Since you wanted the spec, here it is (from ยง15.25 Conditional Operator ? :, the last sentence of the section):
The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.
Since you wanted the spec, here it is (from ยง15.25 Conditional Operator ? :, the last sentence of the section):
The operand expression not chosen is not evaluated for that particular evaluation of the conditional expression.