Fix forcing of VmValue during error rendering (#1629)

Author: stackoverflow

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorElementNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@
 import org.pkl.core.runtime.VmClass;
 import org.pkl.core.runtime.VmDynamic;
 import org.pkl.core.runtime.VmListing;
+import org.pkl.core.runtime.VmUtils;
 
 @ImportStatic(BaseModule.class)
 public abstract class GeneratorElementNode extends GeneratorMemberNode {
@@ -63,6 +64,7 @@ protected void evalListingClass(VirtualFrame frame, VmClass parent, ObjectData d
   @SuppressWarnings("unused")
   void fallback(VirtualFrame frame, Object parent, ObjectData data) {
     CompilerDirectives.transferToInterpreter();
-    throw exceptionBuilder().evalError("objectCannotHaveElement", parent).build();
+    var parentClass = parent instanceof VmClass ? (VmClass) parent : VmUtils.getClass(parent);
+    throw exceptionBuilder().evalError("objectCannotHaveElement", parentClass).build();
   }
 }

pkl-core/src/main/java/org/pkl/core/util/ErrorMessages.java

@@ -20,10 +20,14 @@
 import java.util.ResourceBundle;
 import java.util.stream.*;
 import org.jspecify.annotations.Nullable;
+import org.pkl.core.runtime.VmValue;
+import org.pkl.core.runtime.VmValueRenderer;
 
 public final class ErrorMessages {
   private ErrorMessages() {}
 
+  private static final VmValueRenderer renderer = VmValueRenderer.singleLine(Integer.MAX_VALUE);
+
   public static String create(String messageName, @Nullable Object... args) {
     var locale = Locale.getDefault();
     String errorMessage =
@@ -33,7 +37,18 @@ public static String create(String messageName, @Nullable Object... args) {
     if (args.length == 0) return errorMessage;
 
     var formatter = new MessageFormat(errorMessage, locale);
-    return formatter.format(args);
+    // TODO: we render VmValues here with VmValueRenderer, but that's not enough to properly
+    //       render all kinds of values, like Pkl Strings, for example
+    @Nullable Object[] actualArgs = new @Nullable Object[args.length];
+    for (var i = 0; i < args.length; i++) {
+      var arg = args[i];
+      if (arg instanceof VmValue) {
+        actualArgs[i] = renderer.render(arg);
+      } else {
+        actualArgs[i] = arg;
+      }
+    }
+    return formatter.format(actualArgs);
   }
 
   public static String createIndented(String messageName, String indent, @Nullable Object... args) {

pkl-core/src/test/files/LanguageSnippetTests/output/listings2/wrongParent.pcf

@@ -1,12 +1,12 @@
 res1 = "Cannot instantiate, or amend an instance of, external class `Int`."
 res2 = "Cannot instantiate, or amend an instance of, external class `List`."
 res3 = "Cannot instantiate, or amend an instance of, external class `List`."
-res4 = "Object of type `new Person {}` cannot have an element."
+res4 = "Object of type `wrongParent#Person` cannot have an element."
 res5 = "Cannot instantiate abstract class `ValueRenderer`."
-res6 = "Object of type `new Mapping {}` cannot have an element."
+res6 = "Object of type `Mapping` cannot have an element."
 res7 = "Cannot instantiate, or amend an instance of, external class `Int`."
 res8 = "Cannot instantiate, or amend an instance of, external class `List`."
 res9 = "Cannot instantiate, or amend an instance of, external class `List`."
-res10 = "Object of type `new Person {}` cannot have an element."
+res10 = "Object of type `wrongParent#Person` cannot have an element."
 res11 = "Cannot instantiate abstract class `ValueRenderer`."
-res12 = "Object of type `new Mapping {}` cannot have an element."
+res12 = "Object of type `Mapping` cannot have an element."

pkl-core/src/test/kotlin/org/pkl/core/util/ErrorMessagesTest.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.util
+
+import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.Assertions.assertThatCode
+import org.junit.jupiter.api.Test
+import org.pkl.core.runtime.VmClass
+import org.pkl.core.runtime.VmValue
+import org.pkl.core.runtime.VmValueConverter
+import org.pkl.core.runtime.VmValueVisitor
+
+class ErrorMessagesTest {
+  private class UnforceableValue : VmValue() {
+    var forced = false
+      private set
+
+    override fun force(allowUndefinedValues: Boolean) {
+      forced = true
+      throw RuntimeException("value must not be forced while rendering an error message")
+    }
+
+    override fun accept(visitor: VmValueVisitor) {
+      visitor.visitString("lazy")
+    }
+
+    override fun <T : Any> accept(converter: VmValueConverter<T>, path: Iterable<Any>): T =
+      throw UnsupportedOperationException()
+
+    override fun equals(obj: Any?): Boolean = this === obj
+
+    override fun toString(): String {
+      force(true)
+      return "lazy"
+    }
+
+    override fun getVmClass(): VmClass = throw UnsupportedOperationException()
+
+    override fun export(): Any = throw UnsupportedOperationException()
+
+    override fun hashCode(): Int = System.identityHashCode(this)
+  }
+
+  @Test
+  fun `renders VmValue arguments without forcing them`() {
+    val value = UnforceableValue()
+
+    lateinit var message: String
+    assertThatCode { message = ErrorMessages.create("cannotIterateOverThisValue", value) }
+      .doesNotThrowAnyException()
+
+    assertThat(value.forced).isFalse()
+    assertThat(message).contains("lazy")
+  }
+}