The check cannot be made at compile time, only at run time, and even if it were possible to make the check at compile time it would still be a bad idea. At some point in the program's 20 year old life time, someone will decide that a number N used somewhere in the program has to become N+1, and N+1 doesn't work. It would be unacceptable for such a trivial change to break the program's compilation, and for a number to carry so much hidden state and meaning.
I don't understand why people are so afraid of compile time errors. They're great! They help us write code and reason about it.
If someone changes some ossified piece of code, and that has the effect that it makes an automatic cast semantically dangerous, I'd be extremely grateful if the compiler scolded me with a helpful error message.
I'm not sure what you mean by "The check cannot be made at compile time, only at run time". Are you talking about a language other than Go? Go is statically typed.
edit: I've been talking about compile time errors that prevent potentially dangerous implicit casts from happening. I think you're talking about something else.
If int64 is the default type for int, then any int will be able to contain values not representable as a float64 (you'd need a float80 or float128 to make it work). To safely convert, a run-time check will be needed.
