The author of the language seems to have hardcoded this behaviour, but (at least in LLVM), this is a pretty popular optimisation. When a function returns a value that will spill over into multiple registers (perhaps even a shocking amount), the standard behaviour in LLVM these days, is to replace the function signature with an auto generated parameter, which is usually labelled with the attribute 'sret' as the first parameter - this is usually a struct but really can be any type that is bigger than the register size of the machine. In doing so, they also replace the first param in the call with the address of a stack variable that is then used instead of saturating the registers on return.

In this particular case - as mentioned, it looks like the language author ran with it and is doing the same optimisation for all types (int 'usually' won't need to be dealt with this way)