As a general rule, if the promisor fails to perform under a contract, a donee third-party beneficiary whose rights have vested can sue:

- The promisor to enforce the contract.

Explanation:

As per the general rule, if a promisor fails to perform under the contract and its conditions the third party beneficiary is equipped with the right to sue the 'promisor for enforcement of the contract'. According to the detrimental reliance, if the donee possesses a valid legal contract, then he/she possesses the right to sue the donor under the charges of non-fulfillment of the conditions of the contract but if there is absence of 'legal detrimental reliance', then the donee beneficiary could not sue due to non-availability of contract and no proof of the promisor's promise.