https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58159
--- Comment #6 from Geoff Romer <gromer at google dot com> --- A Chromium maintainer privately pointed out a use case that would be thwarted by a check like this: basically, unique_ptr is used to hold pointers from a legacy API, using a custom deleter that decrements a reference count rather than actually destroying the object. If the legacy API returns the same pointer from multiple calls, this can lead to reset() being legitimately called with a pointer equal to the stored value. So on reflection, I agree this can only be done for default_delete.
