go/types: better error message for invalid fallthrough case

Now matches the gc compiler.

Fixes #15594.

Change-Id: I9f3942367bc0acf883c6216b8ca44820832f5fe3
Reviewed-on: https://go-review.googlesource.com/27241
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/stmt.go

@@ -68,13 +68,19 @@ func (check *Checker) usage(scope *Scope) {
 }
 
 // stmtContext is a bitset describing which
-// control-flow statements are permissible.
+// control-flow statements are permissible,
+// and provides additional context information
+// for better error messages.
 type stmtContext uint
 
 const (
+	// permissible control-flow statements
 	breakOk stmtContext = 1 << iota
 	continueOk
 	fallthroughOk
+
+	// additional context information
+	finalSwitchCase
 )
 
 func (check *Checker) simpleStmt(s ast.Stmt) {
@@ -292,7 +298,7 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		}(check.scope)
 	}
 
-	inner := ctxt &^ fallthroughOk
+	inner := ctxt &^ (fallthroughOk | finalSwitchCase)
 	switch s := s.(type) {
 	case *ast.BadStmt, *ast.EmptyStmt:
 		// ignore
@@ -454,7 +460,11 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			}
 		case token.FALLTHROUGH:
 			if ctxt&fallthroughOk == 0 {
-				check.error(s.Pos(), "fallthrough statement out of place")
+				msg := "fallthrough statement out of place"
+				if ctxt&finalSwitchCase != 0 {
+					msg = "cannot fallthrough final case in switch"
+				}
+				check.error(s.Pos(), msg)
 			}
 		default:
 			check.invalidAST(s.Pos(), "branch statement: %s", s.Tok)
@@ -523,6 +533,8 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 			inner := inner
 			if i+1 < len(s.Body.List) {
 				inner |= fallthroughOk
+			} else {
+				inner |= finalSwitchCase
 			}
 			check.stmtList(inner, clause.Body)
 			check.closeScope()

src/go/types/testdata/stmt0.src

@@ -536,7 +536,7 @@ func switches1() {
 	default:
 		fallthrough; ;
 	case 4:
-		fallthrough /* ERROR "fallthrough statement out of place" */
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
 	}
 
 	var y interface{}
@@ -573,7 +573,7 @@ func switches1() {
 		goto L6
 		goto L7
 		goto L8
-		L6: L7: L8: fallthrough /* ERROR "fallthrough statement out of place" */
+		L6: L7: L8: fallthrough /* ERROR "cannot fallthrough final case in switch" */
 	}
 
 	switch x {
@@ -589,6 +589,14 @@ func switches1() {
 		fallthrough /* ERROR "fallthrough statement out of place" */
 		{ /* empty block is not an empty statement */ }; ;
 	default:
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	switch x {
+	case 0:
+		{
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
 	}
 }