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

[griesemer]

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

[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

[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.

[aclements]

Bumping to 1.21.

[griesemer]

Too late for 1.21.

[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.

[griesemer]

Too late for 1.23.