cmd/compile: should accept converting slices of named byte types to strings

[josharian]

package p

type B byte

func f() {
	var x []B
	_ = string(x)
}

go/types accepts this code. Go 1.12, 1.13, and 1.14beta1 all reject it with:

x.go:7:12: cannot use x (type []B) as type []byte in argument to runtime.slicebytetostring

Noticed while looking at #36890.

Easy fix, will plan to do for 1.15.

[networkimprov]

Also needs fix for rune. Playground said:

cannot use r (type []R) as type []rune in argument to runtime.slicerunetostring

[josharian]

Actually, go/types may be wrong here. From the spec, on conversions:

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.
x is a string and T is a slice of bytes or runes.

It doesn’t say that T is a slice whose elements have underlying type byte or rune.

cc @griesemer for spec clarification

cc @ianlancetaylor for gccgo

[dotaheor]

dup of #23536?

[josharian]

Yes, it appears so. Unfortunately it doesn’t appear that that issue was brought to a conclusion.