## 无法实现的is表述(Unreachable is statement)

### 一、简介

SpinalHDL会检查并确保`switch`中的所有`is`语句是可实现的

### 二、例子

下述代码：
```Scala
class TopLevel extends Component {
  val sel = UInt(2 bits)
  val result = UInt(4 bits)
  switch(sel) {
    is(0){ result := 4 }
    is(1){ result := 6 }
    is(2){ result := 8 }
    is(3){ result := 9 }
    is(0){ result := 2 } // 复制 is 语句!
  }
}
```
会报错：
```
UNREACHABLE IS STATEMENT in the switch statement at
  ***
  Source file location of the is statement definition via the stack trace
  ***
```
可修复为：
```Scala
class TopLevel extends Component {
  val sel = UInt(2 bits)
  val result = UInt(4 bits)
  switch(sel) {
    is(0){ result := 4 }
    is(1){ result := 6 }
    is(2){ result := 8 }
    is(3){ result := 9 }
  }
}
```