Support KClass.primaryConstructor

compiler/testData/codegen/boxWithStdlib/reflection/constructors/primaryConstructor.kt

@@ -0,0 +1,52 @@
+import kotlin.test.assertNull
+import kotlin.test.assertNotNull
+import kotlin.reflect.*
+
+class OnlyPrimary
+
+class PrimaryWithSecondary(val s: String) {
+    constructor(x: Int) : this(x.toString())
+
+    override fun toString() = s
+}
+
+class OnlySecondary {
+    constructor(s: String)
+}
+
+class TwoSecondaries {
+    constructor(s: String)
+    constructor(d: Double)
+}
+
+enum class En
+
+interface I
+object O
+class C {
+    companion object
+}
+
+fun box(): String {
+    val p1 = OnlyPrimary::class.primaryConstructor
+    assertNotNull(p1)
+    assert(p1!!.call() is OnlyPrimary)
+
+    val p2 = PrimaryWithSecondary::class.primaryConstructor
+    assertNotNull(p2)
+    assert(p2!!.call("beer").toString() == "beer")
+
+    val p3 = OnlySecondary::class.primaryConstructor
+    assertNull(p3)
+
+    val p4 = TwoSecondaries::class.primaryConstructor
+    assertNull(p4)
+
+    assertNotNull(En::class.primaryConstructor) // TODO: maybe primaryConstructor should be null for enum classes
+
+    assertNull(I::class.primaryConstructor)
+    assertNull(O::class.primaryConstructor)
+    assertNull(C.Companion::class.primaryConstructor)
+
+    return "OK"
+}

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

@@ -2934,6 +2934,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
                 doTestWithStdlib(fileName);
             }
 
+            @TestMetadata("primaryConstructor.kt")
+            public void testPrimaryConstructor() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/constructors/primaryConstructor.kt");
+                doTestWithStdlib(fileName);
+            }
+
             @TestMetadata("simpleGetConstructors.kt")
             public void testSimpleGetConstructors() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/constructors/simpleGetConstructors.kt");

core/reflection.jvm/src/kotlin/reflect/KClassExtensions.kt

@@ -16,7 +16,19 @@
 
 package kotlin.reflect
 
+import org.jetbrains.kotlin.descriptors.ConstructorDescriptor
 import kotlin.reflect.jvm.internal.KClassImpl
+import kotlin.reflect.jvm.internal.KFunctionImpl
+
+/**
+ * Returns the primary constructor of this class, or `null` if this class has no primary constructor.
+ * See the [Kotlin language documentation](http://kotlinlang.org/docs/reference/classes.html#constructors)
+ * for more information.
+ */
+public val <T : Any> KClass<T>.primaryConstructor: KFunction<T>?
+    get() = (this as KClassImpl<T>).constructors.firstOrNull {
+        ((it as KFunctionImpl).descriptor as ConstructorDescriptor).isPrimary
+    }
 
 /**
  * Returns all functions declared in this class.