go/types: trailing semis are ok after valid fallthrough

Fixes #15376.

Change-Id: I9ece80f26b83be129671c961120c157da2ac0079
Reviewed-on: https://go-review.googlesource.com/22270
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/stmt.go

@@ -83,9 +83,19 @@ func (check *Checker) simpleStmt(s ast.Stmt) {
 	}
 }
 
+func trimTrailingEmptyStmts(list []ast.Stmt) []ast.Stmt {
+	for i := len(list); i > 0; i-- {
+		if _, ok := list[i-1].(*ast.EmptyStmt); !ok {
+			return list[:i]
+		}
+	}
+	return nil
+}
+
 func (check *Checker) stmtList(ctxt stmtContext, list []ast.Stmt) {
 	ok := ctxt&fallthroughOk != 0
 	inner := ctxt &^ fallthroughOk
+	list = trimTrailingEmptyStmts(list) // trailing empty statements are "invisible" to fallthrough analysis
 	for i, s := range list {
 		inner := inner
 		if ok && i+1 == len(list) {

src/go/types/testdata/stmt0.src

@@ -531,16 +531,18 @@ func switches1() {
 	case 1:
 		fallthrough
 	case 2:
-	default:
-		fallthrough
+		fallthrough; ; ; // trailing empty statements are ok
 	case 3:
+	default:
+		fallthrough; ;
+	case 4:
 		fallthrough /* ERROR "fallthrough statement out of place" */
 	}
 
 	var y interface{}
 	switch y.(type) {
 	case int:
-		fallthrough /* ERROR "fallthrough statement out of place" */
+		fallthrough /* ERROR "fallthrough statement out of place" */ ; ; ;
 	default:
 	}
 
@@ -554,7 +556,7 @@ func switches1() {
 	switch x {
 	case 0:
 		goto L1
-		L1: fallthrough
+		L1: fallthrough; ;
 	case 1:
 		goto L2
 		goto L3
@@ -576,9 +578,16 @@ func switches1() {
 
 	switch x {
 	case 0:
+		fallthrough; ;
+	case 1:
 		{
 			fallthrough /* ERROR "fallthrough statement out of place" */
 		}
+	case 2:
+		fallthrough
+	case 3:
+		fallthrough /* ERROR "fallthrough statement out of place" */
+		{ /* empty block is not an empty statement */ }; ;
 	default:
 	}
 }