Improve error messages for lambdas when the number of arguments is wrong

modules/lang-painless/src/main/java/org/elasticsearch/painless/Def.java

@@ -341,6 +341,12 @@ public final class Def {
                                                                  MethodHandle.class);
                  handle = (MethodHandle) accessor.invokeExact();
              } catch (NoSuchFieldException | IllegalAccessException e) {
+                 // is it a synthetic method? If we generated the method ourselves, be more helpful. It can only fail
+                 // because the arity does not match the expected interface type.
+                 if (call.contains("$")) {
+                     throw new IllegalArgumentException("Incorrect number of parameters for [" + interfaceMethod.name + 
+                                                        "] in [" + clazz.clazz + "]");
+                 }
                  throw new IllegalArgumentException("Unknown call [" + call + "] with [" + arity + "] arguments.");
              }
              ref = new FunctionRef(clazz, interfaceMethod, handle, captures);

modules/lang-painless/src/main/java/org/elasticsearch/painless/FunctionRef.java

@@ -176,7 +176,12 @@ public class FunctionRef {
      * If the interface expects a primitive type to be returned, we can't return Object,
      * But we can set SAM to the wrapper version, and a cast will take place 
      */
-    private static MethodType adapt(MethodType expected, MethodType actual) {
+    private MethodType adapt(MethodType expected, MethodType actual) {
+        // add some checks, now that we've set everything up, to deliver exceptions as early as possible.
+        if (expected.parameterCount() != actual.parameterCount()) {
+            throw new IllegalArgumentException("Incorrect number of parameters for [" + invokedName + 
+                                               "] in [" + invokedType.returnType() + "]");
+        }
         if (expected.returnType().isPrimitive() && actual.returnType() == Object.class) {
             actual = actual.changeReturnType(MethodType.methodType(expected.returnType()).wrap().returnType());
         }

modules/lang-painless/src/test/java/org/elasticsearch/painless/FunctionRefTests.java

@@ -162,4 +162,18 @@ public class FunctionRefTests extends ScriptTestCase {
             exec("List l = new ArrayList(); l.add(2); l.add(1); l.sort(String::startsWith); return l.get(0);");
         });
     }
+    
+    public void testWrongArity() {
+        IllegalArgumentException expected = expectScriptThrows(IllegalArgumentException.class, () -> {
+            exec("Optional.empty().orElseGet(String::startsWith);");
+        });
+        assertTrue(expected.getMessage().contains("Unknown reference"));
+    }
+    
+    public void testWrongArityDef() {
+        IllegalArgumentException expected = expectScriptThrows(IllegalArgumentException.class, () -> {
+            exec("def y = Optional.empty(); return y.orElseGet(String::startsWith);");
+        });
+        assertTrue(expected.getMessage().contains("Unknown reference"));
+    }
 }

modules/lang-painless/src/test/java/org/elasticsearch/painless/LambdaTests.java

@@ -142,4 +142,18 @@ public class LambdaTests extends ScriptTestCase {
         assertEquals(1, exec("boolean x = false; int y = 1;" +
                              "return Optional.empty().orElseGet(() -> x ? 5 : Optional.empty().orElseGet(() -> y));"));
     }
+    
+    public void testWrongArity() {
+        IllegalArgumentException expected = expectScriptThrows(IllegalArgumentException.class, () -> {
+            exec("Optional.empty().orElseGet(x -> x);");
+        });
+        assertTrue(expected.getMessage().contains("Incorrect number of parameters"));
+    }
+    
+    public void testWrongArityDef() {
+        IllegalArgumentException expected = expectScriptThrows(IllegalArgumentException.class, () -> {
+            exec("def y = Optional.empty(); return y.orElseGet(x -> x);");
+        });
+        assertTrue(expected.getMessage(), expected.getMessage().contains("Incorrect number of parameters"));
+    }
 }