ConeFlexibleType: statically require to have simple bounds

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/SamResolution.kt

@@ -58,8 +58,8 @@ class FirSamResolverImpl(
         return when (type) {
             is ConeClassLikeType -> getFunctionTypeForPossibleSamType(type.fullyExpandedType(session))
             is ConeFlexibleType -> ConeFlexibleType(
-                getFunctionTypeForPossibleSamType(type.lowerBound) ?: return null,
-                getFunctionTypeForPossibleSamType(type.upperBound) ?: return null,
+                getFunctionTypeForPossibleSamType(type.lowerBound)?.lowerBoundIfFlexible() ?: return null,
+                getFunctionTypeForPossibleSamType(type.upperBound)?.upperBoundIfFlexible() ?: return null,
             )
             is ConeClassErrorType, is ConeStubType -> null
             // TODO: support those types as well

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

@@ -92,7 +92,7 @@ internal object CreateFreshTypeVariableSubstitutorStage : ResolutionStage() {
         session: FirSession,
     ): ConeKotlinType {
         return if (typeParameter.shouldBeFlexible(session.typeContext)) {
-            val notNullType = type.withNullability(ConeNullability.NOT_NULL, session.typeContext)
+            val notNullType = type.withNullability(ConeNullability.NOT_NULL, session.typeContext) as ConeSimpleKotlinType
             ConeFlexibleType(notNullType, notNullType.withNullability(ConeNullability.NULLABLE, session.typeContext))
         } else {
             type