strings: Map does not process invalid UTF-8 sequences correctly

[martisch]

go tip

The new strings.Map implementation introduced in http://golang.org/cl/33201/ does not correctly handle strings with invalid UTF-8 sequences.
This is due to a lack of differentiating between decoding an invalid sequence and the RuneError rune.

https://play.golang.org/p/A-UxFJNWAb

	replaceNotLatin := func(r rune) rune {
		if unicode.Is(unicode.Latin, r) {
			return r
		}
		return '?'
	}
	fmt.Println(strings.Map(replaceNotLatin, "Hello\255World"))

should print "Hello?World" but prints "Hello?rld".

[gopherbot]

CL https://golang.org/cl/37597 mentions this issue.