go/printer: test that formatted code is parseable

- Added test case for issue 1542.

Fixes #1542.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5645080

src/pkg/go/printer/printer_test.go

@@ -67,6 +67,13 @@ func runcheck(t *testing.T, source, golden string, mode checkMode) {
 	}
 	res := buf.Bytes()
 
+	// formatted source must be valid
+	if _, err := parser.ParseFile(fset, "", res, 0); err != nil {
+		t.Error(err)
+		t.Logf("\n%s", res)
+		return
+	}
+
 	// update golden files if necessary
 	if *update {
 		if err := ioutil.WriteFile(golden, res, 0644); err != nil {

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

@@ -404,7 +404,7 @@ func _() {
 	*/
 }
 
-// Some interesting interspersed comments
+// Some interesting interspersed comments.
 func _( /* this */ x /* is */ /* an */ int) {
 }
 
@@ -428,6 +428,26 @@ func _() {
 	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */ }
 }
 
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2	/*jasldf*/}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}/*jasldf
+	 */
+
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}// jasldf 
+
+	_ = a
+}
+
 // Comments immediately adjacent to punctuation (for which the go/printer
 // may only have estimated position information) must remain after the punctuation.
 func _() {

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

@@ -410,7 +410,7 @@ func _() {
 }
 
 
-// Some interesting interspersed comments
+// Some interesting interspersed comments.
 func _(/* this */x/* is *//* an */ int) {
 }
 
@@ -432,6 +432,26 @@ func _() {
 	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
 }
 
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2, /*jasldf*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, /*jasldf
+						*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, // jasldf 
+	}
+	_ = a
+}
 
 // Comments immediately adjacent to punctuation (for which the go/printer
 // may only have estimated position information) must remain after the punctuation.