Vararg Patterns

The syntax of vararg patterns has changed. In the new syntax one writes varargs in patterns exactly like one writes them in expressions, using a : _* type annotation:

xs match {
  case List(1, 2, xs: _*) => println(xs)    // binds xs
  case List(1, _ : _*) =>                   // wildcard pattern
}

The old syntax, which is shorter but less regular, is no longer supported.

/*!*/ case List(1, 2, xs @ _*)       // syntax error
/*!*/ case List(1, 2, _*) => ...     // syntax error

The change to the grammar is:

 SimplePattern ::=  ‘_’
                 |  varid
                 |  Literal
                 |  StableId
                 |  StableId ‘(’ [Patterns ‘)’
-                |  StableId ‘(’ [Patterns ‘,’] [varid ‘@’] ‘_’ ‘*’ ‘)’
+                |  StableId ‘(’ [Patterns ‘,’] (varid | ‘_’) ‘:’ ‘_’ ‘*’ ‘)’
                 |  ‘(’ [Patterns] ‘)’
                 |  XmlPattern

Compatibility considerations

Under the -language:Scala2Compat option, Dotty will accept both the old and the new syntax. A migration warning will be emitted when the old syntax is encountered.