What is the tax base of a loan?

According to the definition: The tax base of a liability is the amount that is taxable in the future for tax purpose or in other words, the amount that can contribute to the future taxable income. So we have a liability: a loan of $100, this amount can not contribute to future taxable income and is not taxable in the future when it is repaid. So why its tax base of liability equals to its carrying value of 100, not 0?

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