Fix #8357: add test

tests/pos/i8357.scala

@@ -0,0 +1,30 @@
+trait FieldInfo {
+  val fieldType: ALL_TYPE
+}
+
+opaque type TypeSymbol = String
+
+trait ConcreteType {
+  val name: String
+  val typeParameters: List[TypeSymbol]
+  val isUnion: Boolean = false
+}
+
+trait UnionContainer {
+  val hasUnion: Boolean
+}
+
+type ALL_TYPE = ConcreteType | TypeSymbol
+
+case class StaticUnionInfo(name: String, typeParameters: List[TypeSymbol]) extends ConcreteType 
+case class AliasInfo(name: String, typeParameters: List[TypeSymbol]) extends ConcreteType
+
+object UnionKind {
+  def unapply(f: FieldInfo): Boolean = 
+    f.fieldType match {
+      case _: StaticUnionInfo => true
+      case t: AliasInfo if t.isUnion =>  true
+      case t: UnionContainer if t.hasUnion => true  // this line here causes crash
+      case _ => false
+    }
+}