Originally Posted by
CKDGM
Technically, 8 USC 1185(b) says "it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport" (emphasis added).
I don't see a definition of "bears" in that portion of 8 USC, so the common meaning of "carries" should apply; if you have it with you (and, presumably, show it to CBP upon request) when departing the US you would seem to be in compliance whether or not it was the passport you showed the airline at check-in.
Sounds reasonable on the face of it (before getting into all the "exceptions" to that clause which aren't really exceptions). However, then what to make of the situation where a dual citizen of the US with a US and foreign passport in possession merely presents their non-US passport when dealing directly with CBP to enter or exit the country? The word has the same meaning in both contexts, that of exiting the US and that of entering the US.