clarify leading infix example

[kitbellew]

started here: 4cc6c06#r138896963

the question is: in this example

foo
???
??? match { case _ => 0 }

why isn't the first instance of ??? considered to be a leading infix operator? it seems to satisfy all conditions mentioned in the documentation:

• symbolic
• starts new line
• not preceded by blank line
• next token is after whitespace
• next token is after minimum indent
• next token can start an expression

[odersky]

I think we want to exclude operator symbols (except prefix operators) from the tokens that can start an expression. The spec should make that clear.

[odersky]

Looking at the code in Scanner confirms this:

        def assumeStartsExpr(lexeme: TokenData) =
          (canStartExprTokens.contains(lexeme.token) || lexeme.token == COLONeol)
          && (!lexeme.isOperator || nme.raw.isUnary(lexeme.name))

[som-snytt]

The backticked identifier is a non-unary operator, so this is rejected:

  val a = 7
  val x = 1
    +
    `a`
    *
    6

  assert(x == 1 + 7 * 6, x)  // was: 1, now: successor(42)

https://github.com/scala/scala/blob/2.13.x/test/files/run/multiLineOps.scala

If accepted, the OP would be foo.???(???) match { }. The following is "huh" not "hibi".

  val y =
    "hi"
    +
    "bye" match { case "bye" => "bi" case _ => "huh" }

Scala 2 tries to closely approximate Scala 3 modulo indentation, but for some reason diverged on this point:

  1
  +
  `a` * 6

is OK but not

  1
  +
  `a` *= 6

where it is trying to determine whether the backticked ident is an operator. Some ancient messaging that is not leading infix aware suggests adding a semicolon.

mlo.scala:10: error: value *= is not a member of Int
possible cause: maybe a semicolon is missing before `value *=`?
  Expression does not convert to assignment because receiver is not assignable.
    `a` *= 6
        ^

It disdains to suggest != etc as seen in Scala 3.

value *= is not a member of Int - did you mean Int.!=? or perhaps Int.<=?

I just spent too many minutes reading my code comment from 2021. For me, that was mid-pandemic and early funemployment, so it was a lot of brain fog ago.

scala/bug#12970