自引用类型和有界通配符之间的不良交互 [英] A bad interaction between self-referential types and bounded wildcards

问题描述

这个案例似乎是Eclipse的Java编译器压缩javac的另一个案例。对我来说唯一的问题是它是否是JLS或javac中的错误。

  interface EndoFunctor< C，FC扩展EndoFunctor< C，FC> > {/*...*/} 
接口代数< C，FC扩展EndoFunctor< ？扩展C，FC> > {/*...*/} 
  

第二行在Eclipse中编译，但无法编译javac，并提示类型参数FC不在其范围内。

声明FC扩展EndoFunctor< ？扩展C，FC>，并且FC上的边界是它扩展了EndoFunctor< D，FC>为推断D，在这种情况下是？扩展C.我认为javac没有意识到通配符在两个上下文中表示相同的未知类型。 Eclipse不会，但！！

显然，以下解决了javac中的问题：

  interface EndoFunctor< C，FC扩展EndoFunctor< ？扩展C，FC> > {/*...*/} 
  

但是这是一个比我想要的接口更宽松的定义。

我也可以尝试

  interface Algebra< C，D扩展C，FC扩展EndoFunctor< D，FC> > 
  

但是这种方法迫使我把这个额外的类型参数D带到任何地方。

该怎么办？

解决方案

该怎么办？ / b>

以下是一些实用的解决方案。

<尝试使用最新的Java 7修补程序版本中的 javac 。我记得在Java 6中只有Java 7中修复了某些javac编译器错误。但我不知道列表。 （并且Java Bugs Database在搜索时没有希望）


• 使用它，并使用您已经找到的两个替代方案之一即工作。

This case seems to be another one where Eclipse's Java compiler crushes javac. The only question for me is whether it's a bug in JLS or javac.

interface EndoFunctor< C, FC extends EndoFunctor< C, FC > > { /*...*/ }
interface Algebra< C, FC extends EndoFunctor< ? extends C, FC > > { /*...*/ }

The second line compiles in Eclipse, but fails to compile in javac with the message that "type parameter FC is not within its bound".

FC is declared to extend EndoFunctor< ? extends C, FC >, and the bound on FC is that it extend EndoFunctor< D, FC > for the inferred D, which in this case is ? extends C. I think javac doesn't realize that the wildcard represents the same unknown type in both contexts. Eclipse does, though!

Apparently the following gets around the problem in javac:

interface EndoFunctor< C, FC extends EndoFunctor< ? extends C, FC > > { /*...*/ }

but this is a looser definition than I want for that interface.

I could also try

interface Algebra< C, D extends C, FC extends EndoFunctor< D, FC > >

but that approach forces me to carry that extra type parameter D through everywhere.

What to do?

解决方案

What to do?

Here are a couple of pragmatic solutions.

• Try using javac from the latest patch release of Java 7. I recall hearing of certain javac compiler bugs in Java 6 that were only fixed in Java 7 ... but I don't know of a list. (And the Java Bugs Database is hopeless at searching ...)

• Put up with it, and use one of the two alternatives that you've already found that "work".

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