Any pallet implementing ReservableCurrency or fungible::MutateHold should ensure that the reserve/hold operation can be followed by a slash or repatriation of the reserved amount without bringing the state becoming invalid. I.e. something like this should always succeed:

Balances::make_free_balance_be(1, 1000);
if Balances::reserve(1, 1000).is_ok() {
	System::inc_consumers(1);
	assert_ok!(Balances::slash_reserved(1, 1000);
	assert_eq!(System::consumers(1), 1);
	assert_eq!(System::providers(1), 1);
}

Right now, this will fail as the balance is both reserved/slashed and used for a provider ref.

The solution is to ensure that any reserved amounts are not considered toward the ED (only the free, unlocked balance); if a transfer, reserve or lock operation were to take the unlocked free balance below the ED then it should fail.