JLine 3: parse errors should be suppressed during JLine-initiated tokenizing

[SethTisue]

for example, when calling a method on the last REPL result, it works but a spurious error is printed, also

not sure how many people even know this feature exists, but:

scala 2.13.1> List(9)
res2: List[Int] = List(9)

scala 2.13.1> .length
res3: Int = 1

whereas on JLine 3 currently:

scala> List(9)
val res1: List[Int] = List(9)

scala> .length       ^
       error: illegal start of definition

val res2: Int = 1

funny that you get the right answer anyway.

the special handling for this is in ReplCompletion#complete

there should really be test coverage.

[SethTisue]

there should really be test coverage

I added a test, but it passes, it doesn't show the failing behavior, so apparently that depends somehow on JLine being in the picture

the special handling for this is in ReplCompletion#complete

well, yes, the handling for tab completion in this style of method invocation is there, but tab completion isn't the problem, completion works fine, it's when the code is compiled/run that the problem comes

[SethTisue]

it's when the code is compiled/run that the problem comes

the error is being emitted when JLine calls back to our code, ScalaParser#parse which fails to suppress errors coming from nsc's parser.

I would expect this would cause other kinds of incorrect code to result in spurious error output, so for example this interaction:

scala 2.13.2-bin-cd56924> )                          ^
                          error: illegal start of definition

                          ^
                          error: illegal start of definition

I'm pretty certain the double-printing of the error has the same underlying cause. I will broaden the issue title

[SethTisue]

I added a commit with a one-line fix to scala/scala#8848

closing on the assumption that PR will be merged.