如何从投影类型推断正确的类型参数? [英] How to infer the right type parameter from a projection type?

问题描述

使用Scala从类型投影中推断正确的类型时，我会遇到一些麻烦.

I have some troubles having Scala to infer the right type from a type projection.

请考虑以下内容:

trait Foo {
  type X
}

trait Bar extends Foo {
  type X = String
}

def baz[F <: Foo](x: F#X): Unit = ???

然后以下代码可以正常编译:

Then the following compiles fine:

val x: Foo#X = ???    
baz(x)

但是以下内容无法编译:

But the following won't compile:

val x: Bar#X = ???    
baz(x)

Scala看到x的底层类型字符串"，但是丢失了x是Bar#X的信息.如果我注释类型，它会很好地工作:

Scala sees the "underlying type String" for x, but has lost the information that x is a Bar#X. It works fine if I annotate the type:

baz[Bar](x)

是否有一种方法可以使Scala推断baz的正确类型参数?
如果没有，什么使它不可能呢?

Is there a way to make Scala infer the right type parameter for baz?
If not, what is the general answer that makes it impossible?

推荐答案

程序通过在上下文中添加此隐式转换来进行编译:

The program compiles by adding this implicit conversion in the context:

implicit def f(x: Bar#X): Foo#X = x

由于这种隐式转换对任何F <: Foo都是正确的，所以我想知道为什么编译器本身不会这样做.

As this implicit conversion is correct for any F <: Foo, I wonder why the compiler does not do that by itself.

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