Golang reflect: verify/ensure that reflect supports the same string/[]byte/[]rune conversions as the spec/implementation

Follow-up on #23814. See https://go-review.googlesource.com/c/go/+/412095/4/test/fixedbugs/issue23814.go for examples.

Comment From: dsnet

See #24746 where it is observed that reflect lets you do:

type namedByte byte
var b []namedByte
reflect.ValueOf(b).Bytes() // analogous to []byte(b), which is invalid according to the Go spec

Comment From: griesemer

@dsnet That conversion is a slice-to-slice conversion which always required that element types be identical (playground); this seems a bit different from the string->bytes/runes and bytes/runes->string conversions, which are special.

That is, I'd argue that reflect is incorrect in that case.

Comment From: aclements

Bumping to 1.21.

Comment From: griesemer

Too late for 1.21.

Comment From: griesemer

We still need to do this but we've lived with this for several releases, so there's no urgency. Bumping to 1.23.

Comment From: griesemer

Too late for 1.23.