## 赋值覆盖(Assignment Overlap)

### 一、简介

SpinalHDL将会检查, 没有任何的信号赋值会完全抹除前面的赋值。

### 二、例子

下列代码：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits)
  a := 42
  a := 66 // Erase the a := 42 assignment
}
```
将会产生如下报错：
```
ASSIGNMENT OVERLAP completely the previous one of (toplevel/a :  UInt[8 bits])
  ***
  Source file location of the a := 66 assignment via the stack trace
  ***
```
可以修复为：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits)
  a := 42
  when(something) {
    a := 66
  }
}
```
但当某些情况下, 如果用户真的需要去覆盖掉之前的赋值(因为在某些时候覆盖是有意义的), 则可以使用如下表达：
```Scala
class TopLevel extends Component {
  val a = UInt(8 bits)
  a := 42
  a.allowOverride
  a := 66
}
```