Deprecate infix type args, as they are dropped in Scala 3

[som-snytt]

Type arguments for infix expressions were added but never included in the specification and never received any support in Scala 3. They are now deprecated in Scala 2 and error under -Xsource:3.

For example, the 4th expression below is deprecated. In general, Scala 3 encourages only symbolic operators as infix operators, and never allows type arguments.

scala> xs.map(_ + 1)
val res1: List[Int] = List(43)

scala> xs map (_ + 1)
val res2: List[Int] = List(43)

scala> xs.map[Int](_ + 1)
val res3: List[Int] = List(43)

scala> xs map[Int] (_ + 1)
val res4: List[Int] = List(43)

[lrytz]

👍 thank you!

[SethTisue]

The mailing list discussion linked from the original PR

link?

[som-snytt]

a1c00ae

The commit message also shows the infix expression in the style of the era.

Today, infix prefers operators, especially for leading infix syntax (which requires backquoted alnum identifiers).

[SethTisue]

@som-snytt would you mind updating the PR description to tell readers, who might be fairly casual users of Scala and haven't followed this stuff, what the change is, in a self-contained way? with an example of what "infix type args" even are?

[som-snytt]

preserving the OP

Under -Xsource:3, error.

The feature was adopted during the dotless craze of 2013, but never taken up by dotty for obvious reasons (naming).

The mailing list discussion linked from the original PR shows only partial consensus.