spec: several sections require core types and don't mention `[]byte | string`

[dominikh]

https://tip.golang.org/ref/spec#Slice_expressions states that

The core type of a must be a string, array, pointer to array, or slice.

https://tip.golang.org/ref/spec#Appending_and_copying_slices states for append that

The core type of s must be a slice of type []E. The values x are passed to a parameter of type ...E and the respective parameter passing rules apply. As a special case, if the core type of s is []byte, append also accepts a second argument with core type string followed by ....

and for copy that

The core types of both arguments must be slices with identical element type. [...] As a special case, if the destination's core type is []byte, copy also accepts a source argument with core type string.

All three passages require that the types have core types. append and copy do allow for two different core types (string and []byte) for the two arguments, but they don't allow []byte | string for the second argument. The compiler, however, special-cases []byte | string in all these places, as can be seen in the following function, which compiles:

func foo[T []byte | string](x T) {
	var dst []byte
	dst = append(dst, x...)
	_ = x[0:1]
	copy(dst, x)
}

/cc @findleyr

[findleyr]

CC @griesemer

[gopherbot]

Change https://go.dev/cl/412234 mentions this issue: spec: document operations which accept []byte|string constrained types