Don't insert semicolons in conditions unless indented

[odersky]

In a statement like this

if foo
    (bar)
then

we would never expect a semicolon to be inserted between foo and bar. The foo
would need to be indented itself for this to happen.

So, the following is OK

if
  val x = ...
  x > 0
then

But the following is not:

if val x = ...
  x > 0

Fixes #12757

[tgodzik]

Looks good, though I have a question about:

def f =
   if
      x.exists
         (x => x == 10) then
      println("Yes")
   else
      println("No")

Do we ignore the indentation after if if we have a single statement?

[odersky]

@tgodzik I don't think there's a special rule for single statements. Your example would parse like this with indent/unindent tokens added:

def f = <INDENT>
   if <INDENT>
      x.exists
         (x => x == 10) <UNINDENT> then <INDENT>
      println("Yes") <UNINDENT>
   else <INDENT>
      println("No") <UNINDENT> <UNINDENT>

In fact you can test this yourself by uncommenting this line in Parser.scala:

    // in.debugTokenStream = true    // uncomment to see the token stream of the standard scanner, but not syntax highlighting

Notes:

1. The then forces an <UNINDENT> to be inserted since otherwise we cannot continue to parse.
2. The reason an <INDENT> is inserted after the then is that the previous indentation region started at width 3, and the new indentation is at 6, so it is indented more. This holds even though the immediately preceding line is indented more.

[tgodzik]

Ok, I kind of assumed that:

An <outdent> is also inserted if the next token following a statement sequence starting with an <indent> closes an indentation region, i.e. is one of then, else, do, catch, finally, yield, }, ), ] or case.

is also at line break, but that is actually not mentioned in the paragraph. We should be able to fix that.

[tgodzik]

Fixing it here scalameta/scalameta#2384