added multiArity bug puzzler

src/bugs/multiArity/Rationale.md

@@ -0,0 +1,33 @@
+The correct answer is **b) 0**.
+
+While IDE shows that “Check for instance is always 'true'” for both `is` expressions,
+only first one will be successful.
+
+The cause of [this bug](https://youtrack.jetbrains.com/issue/KT-24067) is that
+instead of generating normal INSTANCEOF (or CHECKCAST)
+compiler inserts a call to `kotlin.jvm.internal.TypeIntrinsics.isFunctionOfArity`
+which internally uses `getFunctionArity` declared as follows:
+
+```java
+public static int getFunctionArity(Object obj) {
+    if (obj instanceof FunctionBase) {
+        return ((FunctionBase) obj).getArity();
+    }
+    else if (obj instanceof Function0) {
+        return 0;
+    }
+    else if (obj instanceof Function1) {
+        return 1;
+    }
+    …
+    else if (obj instanceof Function22) {
+        return 22;
+    }
+    else {
+        return -1;
+    }
+}
+```
+
+Thus, Kotlin compiler expects functions to have only one arity.
+

src/bugs/multiArity/multiArity.kts

@@ -0,0 +1,16 @@
+package bugs.multiArity
+// by Mike Gorünov @Miha-x64
+
+object MultiFunction : () -> Unit, (Unit) -> Unit {
+  override fun invoke() {}
+  override fun invoke(p1: Unit) {}
+}
+
+if (MultiFunction is () -> Unit) print(0)
+if (MultiFunction is (Unit) -> Unit) print(1)
+
+// What will it print?
+// a) nothing
+// b) 0
+// c) 1
+// d) 01