Asking for extension is Technically a default. It logically follows then, Technically the US is bankrupt.
There has not been any time in the history of the U.S. where the U.S. government did not repay the original capital on a U.S. bond back to the investor.
In addition, there has never been a request for an extension of repayment as per above.
However, you are correct that technically the U.S. is bankrupt....depending upon how you define "bankrupt":
-if it is assets - liabilities......bankrupt.
-if it is defaulting on a repayment.......not bankrupt as there has never been a default.