c17_updated_proposed_fdis.pdf § 6.7.9 point 19:

... all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

and point 10:

If an object that has static or thread storage duration is not initialized explicitly, then: [...] — if it has arithmetic type, it is initialized to (positive or unsigned) zero

Or latest draft § 6.7.10 points 20 and 11