go/printer: parenthesize type in <-((<-chan int)(nil))

When printing synthetic syntax (not created by the parser),
the tree for <-((<-chan int)(nil)) without any ParenExpr nodes
was misprinted so that it was parsed back as a receive of
a receive. This changes emits parens around the channel type.

Fixes #63362

Change-Id: I2041ced224f0bca001cee5d37f7a127265d21020
Reviewed-on: https://go-review.googlesource.com/c/go/+/532556
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/printer/nodes.go

@@ -974,15 +974,24 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		if len(x.Args) > 1 {
 			depth++
 		}
-		var wasIndented bool
-		if _, ok := x.Fun.(*ast.FuncType); ok {
-			// conversions to literal function types require parentheses around the type
-			p.print(token.LPAREN)
-			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
-			p.print(token.RPAREN)
-		} else {
-			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+
+		// Conversions to literal function types or <-chan
+		// types require parentheses around the type.
+		paren := false
+		switch t := x.Fun.(type) {
+		case *ast.FuncType:
+			paren = true
+		case *ast.ChanType:
+			paren = t.Dir == ast.RECV
 		}
+		if paren {
+			p.print(token.LPAREN)
+		}
+		wasIndented := p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+		if paren {
+			p.print(token.RPAREN)
+		}
+
 		p.setPos(x.Lparen)
 		p.print(token.LPAREN)
 		if x.Ellipsis.IsValid() {

src/go/printer/printer_test.go

@@ -604,6 +604,29 @@ func f()
 	}
 }
 
+// TestChanType tests that the tree for <-(<-chan int), without
+// ParenExpr, is correctly formatted with parens.
+// Test case for issue #63362.
+func TestChanType(t *testing.T) {
+	expr := &ast.UnaryExpr{
+		Op: token.ARROW,
+		X: &ast.CallExpr{
+			Fun: &ast.ChanType{
+				Dir:   ast.RECV,
+				Value: &ast.Ident{Name: "int"},
+			},
+			Args: []ast.Expr{&ast.Ident{Name: "nil"}},
+		},
+	}
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, expr); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := buf.String(), `<-(<-chan int)(nil)`; got != want {
+		t.Fatalf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
 type limitWriter struct {
 	remaining int
 	errCount  int