已删除：自动应用

以前在不提供参数调用零元方法时，一个空的参数列表 () 会被隐式插入。例如：

def next(): T = ...
next     // is expanded to next()

在 Scala 3 中，这个惯用法是错误的。

next
^
missing arguments for method next

在 Scala 3 中，应用语法必须完全遵循参数语法。在 Java 中定义的方法， 以及正在重写 Java 中定义的方法的方法是例外。对这种情况宽容的原因是， 如果没有例外，则所有人都要用这种写法

xs.toString().length()

替代

xs.toString.length

后者是符合 Scala 惯用法的，因为它符合统一访问原则。 这一原则指出，应该能够将对象成员从字段修改为无副作用的方法， 也能够再修改回来，而不影响访问该成员的客户端。 因此，Scala 鼓励定义没有 () 参数列表的“属性”方法， 而带有副作用的方法应该带有 () 参数列表。 在 Java 中定义的方法不能做出这种区分；对于它们来说， () 总是必须的。因此 Scala 通过允许无参数引用在用户侧解决了这个问题。 But where Scala allows that freedom for all method references, Scala 3 restricts it to references of external methods that are not defined themselves in Scala 3.

For reasons of backwards compatibility, Scala 3 for the moment also auto-inserts () for nullary methods that are defined in Scala 2, or that override a method defined in Scala 2. It turns out that, because the correspondence between definition and call was not enforced in Scala so far, there are quite a few method definitions in Scala 2 libraries that use () in an inconsistent way. For instance, we find in scala.math.Numeric

def toInt(): Int

whereas toInt is written without parameters everywhere else. Enforcing strict parameter correspondence for references to such methods would project the inconsistencies to client code, which is undesirable. So Scala 3 opts for more leniency when type-checking references to such methods until most core libraries in Scala 2 have been cleaned up.

Stricter conformance rules also apply to overriding of nullary methods. It is no longer allowed to override a parameterless method by a nullary method or vice versa. Instead, both methods must agree exactly in their parameter lists.

class A {
   def next(): Int
}

class B extends A {
   def next: Int // overriding error: incompatible type
}

Methods overriding Java or Scala 2 methods are again exempted from this requirement.

Migrating code

Existing Scala code with inconsistent parameters can still be compiled in Scala 3 under -source 3.0-migration. When paired with the -rewrite option, the code will be automatically rewritten to conform to Scala 3’s stricter checking.

Reference

For more information, see Issue #2570 and PR #2716.