隐式参数中断类型推断还是不足以解决它们的解析? [英] Implicit parameters break type inference or inference does not suffice for their resolution?

问题描述

类型推断在此示例中工作正常.类型推断规则(从左到右以及跨参数列表)似乎很满意，但是关于隐式的某些事情打破了它.

Type inference works fine in this example until I add the implicit ordering evidence. Type inference rules (from left to right & across parameter lists) seem to be satisfied, but there is something in regards to the implicit that breaks it.

case class Insert[I, O : Ordering](arg: I)
def execute[I,O](req: Insert[I,O]): O = null.asInstanceOf[O]
val result: Int = execute(Insert("test"))

Error:(5, 39) diverging implicit expansion for type Ordering[O]
starting with method Tuple9 in object Ordering
lazy val result: Int = execute(Insert("test"));}
                                     ^

这可以编译并正常工作:

This compiles and works fine:

case class Insert[I, O](arg: I)
def execute[I,O](req: Insert[I,O]): O = null.asInstanceOf[O]
val result: Int = execute(Insert("test"))

因此，要么类型推断不足以实现隐式解析，要么隐式解析破坏了类型推断.

So either type inference isn't sufficient for implicit resolution OR implicit resolution breaks type inference.

我猜想推断出O类型，但是当发生隐式解析时，它将视为Nothing，换句话说，它将视为我未在val result: Int中指定Int.是虫子吗?

I guess the O type is inferred but when implicit resolution is happening, it sees it as Nothing, in other words, it sees it as if I didn't specified Int in val result: Int. Is it a bug?

推荐答案

此处的问题是scala无法推断O类型，因为它不在Insert

The issue here is that scala cannot infer the O type because it is not present in Insert

// I is in the parameter list but there is no O to be found
case class Insert[I, O](arg: I)(implicit evidence: Ordering[O]) 

这使编译器别无选择，只能推断O为Nothing.创建Insert的实例将无法编译.

This leaves the compiler no choice but to infer O to be Nothing. Creating an instance of Insert will then fail to compile.

scala> val i = Insert(3)
<console>:9: error: diverging implicit expansion for type Ordering[O]
starting with method Tuple9 in object Ordering
   val i = Insert(3)

diverging implicit expansion错误是Scala试图找到适用于这种情况的隐式函数，并陷入了循环之中.这是一条红鲱鱼.

The diverging implicit expansion error is scala attempting to find an implicit that works for this case and getting caught in a cycle. It is a red herring.

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