当名义类需要访问修改时，无法使用交集类型

问题描述

接口：

interface PublicCloneable {
    Object clone();
}

interface HasPosition {
    // doesn't matter
}

尝试使用交叉点类型：

@SuppressWarnings("unchecked")
<E extends PublicCloneable & HasPosition> E cloneAndIncrementPosition(E elem) {
    final E clone = (E)elem.clone();
    // rest omitted
}

尝试使用<$ c $进行编译c> javac 1.8.0_60 ：

$ javac xx.java
xx.java:13: error: clone() in Object cannot implement clone() in PublicCloneable
    <E extends PublicCloneable & HasPosition> E cloneAndIncrementPosition(E elem) {
     ^
  attempting to assign weaker access privileges; was public
xx.java:14: error: clone() has protected access in Object
        final E clone = (E)elem.clone();
                               ^
2 errors

为什么这个交集类型对javac无效？

Why this intersection type is invalid for javac?

推荐答案

这看起来像是一个javac bug。

This looks like a javac bug.

如果Ck是Object，则会产生一个名义界面...直接超接口T1'，...，Tn'

If Ck is Object, a notional interface is induced ... has direct superinterfaces T1', ..., Tn'

因此，对于 PublicCloneable& HasPosition ，引入了一个名义界面，扩展了它们，这应该没问题。

Therefore, for PublicCloneable & HasPosition, a notional interface is introduced, extending both of them, which should be OK.

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