go/printer, gofmt: print "select {}" on one line

No impact on existing sources under src, misc.

Fixes #1959.

R=r, rsc, r
CC=golang-dev
https://golang.org/cl/4589056

src/pkg/go/printer/nodes.go

@@ -1158,8 +1158,14 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool, multiLine *bool) {
 
 	case *ast.SelectStmt:
 		p.print(token.SELECT, blank)
-		p.block(s.Body, 0)
-		*multiLine = true
+		body := s.Body
+		if len(body.List) == 0 && !p.commentBefore(p.fset.Position(body.Rbrace)) {
+			// print empty select statement w/o comments on one line
+			p.print(body.Lbrace, token.LBRACE, body.Rbrace, token.RBRACE)
+		} else {
+			p.block(body, 0)
+			*multiLine = true
+		}
 
 	case *ast.ForStmt:
 		p.print(token.FOR)

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

@@ -111,6 +111,21 @@ func _() {
 }
 
 
+// Formatting of selected select statements.
+func _() {
+	select {}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */
+	}
+	select {	/* this comment should be tab-aligned */
+	}
+	select {	// this comment should be tab-aligned
+	}
+	select {
+	case <-c:
+	}
+}
+
+
 // Formatting of for-statement headers.
 func _() {
 	for {

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

@@ -91,6 +91,19 @@ func _() {
 }
 
 
+// Formatting of selected select statements.
+func _() {
+	select {
+	}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */ }
+	select { /* this comment should be tab-aligned */
+	}
+	select { // this comment should be tab-aligned
+	}
+	select { case <-c: }
+}
+
+
 // Formatting of for-statement headers.
 func _() {
 	for{}