FIR types: relax requirements around flexible types

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

@@ -109,6 +109,13 @@ abstract class ConeTypeParameterType : ConeLookupTagBasedType() {
 
 class ConeFlexibleType(val lowerBound: ConeKotlinType, val upperBound: ConeKotlinType) : ConeKotlinType(),
     FlexibleTypeMarker {
+
+    init {
+        val message = { "Bounds violation: $lowerBound, $upperBound" }
+        require(lowerBound is SimpleTypeMarker, message)
+        require(upperBound is SimpleTypeMarker, message)
+    }
+
     override val typeArguments: Array<out ConeKotlinTypeProjection>
         get() = emptyArray()
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt

@@ -41,8 +41,8 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext,
     }
 
     override fun createFlexibleType(lowerBound: SimpleTypeMarker, upperBound: SimpleTypeMarker): KotlinTypeMarker {
-        require(lowerBound is ConeLookupTagBasedType)
+        require(lowerBound is ConeKotlinType)
-        require(upperBound is ConeLookupTagBasedType)
+        require(upperBound is ConeKotlinType)
 
         return ConeFlexibleType(lowerBound, upperBound)
     }