Why would you use the ternary operator without assigning a value for the “true” condition (x = x ?: 1)

Question

In the Android open-source qemu code I ran across this line of code:

machine->max_cpus = machine->max_cpus ?: 1; /* Default to UP */

Answer 1

The K&R BNF shows an expression is required between "?" and ":". I don't think gcc should be compiling that without a diagnostic.