Painless: improve error message on non-constant

modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultConstantFoldingOptimizationPhase.java

@@ -841,12 +841,17 @@ private void replaceCallWithConstant(
         Object[] args = new Object[irInvokeCallMemberNode.getArgumentNodes().size()];
         for (int i = 0; i < irInvokeCallMemberNode.getArgumentNodes().size(); i++) {
             ExpressionNode argNode = irInvokeCallMemberNode.getArgumentNodes().get(i);
-            if (argNode instanceof ConstantNode == false) {
+            IRDConstant constantDecoration = argNode.getDecoration(IRDConstant.class);
+            if (constantDecoration == null) {
                 // TODO find a better string to output
                 throw irInvokeCallMemberNode.getLocation()
-                    .createError(new IllegalArgumentException("all arguments must be constant but the [" + (i + 1) + "] argument isn't"));
+                    .createError(
+                        new IllegalArgumentException(
+                            "all arguments to [" + javaMethod.getName() + "] must be constant but the [" + (i + 1) + "] argument isn't"
+                        )
+                    );
             }
-            args[i] = ((ConstantNode) argNode).getDecorationValue(IRDConstant.class);
+            args[i] = constantDecoration.getValue();
         }
         Object result;
         try {

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

@@ -211,7 +211,7 @@ public void testClassMethodCompileTimeOnlyVariableParams() {
             IllegalArgumentException.class,
             () -> scriptEngine.compile(null, "def a = 2; classMul(2, a)", BindingsTestScript.CONTEXT, Collections.emptyMap())
         );
-        assertThat(e.getMessage(), equalTo("all arguments must be constant but the [2] argument isn't"));
+        assertThat(e.getMessage(), equalTo("all arguments to [classMul] must be constant but the [2] argument isn't"));
     }
 
     public void testClassMethodCompileTimeOnlyThrows() {
@@ -240,7 +240,7 @@ public void testInstanceMethodCompileTimeOnlyVariableParams() {
             IllegalArgumentException.class,
             () -> scriptEngine.compile(null, "def a = 2; instanceMul(a, 2)", BindingsTestScript.CONTEXT, Collections.emptyMap())
         );
-        assertThat(e.getMessage(), equalTo("all arguments must be constant but the [1] argument isn't"));
+        assertThat(e.getMessage(), equalTo("all arguments to [instanceMul] must be constant but the [1] argument isn't"));
     }
 
     public void testCompileTimeOnlyParameterFoldedToConstant() {

x-pack/plugin/src/test/resources/rest-api-spec/test/runtime_fields/250_grok.yml

@@ -80,7 +80,7 @@ fetch:
 ---
 mutable pattern:
   - do:
-      catch: /all arguments must be constant but the \[1\] argument isn't/
+      catch: /all arguments to \[grok\] must be constant but the \[1\] argument isn't/
       search:
         index: http_logs
         body: