cmd/api: fix signatures like func(x, y, z int)

Fixes writing of function parameter, result lists which
consist of multiple named or unnamed items with same type.

Fixes #4011.

R=golang-dev, bsiegert, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6475062

src/cmd/api/goapi.go

@@ -1017,18 +1017,38 @@ func (w *Walker) walkFuncDecl(f *ast.FuncDecl) {
 
 func (w *Walker) funcSigString(ft *ast.FuncType) string {
 	var b bytes.Buffer
+	writeField := func(b *bytes.Buffer, f *ast.Field) {
+		if n := len(f.Names); n > 1 {
+			for i := 0; i < n; i++ {
+				if i > 0 {
+					b.WriteString(", ")
+				}
+				b.WriteString(w.nodeString(w.namelessType(f.Type)))
+			}
+		} else {
+			b.WriteString(w.nodeString(w.namelessType(f.Type)))
+		}
+	}
 	b.WriteByte('(')
 	if ft.Params != nil {
 		for i, f := range ft.Params.List {
 			if i > 0 {
 				b.WriteString(", ")
 			}
-			b.WriteString(w.nodeString(w.namelessType(f.Type)))
+			writeField(&b, f)
 		}
 	}
 	b.WriteByte(')')
 	if ft.Results != nil {
-		if nr := len(ft.Results.List); nr > 0 {
+		nr := 0
+		for _, f := range ft.Results.List {
+			if n := len(f.Names); n > 1 {
+				nr += n
+			} else {
+				nr++
+			}
+		}
+		if nr > 0 {
 			b.WriteByte(' ')
 			if nr > 1 {
 				b.WriteByte('(')
@@ -1037,7 +1057,7 @@ func (w *Walker) funcSigString(ft *ast.FuncType) string {
 				if i > 0 {
 					b.WriteString(", ")
 				}
-				b.WriteString(w.nodeString(w.namelessType(f.Type)))
+				writeField(&b, f)
 			}
 			if nr > 1 {
 				b.WriteByte(')')

src/cmd/api/testdata/src/pkg/p3/golden.txt

+pkg p3, method (*ThirdBase) GoodPlayer() (int, int, int)
+pkg p3, func BadHop(int, int, int) (bool, bool, *ThirdBase, *ThirdBase, error)
+pkg p3, type ThirdBase struct

src/cmd/api/testdata/src/pkg/p3/p3.go

+package p3
+
+type ThirdBase struct{}
+
+func (tb *ThirdBase) GoodPlayer() (i, j, k int)
+func BadHop(i, j, k int) (l, m bool, n, o *ThirdBase, err error)