Missing indent says `not a legal formal parameter for a function literal`

[som-snytt]

Compiler version

3.7

Minimized example

def f =
  Option("hello").map: s =>
  s + ", world"

Output Error/Warning message

-- Error: badfunc.scala:3:2 ----------------------------------------------------
3 |  Option("hello").map: s =>
  |  ^^^^^^^^^^^^^^^^^^^^^^
  |  not a legal formal parameter for a function literal
-- [E006] Not Found Error: badfunc.scala:4:2 -----------------------------------
4 |  s + ", world"
  |  ^
  |  Not found: s
  |
  | longer explanation available when compiling with `-explain`
2 errors found

Why this Error/Warning was not helpful

The "real" error is the missing indent.

The "real" code was a slightly longer expression with another buried error:

    allToolArgs.get(ToolName.Target)
      .map("." + _).or(Some("")).map: platform =>
      JFile(s"${dir.getPath}${platform}.check")

That is, it's not hard for the "real, obvious" error to become obscured.

Suggested improvement

Tell me the "obvious" error. This is not a position for a function literal (perhaps expected type is not a function), so maybe that results in extra help.

Almost any text that patently does not look like a formal parameter may suffer from a missing indent after "colon arrow EOL" (to mean this special syntax for the parameter trailing on the same line as colon).

Maybe it's feasible to re-parse with an indent injected.

I was "amazed" to stumble across:

def f =
  Option("hello").map( s =>
    println(s"appending")
    s + ", world"
  )

which works now because of indentation of the lambda body. (In Scala 2, one is perpetually choosing whether to switch from parens to braces in order to add a line of debug.)

But an unindented expression also works:

def f =
  Option("hello").map( s =>
  s + ", world"
  )

Yet it's not sufficient to omit the parens without adding an indent. (Maybe that is a bug. Edit: as specified, since colon token always requires an indent token.) Currently, that is a bit tricky. Normally, x => expr.