Fix memory leak caused by invalid KTypeWrapper's equals method (#2274)

* Fix KTypeWrapper equality bug to avoid memory leak

* Optimize imports of CachingTest.kt

Author: jooohn

core/jvmMain/src/kotlinx/serialization/internal/Caching.kt

@@ -170,7 +170,7 @@ private class KTypeWrapper(private val origin: KType) : KType {
 
     override fun equals(other: Any?): Boolean {
         if (other == null) return false
-        if (origin != other) return false
+        if (origin != (other as? KTypeWrapper)?.origin) return false
 
         val kClassifier = classifier
         if (kClassifier is KClass<*>) {

core/jvmTest/src/kotlinx/serialization/CachingTest.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization
+
+import kotlinx.serialization.internal.*
+import kotlinx.serialization.modules.*
+import org.junit.Test
+import kotlin.reflect.*
+import kotlin.test.*
+
+class CachingTest {
+    @Test
+    fun testCache() {
+        var factoryCalled = 0
+
+        val cache = createCache {
+            factoryCalled += 1
+            it.serializerOrNull()
+        }
+
+        repeat(10) {
+            cache.get(typeOf<String>().kclass())
+        }
+
+        assertEquals(1, factoryCalled)
+    }
+
+    @Test
+    fun testParameterizedCache() {
+        var factoryCalled = 0
+
+        val cache = createParametrizedCache { clazz, types ->
+            factoryCalled += 1
+            val serializers = EmptySerializersModule().serializersForParameters(types, true)!!
+            clazz.parametrizedSerializerOrNull(types, serializers)
+        }
+
+        repeat(10) {
+            cache.get(typeOf<Map<*, *>>().kclass(), listOf(typeOf<String>(), typeOf<String>()))
+        }
+
+        assertEquals(1, factoryCalled)
+    }
+}