Reflection: create synthetic classes instead of throwing UOE

... for Kotlin-generated classes which do not correspond to a "class"
from the Kotlin language's point of view. For example, Kotlin lambdas,
file facade classes, multifile class facade/part classes, WhenMappings,
DefaultImpls. They can be distinguished from normal classes by the value
of `KotlinClassHeader.Kind` (which is the same as `Metadata.kind`).

Another theoretical option would be to throw exception at the point
where the `::class` expression is used, if the expression's type on the
left-hand side is a synthetic class. But we can't really do that since
it'll affect performance of most `<expression>::class` expressions.

So, construct a fake synthetic class instead, without any members except
equals/hashCode/toString, and without any non-trivial modifiers. It kind
of contradicts the general idea that kotlin-reflect presents anything
exactly the same as the compiler sees it, but arguably it's worth it to
avoid unexpected exceptions like in KT-41373.

In the newly added test, Java lambda check is muted but it should work
exactly the same as for Kotlin lambdas and other synthetic classes. It's
fixed in a subsequent commit.

 #KT-41373 In Progress

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KClassImpl.kt

@@ -20,7 +20,10 @@ import org.jetbrains.kotlin.builtins.CompanionObjectMapping
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.builtins.isMappedIntrinsicCompanionObject
 import org.jetbrains.kotlin.descriptors.*
+import org.jetbrains.kotlin.descriptors.impl.ClassDescriptorImpl
+import org.jetbrains.kotlin.descriptors.impl.EmptyPackageFragmentDescriptor
 import org.jetbrains.kotlin.descriptors.runtime.components.ReflectKotlinClass
+import org.jetbrains.kotlin.descriptors.runtime.components.RuntimeModuleData
 import org.jetbrains.kotlin.descriptors.runtime.structure.functionClassArity
 import org.jetbrains.kotlin.descriptors.runtime.structure.wrapperByPrimitive
 import org.jetbrains.kotlin.incremental.components.NoLookupLocation
@@ -32,6 +35,7 @@ import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.resolve.DescriptorUtils
 import org.jetbrains.kotlin.resolve.descriptorUtil.builtIns
+import org.jetbrains.kotlin.resolve.scopes.GivenFunctionsMemberScope
 import org.jetbrains.kotlin.resolve.scopes.MemberScope
 import org.jetbrains.kotlin.serialization.deserialization.MemberDeserializer
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.DeserializedClassDescriptor
@@ -49,6 +53,7 @@ internal class KClassImpl<T : Any>(
         val descriptor: ClassDescriptor by ReflectProperties.lazySoft {
             val classId = classId
             val moduleData = data.value.moduleData
+            val module = moduleData.module
 
             val descriptor =
                 if (classId.isLocal && jClass.isAnnotationPresent(Metadata::class.java)) {
@@ -56,10 +61,10 @@ internal class KClassImpl<T : Any>(
                     // `module.findClassAcrossModuleDependencies`.
                     moduleData.deserialization.deserializeClass(classId)
                 } else {
-                    moduleData.module.findClassAcrossModuleDependencies(classId)
+                    module.findClassAcrossModuleDependencies(classId)
                 }
 
-            descriptor ?: reportUnresolvedClass()
+            descriptor ?: createSyntheticClassOrFail(classId, moduleData)
         }
 
         val annotations: List<Annotation> by ReflectProperties.lazySoft { descriptor.computeAnnotations() }
@@ -310,29 +315,38 @@ internal class KClassImpl<T : Any>(
         }
     }
 
-    private fun reportUnresolvedClass(): Nothing {
+    private fun createSyntheticClassOrFail(classId: ClassId, moduleData: RuntimeModuleData): ClassDescriptor {
         when (val kind = ReflectKotlinClass.create(jClass)?.classHeader?.kind) {
-            KotlinClassHeader.Kind.FILE_FACADE, KotlinClassHeader.Kind.MULTIFILE_CLASS, KotlinClassHeader.Kind.MULTIFILE_CLASS_PART -> {
-                throw UnsupportedOperationException(
-                    "Packages and file facades are not yet supported in Kotlin reflection. " +
-                            "Meanwhile please use Java reflection to inspect this class: $jClass"
-                )
-            }
-            KotlinClassHeader.Kind.SYNTHETIC_CLASS -> {
-                throw UnsupportedOperationException(
-                    "This class is an internal synthetic class generated by the Kotlin compiler, such as an anonymous class " +
-                            "for a lambda, a SAM wrapper, a callable reference, etc. It's not a Kotlin class or interface, so the reflection " +
-                            "library has no idea what declarations it has. Please use Java reflection to inspect this class: $jClass"
-                )
-            }
+            KotlinClassHeader.Kind.FILE_FACADE,
+            KotlinClassHeader.Kind.MULTIFILE_CLASS,
+            KotlinClassHeader.Kind.MULTIFILE_CLASS_PART,
+            KotlinClassHeader.Kind.SYNTHETIC_CLASS ->
+                return createSyntheticClass(classId, moduleData)
             KotlinClassHeader.Kind.UNKNOWN -> {
                 // Should not happen since ABI-related exception must have happened earlier
                 throw KotlinReflectionInternalError("Unknown class: $jClass (kind = $kind)")
             }
             KotlinClassHeader.Kind.CLASS, null -> {
                 // Should not happen since a proper Kotlin- or Java-class must have been resolved
-                throw KotlinReflectionInternalError("Unresolved class: $jClass")
+                throw KotlinReflectionInternalError("Unresolved class: $jClass (kind = $kind)")
             }
         }
     }
+
+    private fun createSyntheticClass(classId: ClassId, moduleData: RuntimeModuleData): ClassDescriptor =
+        ClassDescriptorImpl(
+            EmptyPackageFragmentDescriptor(moduleData.module, classId.packageFqName),
+            classId.shortClassName,
+            Modality.FINAL,
+            ClassKind.CLASS,
+            listOf(moduleData.module.builtIns.any.defaultType),
+            SourceElement.NO_SOURCE,
+            false,
+            moduleData.deserialization.storageManager,
+        ).also { descriptor ->
+            descriptor.initialize(object : GivenFunctionsMemberScope(moduleData.deserialization.storageManager, descriptor) {
+                // Don't declare any functions in this class descriptor, only inherit equals/hashCode/toString from Any.
+                override fun computeDeclaredFunctions(): List<FunctionDescriptor> = emptyList()
+            }, emptySet(), null)
+        }
 }