go/printer: don't reduce nesting depth twice if parentheses are present around an expr

No impact on formatting on our repos.

Fixes #8021.

LGTM=adonovan
R=adonovan, dvyukov
CC=golang-codereviews
https://golang.org/cl/142020043

src/go/printer/nodes.go

@@ -736,7 +736,7 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		if _, hasParens := x.X.(*ast.ParenExpr); hasParens {
 			// don't print parentheses around an already parenthesized expression
 			// TODO(gri) consider making this more general and incorporate precedence levels
-			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth
+			p.expr0(x.X, depth)
 		} else {
 			p.print(token.LPAREN)
 			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth

src/go/printer/testdata/expressions.golden

@@ -94,6 +94,11 @@ func _() {
 	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
 	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
 
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
 	// the parser does not restrict expressions that may appear as statements
 	true
 	42

src/go/printer/testdata/expressions.input

@@ -95,6 +95,11 @@ func _() {
 	_ = Open(dpath + "/file", O_WRONLY | O_CREAT, 0666)
 	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
 
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1) + (((((1) + (((((((1) * (((((1) + (1))) + (1))))) + (1))) * (1))))) + (1))))]]
+
 	// the parser does not restrict expressions that may appear as statements
 	true
 	42

src/go/printer/testdata/expressions.raw

@@ -94,6 +94,11 @@ func _() {
 	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
 	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
 
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
 	// the parser does not restrict expressions that may appear as statements
 	true
 	42

src/go/printer/testdata/statements.golden

@@ -444,7 +444,7 @@ func _() {
 	}
 	if x == a+b*(T{42}[0]) {
 	}
-	if x == a+(b * (T{42}[0])) {
+	if x == a+(b*(T{42}[0])) {
 	}
 	if x == a+b*(T{42}[0]) {
 	}