This is actually a good point, but what about Unicode? Will `(UTF8String, Float64)` emit another function?
$ julia -e 'f(a, b) = @sprintf("this is a %s %15.1f", a, b); code_native(f, (AbstractString, Float64))' | wc -l
WARNING: Returned code may not match what actually runs.
628
$ julia -e 'f(a, b) = @sprintf("this is a %s %15.1f", a, b); code_native(f, (ASCIIString, Float64))' | wc -l
194
$ julia -e 'f(a, b) = @sprintf("this is a %s %15.1f", a, b); code_native(f, (UTF8String, Float64))' | wc -l
194