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：

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：

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 }
  }
}