Add error hint when accessing the default value of a union type with no default

pkl-core/src/main/java/org/pkl/core/ast/member/DefaultPropertyBodyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 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.
@@ -19,6 +19,9 @@
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
+import org.pkl.core.ast.type.TypeNode.ConstrainedTypeNode;
+import org.pkl.core.ast.type.TypeNode.TypeAliasTypeNode;
+import org.pkl.core.ast.type.TypeNode.UnionTypeNode;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.VmUtils;
 import org.pkl.core.util.Nullable;
@@ -50,8 +53,30 @@ public Object executeGeneric(VirtualFrame frame) {
     }
 
     CompilerDirectives.transferToInterpreter();
+
+    // attempt to give a hint when a union type is missing a default
+    String aliasName = null;
+    String unionTypeSource = null;
+    if (typeNode != null) {
+      var tn = typeNode.getTypeNode();
+      while (true) {
+        if (tn instanceof TypeAliasTypeNode typeAlias) {
+          aliasName = typeAlias.getVmTypeAlias().getSimpleName();
+          tn = typeAlias.getVmTypeAlias().getTypeNode();
+        } else if (tn instanceof ConstrainedTypeNode constrained) {
+          tn = constrained.getBaseTypeNode();
+        } else {
+          break;
+        }
+      }
+      if (tn instanceof UnionTypeNode union && !union.getHasDefaultElement()) {
+        unionTypeSource = union.getSourceSection().getCharacters().toString();
+      }
+    }
+
     throw exceptionBuilder()
-        .undefinedPropertyValue(propertyName, VmUtils.getReceiver(frame))
+        .undefinedPropertyValue(
+            propertyName, VmUtils.getReceiver(frame), unionTypeSource, aliasName)
         .build();
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -810,6 +810,10 @@ public TypeNode[] getElementTypeNodes() {
       return elementTypeNodes;
     }
 
+    public boolean getHasDefaultElement() {
+      return defaultIndex != -1;
+    }
+
     @Override
     public boolean isNoopTypeCheck() {
       return skipElementTypeChecks;
@@ -2598,6 +2602,10 @@ public Object executeAndSet(VirtualFrame frame, Object value) {
       return childNode.createDefaultValue(language, headerSection, qualifiedName);
     }
 
+    public TypeNode getBaseTypeNode() {
+      return childNode;
+    }
+
     public SourceSection getBaseTypeSection() {
       return childNode.getSourceSection();
     }

pkl-core/src/main/java/org/pkl/core/runtime/VmExceptionBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 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.
@@ -124,9 +124,19 @@ public VmExceptionBuilder undefinedValue() {
     return withExternalMessage("undefinedValue");
   }
 
-  public VmExceptionBuilder undefinedPropertyValue(Identifier propertyName, Object receiver) {
+  public VmExceptionBuilder undefinedPropertyValue(
+      Identifier propertyName,
+      Object receiver,
+      @Nullable String unionTypeSource,
+      @Nullable String typeAliasName) {
     kind = VmException.Kind.UNDEFINED_VALUE;
     this.receiver = receiver;
+    if (unionTypeSource != null && typeAliasName != null) {
+      return withExternalMessage(
+          "undefinedPropertyValueUnionAlias", propertyName, unionTypeSource, typeAliasName);
+    } else if (unionTypeSource != null) {
+      return withExternalMessage("undefinedPropertyValueUnion", propertyName, unionTypeSource);
+    }
     return withExternalMessage("undefinedPropertyValue", propertyName);
   }
 

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -436,6 +436,16 @@ Undefined value.
 undefinedPropertyValue=\
 Tried to read property `{0}` but its value is undefined.
 
+undefinedPropertyValueUnion=\
+Tried to read property `{0}` but its value is undefined.\n\
+Union type `{1}` has no default member denoted by `*`.\n\
+Either update the type to specify a default member or explicitly assign a value.
+
+undefinedPropertyValueUnionAlias=\
+Tried to read property `{0}` but its value is undefined.\n\
+Union type `{1}` (from type alias `{2}`) has no default member denoted by `*`.\n\
+Either update the type to specify a default member or explicitly assign a value.
+
 cannotFindModule=\
 Cannot find module `{0}`.
 

pkl-core/src/test/files/LanguageSnippetTests/input/errors/noDefault3.pkl

@@ -0,0 +1,3 @@
+local typealias B = Listing<String> | String
+
+b: B

pkl-core/src/test/files/LanguageSnippetTests/input/errors/noDefault4.pkl

@@ -0,0 +1,4 @@
+local typealias B = Listing<String> | String
+local typealias C = B(true)
+
+c: C

pkl-core/src/test/files/LanguageSnippetTests/input/errors/noDefault5.pkl

@@ -0,0 +1,3 @@
+// should not get "union with no default" diagnostic here (since there is a default)
+// and lack of default value is due to String's lack of default value
+foo: *String|Listing<String>

pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault2.err

@@ -1,5 +1,7 @@
 –– Pkl Error ––
 Tried to read property `foo` but its value is undefined.
+Union type `Foo|"baz"` has no default member denoted by `*`.
+Either update the type to specify a default member or explicitly assign a value.
 
 x | foo: Foo|"baz"
     ^^^

pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault3.err

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Tried to read property `b` but its value is undefined.
+Union type `Listing<String> | String` (from type alias `B`) has no default member denoted by `*`.
+Either update the type to specify a default member or explicitly assign a value.
+
+x | b: B
+    ^
+at noDefault3#b (file:///$snippetsDir/input/errors/noDefault3.pkl)
+
+The above error occurred when rendering path `b` of module `file:///$snippetsDir/input/errors/noDefault3.pkl`.
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault4.err

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Tried to read property `c` but its value is undefined.
+Union type `Listing<String> | String` (from type alias `B`) has no default member denoted by `*`.
+Either update the type to specify a default member or explicitly assign a value.
+
+x | c: C
+    ^
+at noDefault4#c (file:///$snippetsDir/input/errors/noDefault4.pkl)
+
+The above error occurred when rendering path `c` of module `file:///$snippetsDir/input/errors/noDefault4.pkl`.
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/noDefault5.err

@@ -0,0 +1,16 @@
+–– Pkl Error ––
+Tried to read property `foo` but its value is undefined.
+
+x | foo: *String|Listing<String>
+    ^^^
+at noDefault5#foo (file:///$snippetsDir/input/errors/noDefault5.pkl)
+
+The above error occurred when rendering path `foo` of module `file:///$snippetsDir/input/errors/noDefault5.pkl`.
+
+xxx | renderer.renderDocument(value)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)
+
+xxx | if (renderer is BytesRenderer) renderer.renderDocument(value) else text.encodeToBytes("UTF-8")
+                                                                         ^^^^
+at pkl.base#Module.output.bytes (pkl:base)