Support KParameter.isOptional

#KT-8825 Fixed

compiler/testData/codegen/boxWithStdlib/reflection/parameters/isOptional.kt

@@ -0,0 +1,24 @@
+import kotlin.test.*
+
+open class A {
+    open fun foo(x: Int, y: Int = 1) {}
+}
+
+class B : A() {
+    override fun foo(x: Int, y: Int) {}
+}
+
+class C : A()
+
+
+fun Int.extFun() {}
+
+fun box(): String {
+    assertEquals(listOf(false, false, true), A::foo.parameters.map { it.isOptional })
+    assertEquals(listOf(false, false, true), B::foo.parameters.map { it.isOptional })
+    assertEquals(listOf(false, false, true), C::foo.parameters.map { it.isOptional })
+
+    assertFalse(Int::extFun.parameters.single().isOptional)
+
+    return "OK"
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java

@@ -3648,6 +3648,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
                 doTestWithStdlib(fileName);
             }
 
+            @TestMetadata("isOptional.kt")
+            public void testIsOptional() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/parameters/isOptional.kt");
+                doTestWithStdlib(fileName);
+            }
+
             @TestMetadata("kinds.kt")
             public void testKinds() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/parameters/kinds.kt");

core/builtins/src/kotlin/reflect/KParameter.kt

@@ -60,4 +60,13 @@ public interface KParameter : KAnnotatedElement {
         /** Ordinary named value parameter. */
         VALUE,
     }
+
+    /**
+     * `true` if this parameter is optional, or `false` otherwise.
+     *
+     * A parameter is optional in any of the two cases:
+     * 1. The default value is provided at the declaration of this parameter.
+     * 2. The parameter is declared in a member function and one of the corresponding parameters in the super functions is optional.
+     */
+    public val isOptional: Boolean
 }

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

@@ -19,6 +19,7 @@ package kotlin.reflect.jvm.internal
 import org.jetbrains.kotlin.descriptors.ParameterDescriptor
 import org.jetbrains.kotlin.descriptors.ValueParameterDescriptor
 import org.jetbrains.kotlin.descriptors.annotations.Annotated
+import org.jetbrains.kotlin.resolve.descriptorUtil.hasDefaultValue
 import kotlin.reflect.KParameter
 import kotlin.reflect.KType
 
@@ -42,6 +43,9 @@ class KParameterImpl(
     override val type: KType
         get() = KTypeImpl(descriptor.type) { callable.caller.parameterTypes[index] }
 
+    override val isOptional: Boolean
+        get() = (descriptor as? ValueParameterDescriptor)?.hasDefaultValue() ?: false
+
     override fun equals(other: Any?) =
             other is KParameterImpl && callable == other.callable && descriptor == other.descriptor