ConeFlexibleType: statically require to have simple bounds

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt

@@ -128,16 +128,10 @@ abstract class ConeClassLikeType : ConeLookupTagBasedType() {
 }
 
 open class ConeFlexibleType(
-    val lowerBound: ConeKotlinType,
-    val upperBound: ConeKotlinType
+    val lowerBound: ConeSimpleKotlinType,
+    val upperBound: ConeSimpleKotlinType
 ) : ConeKotlinType(), FlexibleTypeMarker {
 
-    init {
-        val message = { "Bounds violation: $lowerBound, $upperBound" }
-        require(lowerBound is SimpleTypeMarker, message)
-        require(upperBound is SimpleTypeMarker, message)
-    }
-
     override val typeArguments: Array<out ConeTypeProjection>
         get() = lowerBound.typeArguments
 
@@ -167,8 +161,19 @@ open class ConeFlexibleType(
 
 }
 
-fun ConeKotlinType.upperBoundIfFlexible() = (this as? ConeFlexibleType)?.upperBound ?: this
-fun ConeKotlinType.lowerBoundIfFlexible() = (this as? ConeFlexibleType)?.lowerBound ?: this
+fun ConeKotlinType.upperBoundIfFlexible(): ConeSimpleKotlinType {
+    return when (this) {
+        is ConeSimpleKotlinType -> this
+        is ConeFlexibleType -> upperBound
+    }
+}
+
+fun ConeKotlinType.lowerBoundIfFlexible(): ConeSimpleKotlinType {
+    return when (this) {
+        is ConeSimpleKotlinType -> this
+        is ConeFlexibleType -> lowerBound
+    }
+}
 
 class ConeCapturedTypeConstructor(
     val projection: ConeTypeProjection,
@@ -264,7 +269,10 @@ data class ConeDefinitelyNotNullType(val original: ConeKotlinType) : ConeSimpleK
     companion object
 }
 
-class ConeRawType(lowerBound: ConeKotlinType, upperBound: ConeKotlinType) : ConeFlexibleType(lowerBound, upperBound), RawTypeMarker
+class ConeRawType(
+    lowerBound: ConeSimpleKotlinType,
+    upperBound: ConeSimpleKotlinType
+) : ConeFlexibleType(lowerBound, upperBound), RawTypeMarker
 
 /*
  * Contract of the intersection type: it is flat. It means that