proposal: Go 2: allow any compatible slice argument to be copied automatically at the call site

[pjebs]

func main() {
	
	var (
		a int = 1
		b int = 2
		c int = 3
	)
	F(a, b, c) // Works
	F(1, 2, 3) // Works
	
	Inums := []interface{}{5, 6, 7}
	F(Inums...) // Works

	nums := []int{5, 6, 7}
	F(nums...) // Doesn't work
}

func F(x ...interface{}) {
	fmt.Println(x...)
}

Compiler Error:

cannot use nums (type []int) as type []interface {} in argument to F

[pjebs]

I think for Go 2, a nice change would be to allow the above example to work as syntax sugar.

I understand they are technically different types (hence the error). I'm not saying the underlying data types or how the underlying data type works should be changed.

My proposal is only from a "verbal" point of view. The compiler just "pretends" each item in the slice was "by hand" written out in such a way that it simulates the way that it currently works.

If the way interfaces work causes issues, then my proposal is purely to consider interface{ } as a special case.

My proposal is also for it to work irrespective of what happens with the generics issue.

[ianlancetaylor]

See the discussion in #18605, especially #18605 (comment) .

[ianlancetaylor]

Adopting this as suggested would cause a subtle change in the handling of ...: if the types match exactly the same slice would be passed, but if the types differ then a new slice would be created and that new slice would be passed.

Also, as a general guideline we try to avoid having a simple statement cause a lot of work, but this could cause a simple call F(s...) to copy a large slice, converting each element of that slice.

It's always possible to write a little helper function to do this kind of work.

For those reasons, we aren't going to adopt this proposal. Thanks for sending it.