Implement common Throwable.addSuppressed/suppressed extensions

Instead of `suppressed` Array we provide `suppressedExceptions` List

#KT-23737

libraries/stdlib/jvm/src/kotlin/internal/PlatformImplementations.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -12,17 +12,29 @@ import kotlin.random.Random
 
 internal open class PlatformImplementations {
 
-    private object ReflectAddSuppressedMethod {
+    private object ReflectThrowable {
         @JvmField
-        public val method: Method? = Throwable::class.java.let { throwableClass ->
-            throwableClass.methods.find {
+        public val addSuppressed: Method?
+        @JvmField
+        public val getSuppressed: Method?
+
+        init {
+            val throwableClass = Throwable::class.java
+            val throwableMethods = throwableClass.methods
+            addSuppressed = throwableMethods.find {
                 it.name == "addSuppressed" && it.parameterTypes.singleOrNull() == throwableClass
             }
+            getSuppressed = throwableMethods.find { it.name == "getSuppressed" }
         }
     }
 
     public open fun addSuppressed(cause: Throwable, exception: Throwable) {
-        ReflectAddSuppressedMethod.method?.invoke(cause, exception)
+        ReflectThrowable.addSuppressed?.invoke(cause, exception)
+    }
+
+    public open fun getSuppressed(exception: Throwable): List<Throwable> {
+        return ReflectThrowable.getSuppressed?.invoke(exception)?.let { (it as Array<Throwable>).asList() }
+            ?: emptyList()
     }
 
     public open fun getMatchResultNamedGroup(matchResult: MatchResult, name: String): MatchGroup? {

libraries/stdlib/jvm/src/kotlin/util/Exceptions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -42,7 +42,23 @@ public val Throwable.stackTrace: Array<StackTraceElement>
     get() = (this as java.lang.Throwable).stackTrace!!
 
 /**
- * When supported by the platform adds the specified exception to the list of exceptions that were
+ * When supported by the platform, adds the specified exception to the list of exceptions that were
  * suppressed in order to deliver this exception.
  */
-public fun Throwable.addSuppressed(exception: Throwable) = IMPLEMENTATIONS.addSuppressed(this, exception)
+@SinceKotlin("1.1")
+public actual fun Throwable.addSuppressed(exception: Throwable) {
+    if (this !== exception)
+        IMPLEMENTATIONS.addSuppressed(this, exception)
+}
+
+/**
+ * Returns a list of all exceptions that were suppressed in order to deliver this exception.
+ *
+ * The list can be empty:
+ * - if no exceptions were suppressed;
+ * - if the platform doesn't support suppressed exceptions;
+ * - if this [Throwable] instance has disabled the suppression.
+ */
+@SinceKotlin("1.4")
+public actual val Throwable.suppressedExceptions: List<Throwable>
+    get() = IMPLEMENTATIONS.getSuppressed(this)