Improve import selector parsing

[som-snytt]

Scala 2 received upgraded parsing of import selectors, including handling underscore, at scala/scala#6218. There may be something to forward-port.

scala> import collection._ => `not a rename, just dross`                                   
1 |import collection._ => `not a rename, just dross`
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^
  |                       ';' expected, but identifier found

scala> import collection._ => ;

scala> 

Comparing Scala 2:

scala> import collection._ => `not a rename, just dross`
                           ^
       error: ';' expected but '=>' found.

The bad syntax was ignored completely until 2.13.

Edit: the Scala 2 change addresses some but not all issues around use of underscore, and the parsing improvement was just fall-out.

[odersky]

scalac 's behavior is not backed by the language spec. I think if we want to change this it has to go through the SIP process, and I for one would vote against the change. No need to get more syntactic options for features that occur rarely anyway.

[som-snytt]

The parser incorrectly accepts

scala> import collection._ => ;

Also worth mentioning that the linked scalac issue is per spec: backquoted underscore is an identifier, but unadorned underscore is a reserved word.

[som-snytt]

This progressed, probably when export was added.

➜  dotty ./dotty-0.12.0/bin/dotr     
Starting dotty REPL...
scala> import collection._ => ;                                                                                      

scala>                                                                                                               
➜  dotty ./dotty-0.13.0/bin/dotr
Starting dotty REPL...
scala> import collection._ => ;                                                                                      

scala>                                                                                                               
➜  dotty ./dotty-0.14.0/bin/dotr
Starting dotty REPL...
scala> import collection._ => ;                                                                                      
1 |import collection._ => ;
  |                    ^^
  |                    end of statement expected

scala>