per [unique.ptr.single.asgn] and [unique.ptr.single.ctor]. If
> std::default_delete is used with std::unique_ptr, then after such
> construction a delete is called on a pointer to base. According to
> [expr.delete] calling a delete on a non similar object without a
> virtual destructor is an undefined behavior.
>
> This patch turns that undefined behavior into static assertions inside
> std::unique_ptr.

The undefined behaviour only happens if the destructor is actually
reached at runtime, but won't these static assertions make it
ill-formed to instantiate these members, even if the UB never happens?

For example, if you ensure that release() is called before
destruction, the undefined delete never happens.