I don't know what inconsistent behavior would make sense here.
There's a
longstanding tradition in Lisp that integer arithmetic just
works, and I don't
see why Emacs would want to fight against that tradition here.
Although there
may be some low-level functions that distinguish between fixnums
and bignums for
efficiency reasons, the vast majority of Lisp functions should
not distinguish
them, and 'format' should be one of those functions.
Maybe we should merge the branch first, let the dust settle and
let
people use the new functionality, then revisit this stuff with
more
experience on our hands.