cmd/cover: fix handling of empty type switch

Just missed a case (ha!) in the tree walk. Dup the code for an empty switch, add test.

Fixes #10163.

Change-Id: I3d50ab6cb450ca21e87213291eaab8cbe924fac5
Reviewed-on: https://go-review.googlesource.com/7641
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/cover/cover.go

@@ -220,6 +220,11 @@ func (f *File) Visit(node ast.Node) ast.Visitor {
 		if n.Body == nil || len(n.Body.List) == 0 {
 			return nil
 		}
+	case *ast.TypeSwitchStmt:
+		// Don't annotate an empty type switch - creates a syntax error.
+		if n.Body == nil || len(n.Body.List) == 0 {
+			return nil
+		}
 	}
 	return f
 }

cmd/cover/testdata/test.go

@@ -22,6 +22,7 @@ func testAll() {
 	testTypeSwitch()
 	testSelect1()
 	testSelect2()
+	testEmptySwitches()
 }
 
 func testSimple() {
@@ -175,3 +176,20 @@ func testSelect2() {
 		}
 	}
 }
+
+// Empty control statements created syntax errors. This function
+// is here just to be sure that those are handled correctly now.
+func testEmptySwitches() {
+	check(LINE, 1)
+	switch 3 {
+	}
+	check(LINE, 1)
+	switch i := (interface{})(3).(int); i {
+	}
+	check(LINE, 1)
+	go func() {
+		check(LINE, 1)
+		select {}
+	}()
+	check(LINE, 1)
+}