Skip bypassing unapply for scala 2 case classes to allow for single-element named tuple in unapply

[aherlihy]

Fixes #23131

From my understanding, for Scala 2 case classes, the compiler will generate an unapply that returns an Option[(...)] which contains the fields of the class as a tuple and this first case:

scala3/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

Lines 346 to 356 in 6146b90

	if (isSyntheticScala2Unapply(unapp.symbol) && caseAccessors.length == args.length)
	def tupleSel(sym: Symbol) =
	// If scrutinee is a named tuple, cast to underlying tuple, so that we can
	// continue to select with _1, _2, ...
	ref(scrutinee).ensureConforms(scrutinee.info.stripNamedTuple).select(sym)
	val isGenericTuple = defn.isTupleClass(caseClass) &&
	!defn.isTupleNType(tree.tpe match { case tp: OrType => tp.join case tp => tp }) // widen even hard unions, to see if it's a union of tuples
	val components =
	if isGenericTuple then caseAccessors.indices.toList.map(tupleApp(_, ref(scrutinee)))
	else caseAccessors.map(tupleSel)
	matchArgsPlan(components, args, onSuccess)

is to bypass this unapply and directly select the case class parameters for performance. However that causes the fixes added by this PR: #22953 to be skipped since the check comes first. I think the simplest fix is to skip the Scala 2 special case for a single-element selector, but not 100% sure.

[smarter]

I think this is fine as a workaround to avoid the runtime crash now, but it really needs a FIXME comment explaining the context and calling for a refactor of the code.

[aherlihy]

@smarter I added a more precise check but I'm not really sure this is the best way to do it

[smarter]

I think this is fine, since .select(nme.get, _.info.isParameterless) appears both here and below in val get = ..., it could be factored out in a method getOfGetMatch or something like that.

[aherlihy]

Sure, I can add a method for that.