go/analysis/passes/printf: fix panic parsing argument index

Fixes golang/go#54828

Change-Id: I516dc83230f6bc96b0ff21f3bbae702f1511e5b0
GitHub-Last-Rev: b26f46a
GitHub-Pull-Request: #395
Reviewed-on: https://go-review.googlesource.com/c/tools/+/426875
Auto-Submit: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Tim King <taking@google.com>
Reviewed-by: Tim King <taking@google.com>

go/analysis/passes/printf/printf.go

@@ -672,12 +672,13 @@ func (s *formatState) parseIndex() bool {
 	s.scanNum()
 	ok := true
 	if s.nbytes == len(s.format) || s.nbytes == start || s.format[s.nbytes] != ']' {
-		ok = false
-		s.nbytes = strings.Index(s.format, "]")
+		ok = false // syntax error is either missing "]" or invalid index.
+		s.nbytes = strings.Index(s.format[start:], "]")
 		if s.nbytes < 0 {
 			s.pass.ReportRangef(s.call, "%s format %s is missing closing ]", s.name, s.format)
 			return false
 		}
+		s.nbytes = s.nbytes + start
 	}
 	arg32, err := strconv.ParseInt(s.format[start:s.nbytes], 10, 32)
 	if err != nil || !ok || arg32 <= 0 || arg32 > int64(len(s.call.Args)-s.firstArg) {

go/analysis/passes/printf/testdata/src/a/a.go

@@ -217,6 +217,7 @@ func PrintfTests() {
 	Printf("%[2]*.[1]*[3]d x", 2, "hi", 4) // want `a.Printf format %\[2]\*\.\[1\]\*\[3\]d uses non-int \x22hi\x22 as argument of \*`
 	Printf("%[0]s x", "arg1")              // want `a.Printf format has invalid argument index \[0\]`
 	Printf("%[0]d x", 1)                   // want `a.Printf format has invalid argument index \[0\]`
+	Printf("%[3]*.[2*[1]f", 1, 2, 3)       // want `a.Printf format has invalid argument index \[2\*\[1\]`
 	// Something that satisfies the error interface.
 	var e error
 	fmt.Println(e.Error()) // ok