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)
 

plugins/kotlinx-serialization/testData/boxIr/contextualFallback.kt

@@ -0,0 +1,22 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+
+@Serializable
+data class Holder<T>(
+    val ok: Boolean,
+    @Contextual
+    val result: T?
+)
+
+fun box(): String {
+    val serializer = serializer<Holder<List<String>>>()
+    val instance = Holder(true, listOf("a", "b"))
+    val encoded = Json.encodeToString(serializer, instance)
+    if (encoded != """{"ok":true,"result":["a","b"]}""") return encoded
+    return "OK"
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java

@@ -51,6 +51,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/boxIr/contextualByDefault.kt");
         }
 
+        @Test
+        @TestMetadata("contextualFallback.kt")
+        public void testContextualFallback() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/contextualFallback.kt");
+        }
+
         @Test
         @TestMetadata("enumsAreCached.kt")
         public void testEnumsAreCached() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java

@@ -49,6 +49,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/contextualByDefault.kt");
     }
 
+    @Test
+    @TestMetadata("contextualFallback.kt")
+    public void testContextualFallback() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/contextualFallback.kt");
+    }
+
     @Test
     @TestMetadata("enumsAreCached.kt")
     public void testEnumsAreCached() throws Exception {