cc @griesemer

Pet the current spec rules on generic conversions, this conversion is not permitted.

Note that the following code does work:

func toString[T []byte|[]rune](slice T) string {
        return string(slice)
}

Maybe the rules need to be relaxed.
@ianlancetaylor for additional input.

Thanks!

As motivation for the rule relaxation, I'll present my use-case.

I have a generic type Iterator[T] that represents a Python-like iterator (it can be any other collection that's generic over the element type), and I want to convert it to a string.

Due to the current rules, I cannot just write the following:

func ToString[T rune | uint8](iter Iterator[T]) string {
	return string(ToSlice(iter))
}

Trying to work around it doesn't work either:

func sliceToString[T []rune | []uint8](slice T) string {
	return string(slice)
}

func ToString[T rune | uint8](iter Iterator[T]) string {
	return sliceToString(ToSlice(iter))
}

And fails with

[]T does not implement []rune|[]uint8

Which is hard for me to understand as T is either rune or uint8, making []T effectively []rune|[]uint8.

This means that I'll have to write 2 toString functions. One for rune and one for uint8.
It's not that bad in that one case, but I think it would also mean that code that is generic over string-elements would either need to be written twice (once for uint8 and once for rune), or that it will need non-generic free-functions for converting back to string.

Thanks. We're not going to change this for 1.18. In the long run I think we should probably permit this, but let's leave real consideration for later. Thanks.

Per the current spec rules on generic conversions, this conversion is not permitted.

@griesemer Could you more specific on which line in the current spec disallows this conversion? I only found this line

A non-constant value x can be converted to type T in any of these cases:

• x is an integer or a slice of bytes or runes and T is a string type.

The line doesn't mention type parameters.

In your example slice is not a slice of bytes or runes, it is a slice of T, which is a type parameter. The rule you mention ("x is an integer or a slice of bytes or runes and T is a string type") doesn't apply. I'm being pedantic, but that is what the spec is: pedantic.

Is this the same problem?

type Bytes []byte

func hos[T Bytes](x []byte) T {
	return x // okay
}

func lay[T []byte](x Bytes) T {
	return x // error
}

func rez[T []byte](x Bytes) T {
	return T(x) // okay
}

[edit]: It is not the same problem. It is an assignment rule problem,