cmd/vet: extra args if any formats are indexed are ok

For example, the following program is valid:

	func main() {
		fmt.Printf("%[1]d", 1, 2, 3)
	}

If any of the formats are indexed, fmt will not complain about unused
extra arguments. See #22867 for more detail.

Make vet follow the same logic, to avoid erroring on programs that would
run without fmt complaining.

Fixes #23564.

Change-Id: Ic9dede5d4c37d1cd4fa24714216944897b5bb7cc
Reviewed-on: https://go-review.googlesource.com/90495
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

@@ -295,6 +295,7 @@ type formatState struct {
 	file         *File
 	call         *ast.CallExpr
 	argNum       int  // Which argument we're expecting to format now.
+	hasIndex     bool // Whether the argument is indexed.
 	indexPending bool // Whether we have an indexed argument that has not resolved.
 	nbytes       int  // number of bytes of the format string consumed.
 }
@@ -319,6 +320,7 @@ func (f *File) checkPrintf(call *ast.CallExpr, name string) {
 	// Hard part: check formats against args.
 	argNum := firstArg
 	maxArgNum := firstArg
+	anyIndex := false
 	for i, w := 0, 0; i < len(format); i += w {
 		w = 1
 		if format[i] != '%' {
@@ -332,6 +334,9 @@ func (f *File) checkPrintf(call *ast.CallExpr, name string) {
 		if !f.okPrintfArg(call, state) { // One error per format is enough.
 			return
 		}
+		if state.hasIndex {
+			anyIndex = true
+		}
 		if len(state.argNums) > 0 {
 			// Continue with the next sequential argument.
 			argNum = state.argNums[len(state.argNums)-1] + 1
@@ -346,6 +351,10 @@ func (f *File) checkPrintf(call *ast.CallExpr, name string) {
 	if call.Ellipsis.IsValid() && maxArgNum >= len(call.Args)-1 {
 		return
 	}
+	// If any formats are indexed, extra arguments are ignored.
+	if anyIndex {
+		return
+	}
 	// There should be no leftover arguments.
 	if maxArgNum != len(call.Args) {
 		expect := maxArgNum - firstArg
@@ -404,6 +413,7 @@ func (s *formatState) parseIndex() bool {
 	arg := int(arg32)
 	arg += s.firstArg - 1 // We want to zero-index the actual arguments.
 	s.argNum = arg
+	s.hasIndex = true
 	s.indexPending = true
 	return true
 }

src/cmd/vet/testdata/print.go

@@ -270,8 +270,9 @@ func PrintfTests() {
 	Printf("%d %[3]d %d %[-2]d x", 1, 2, 3, 4)            // ERROR "Printf format has invalid argument index \[-2\]"
 	Printf("%d %[3]d %d %[2234234234234]d x", 1, 2, 3, 4) // ERROR "Printf format has invalid argument index \[2234234234234\]"
 	Printf("%d %[3]d %-10d %[2]d x", 1, 2, 3)             // ERROR "Printf format %-10d reads arg #4, but call has only 3 args"
-	Printf("%d %[3]d %d %[2]d x", 1, 2, 3, 4, 5)          // ERROR "Printf call needs 4 args but has 5 args"
 	Printf("%[1][3]d x", 1, 2)                            // ERROR "Printf format %\[1\]\[ has unknown verb \["
+	Printf("%[1]d x", 1, 2)                               // OK
+	Printf("%d %[3]d %d %[2]d x", 1, 2, 3, 4, 5)          // OK
 
 	// wrote Println but meant Fprintln
 	Printf("%p\n", os.Stdout)   // OK