Implement a Tasty CompilerInterface checker

[anatoliykmetyuk]

The CompilerInterface implementation has its Term defined to be a Tree:

https://github.com/lampepfl/dotty/blob/b5f2f1831a17efc0bef80742a9b992134dc7061b/compiler/src/dotty/tools/dotc/tastyreflect/ReflectionCompilerInterface.scala#L226

The fact that a Term can be any tree at all makes the interface unsafe: non-terms may potentially be returned in place of terms when matching on trees. Implementing runtime checks is performance-costly – however we can implement such checks under a separate compiler flag and run them from the CI.

One possible implementation would involve a wrapper around the standard compiler interface:

class CheckingCompilerInterface(val ci: CompilerInterface)
  export ci._
  def matchStatement(tree: Tree)(given ctx: Context): Option[Statement] =
    verifyStatement(ci.matchStatement(tree))
  /* ... */

The verifyStatement method above should do the following:

• Call the more specific matchers of all the subtypes of Statement
• Check that exactly one of them returns a Some

See #7503 for an example (test case) of where things go wrong without such checks.

[nicolasstucki]

We started adding assertions within the reflection API directly behind the YCheck flags. This has the advantage of catching errors early. Other checks should be done in normal Ycheck.