go/printer, gofmt: don't print unneeded parentheses around parameter types

Fixes #4624.

R=rsc
CC=golang-dev
https://golang.org/cl/7058052

src/pkg/go/printer/nodes.go

@@ -307,7 +307,7 @@ func (p *printer) parameters(fields *ast.FieldList) {
 				p.print(blank)
 			}
 			// parameter type
-			p.expr(par.Type)
+			p.expr(stripParensAlways(par.Type))
 			prevLine = parLineEnd
 		}
 		// if the closing ")" is on a separate line from the last parameter,
@@ -336,7 +336,7 @@ func (p *printer) signature(params, result *ast.FieldList) {
 		p.print(blank)
 		if n == 1 && result.List[0].Names == nil {
 			// single anonymous result; no ()'s
-			p.expr(result.List[0].Type)
+			p.expr(stripParensAlways(result.List[0].Type))
 			return
 		}
 		p.parameters(result)
@@ -959,6 +959,13 @@ func stripParens(x ast.Expr) ast.Expr {
 	return x
 }
 
+func stripParensAlways(x ast.Expr) ast.Expr {
+	if x, ok := x.(*ast.ParenExpr); ok {
+		return stripParensAlways(x.X)
+	}
+	return x
+}
+
 func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, post ast.Stmt) {
 	p.print(blank)
 	needsBlank := false

src/pkg/go/printer/printer.go

@@ -1230,7 +1230,7 @@ func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{
 	}
 
 	// flush tabwriter, if any
-	if tw, _ := (output).(*tabwriter.Writer); tw != nil {
+	if tw, _ := output.(*tabwriter.Writer); tw != nil {
 		err = tw.Flush()
 	}
 

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

@@ -887,3 +887,28 @@ type _ interface {
 		r string,
 		x ...int)
 }
+
+// omit superfluous parentheses in parameter lists
+func _(int)
+func _(int)
+func _(x int)
+func _(x int)
+func _(x, y int)
+func _(x, y int)
+
+func _() int
+func _() int
+func _() int
+
+func _() (x int)
+func _() (x int)
+func _() (x int)
+
+// special cases: some channel types require parentheses
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))

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

@@ -896,3 +896,28 @@ p, q,
 r string,
 		x ...int)
 }
+
+// omit superfluous parentheses in parameter lists
+func _((int))
+func _((((((int))))))
+func _(x (int))
+func _(x (((((int))))))
+func _(x, y (int))
+func _(x, y (((((int))))))
+
+func _() (int)
+func _() ((int))
+func _() ((((((int))))))
+
+func _() (x int)
+func _() (x (int))
+func _() (x (((((int))))))
+
+// special cases: some channel types require parentheses
+func _(x chan(<-chan int))
+func _(x (chan(<-chan int)))
+func _(x ((((chan(<-chan int))))))
+
+func _(x chan<-(chan int))
+func _(x (chan<-(chan int)))
+func _(x ((((chan<-(chan int))))))