When an elderly person drafts a will leaving all their property to the eldest son and makes no mention whatsoever of their younger son, who is disabled and unable to care for himself, the determination of the will's validity directly affects the survival security of this vulnerable heir. Judicial standards are clear: if a will fails to reserve a necessary share for an heir who lacks the ability to work and has no source of livelihood, the will is not entirely invalid; rather, only the portion disposing of that necessary share is invalid. The necessary share must first be set aside before the remainder of the will is executed.

  Pursuant to Article 1141 of the Civil Code, a will shall reserve a necessary portion of the estate for heirs who lack the ability to work and have no source of livelihood. This system is known as the "mandatory share," representing a statutory limitation on testamentary freedom. Its purpose is to safeguard the basic subsistence rights of such special heirs and prevent testators from shifting their support and upbringing obligations onto society through testamentary dispositions.

  The application of the mandatory share requires that two conditions be met simultaneously: first, the heir lacks the ability to work, such as being a minor, severely disabled, or elderly and physically frail to the point of losing labor capacity; second, the heir has no means of livelihood, including no fixed income, no social security pension coverage, and no other sources of support. Both conditions are indispensable. Heirs who are adults and healthy but unemployed, or disabled heirs with pension security, do not fall within the scope of protection under the mandatory share. In judicial practice, courts determine eligibility by comprehensively considering evidence of disability level, income status, and social security records.

  The legal effect of a will is partial invalidity rather than total invalidity. When adjudicating an estate, the court first sets aside a necessary share from the total estate for the heir in question, and then distributes the remaining portion in accordance with the provisions of the will. The amount of this necessary share is determined based on the heir's actual living needs, the basic cost of living in the locality, the size of the estate, and the circumstances of other heirs; it is generally limited to what is sufficient to maintain a basic standard of living and does not entail an equal division among heirs.

  It is also necessary to ensure coordination with other legal systems: the reserved portion applies to testamentary succession and legacies; if property is disposed of through a legacy-support agreement, the reserved portion for heirs who lack the ability to work and have no means of livelihood must not be deprived; if such an heir voluntarily waives their right of succession, this constitutes a disposition of their rights, and withholding the reserved portion is no longer necessary.

The testator shall assess the work capacity and sources of livelihood of all heirs and proactively allocate necessary shares to vulnerable heirs in the will. If an heir claims a mandatory share, they must submit supporting documents such as a disability certificate, a labor capacity assessment, and proof of no income.

  The specific amount of the mandatory share is determined in practice by the local per capita consumer expenditure of residents from the previous year, the heir's actual age and expected remaining lifespan, and the total value of the estate. It is typically set within a range sufficient to sustain the heir's basic livelihood for a certain number of years, rather than being calculated according to statutory inheritance proportions. Where the estate is relatively small and the mandatory share constitutes an excessively high proportion, courts seek a balance between safeguarding survival rights and respecting the testator's testamentary intentions, striving to preserve the validity of the testamentary dispositions to the greatest extent possible. If a testator wishes to dispose of property in a concentrated manner, they should first calculate the amount of the mandatory share and make arrangements for the remaining assets after reserving this necessary portion, thereby ensuring the smooth execution of the will.

  In summary, if a will fails to reserve the mandatory share, only that portion of the disposition is invalid; the necessary share must first be set aside before the will is executed. This system represents the balance between testamentary freedom and the guarantee of survival, and testators must approach it with due care.