go/printer, gofmt: avoid extra final comma in multi-line signatures

The parameter list layout function was incorrectly computing the
end of the previous line in cases where a parameter type spanned
multiple lines. As a result, an extra (valid, but not needed)
comma was introduced before the paremeter list's closing parenthesis.

Fixes #4533.

R=rsc
CC=golang-dev
https://golang.org/cl/7674044

src/pkg/go/printer/nodes.go

@@ -271,12 +271,12 @@ func (p *printer) parameters(fields *ast.FieldList) {
 			// if there are multiple parameter names for this par
 			// or the type is on a separate line)
 			var parLineBeg int
-			var parLineEnd = p.lineFor(par.Type.Pos())
 			if len(par.Names) > 0 {
 				parLineBeg = p.lineFor(par.Names[0].Pos())
 			} else {
-				parLineBeg = parLineEnd
+				parLineBeg = p.lineFor(par.Type.Pos())
 			}
+			var parLineEnd = p.lineFor(par.Type.End())
 			// separating "," if needed
 			needsLinebreak := 0 < prevLine && prevLine < parLineBeg
 			if i > 0 {

src/pkg/go/printer/testdata/declarations.golden

@@ -912,3 +912,28 @@ func _(x chan (<-chan int))
 func _(x chan<- (chan int))
 func _(x chan<- (chan int))
 func _(x chan<- (chan int))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+})	// no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x	int
+	y	int
+})	// no extra comma between } and )

src/pkg/go/printer/testdata/declarations.input

@@ -921,3 +921,28 @@ func _(x ((((chan(<-chan int))))))
 func _(x chan<-(chan int))
 func _(x (chan<-(chan int)))
 func _(x ((((chan<-(chan int))))))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+}) // no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x int
+	y int
+}) // no extra comma between } and )