Make `opaque` a soft keyword?

[liufengyun]

Fastparse has a method opaque, which is used in other projects, like Scalapb. After #5300 , the benchmark test for ScalaPB breaks. Can we make opaque a soft keyword like inline?

https://github.com/lihaoyi/fastparse/blob/a36e2cf1b82be967403a23cde3a813e5d4d3f741/fastparse/src/fastparse/package.scala#L337

https://github.com/liufengyun/ScalaPB/blob/a561d5cd107ba4ef8c3a32f57719a2429970b692/scalapb-runtime/shared/src/main/scala/com/trueaccord/scalapb/textformat/Basics.scala#L36

[allanrenucci]

I believe we should first fix "soft keyword", otherwise it is not very helpful. See #5145

[liufengyun]

@allanrenucci I think the example in #5145 is a corner case. The current implementation already helps a lot, it covers most (if not all) practical usage.

class Test {
  def foo(x: Int): Int = {
    val inline = 3
    5 + inline         // ok
    inline + 5         // ok
    // inline            // error
  }
}

[allanrenucci]

I disagree. If it does not cover all cases, I think we should disallow it. It will be very confusing for users to have a small subset of keywords that can be used as an identifier but only if it does not start a statement.

I can imagine people using the name opaque in an API without knowing about this corner case.

Also, at the time we discussed #5145, @odersky said it would be fairly easy to fix. Not sure why he closed the issue. Let's bring this up in the meeting

[allanrenucci]

Fixed in #5471