compiler: allow to use switch without tag

pkg/compiler/codegen.go

@@ -578,9 +578,13 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.SwitchStmt:
+		eqOpcode := opcode.EQUAL
+		if n.Tag != nil {
 			ast.Walk(c, n.Tag)
-
-		eqOpcode, _ := convertToken(token.EQL, c.typeOf(n.Tag))
+			eqOpcode, _ = convertToken(token.EQL, c.typeOf(n.Tag))
+		} else {
+			emit.Bool(c.prog.BinWriter, true)
+		}
 		switchEnd, label := c.generateLabel(labelEnd)
 
 		lastSwitch := c.currentSwitch

pkg/compiler/switch_test.go

@@ -18,6 +18,21 @@ var switchTestCases = []testCase{
 		}`,
 		big.NewInt(2),
 	},
+	{
+		"switch with no tag",
+		`package main
+		func f() bool { return false }
+		func Main() int {
+			switch {
+			case f():
+				return 1
+			case true:
+				return 2
+			}
+			return 3
+		}`,
+		big.NewInt(2),
+	},
 	{
 		"simple switch fail",
 		`package main