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POScala type inference with generics and covariance, possible issue in scalac
 

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  1. POScala type inference with generics and covariance, possible issue in scalac
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    <p>I have a short list of questions that I've found today. I'm a scala novice, therefore my questions may be trivial.</p> <p>Let's assume we have a class like this one:</p> <pre><code>abstract class A[+T] { def foo[S &gt;: T](x: S): String } </code></pre> <p>And we should provide useful child class with some functionality.</p> <p>1) My first attempt looks like this:</p> <pre><code>class B extends A[String] { def foo(x: String) = x } </code></pre> <p>but scalac compiler disagrees with message:</p> <pre><code>xxx@yyy:~$ scalac covariant.scala covariant.scala:3: error: class B needs to be abstract, since method foo in class A of type [S &gt;: String](x: S)String is not defined class B extends A[String] { def foo(x: String) = x } ^ </code></pre> <p>First of all, why scalac didn't infer foo's generic type parameter, is it really a complex task?</p> <p>2) Next try looks better and should be accepted, as I think:</p> <pre><code>class B extends A[String] { def foo[String](x: String) = x } </code></pre> <p>but now compiler made my eyes wide open:</p> <pre><code>covariant.scala:3: error: type mismatch; found : String(in method foo) required: java.lang.String class B extends A[String] { def foo[String](x: String) = x } ^ </code></pre> <p>It looks like an issue with that mismatch of String and java.lang.String. This is a second question: Is it really a bug?</p> <p>3) Finally I've decided to switch String type to Date as parameter:</p> <pre><code>import java.util.Date class B extends A[Date] { def foo[Date](x: Date) = x.toString } </code></pre> <p>It have been compiled without any warnings. So last question is: Why? What is the difference between my second and third snippets?</p> <p>By the way, scalac version is <code>2.9.1.final</code></p>
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