Fixed missing fallback serializer in contextual serializer

Fixes Kotlin/kotlinx.serialization#2158

Merge-request: KT-MR-8338
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/BaseIrGenerator.kt

@@ -467,6 +467,14 @@ abstract class BaseIrGenerator(private val currentClass: IrClass, final override
                 typeArgs = listOf(kType)
             }
             contextSerializerId -> {
+                // don't create an instance if the serializer is being created for the cache
+                if (genericIndex == null && kType.genericIndex != null) {
+                    // if context serializer parametrized by generic type (kType.genericIndex != null)
+                    // and generic types are not allowed (always genericIndex == null for cache)
+                    // then serializer can't be cached
+                    return null
+                }
+
                 args = listOf(classReference(kType.classOrUpperBound()!!))
                 typeArgs = listOf(kType)