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

Author: HT154

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 +53,36 @@ public Object executeGeneric(VirtualFrame frame) {
     }
 
     CompilerDirectives.transferToInterpreter();
-    throw exceptionBuilder()
-        .undefinedPropertyValue(propertyName, VmUtils.getReceiver(frame))
-        .build();
+    var builder =
+        exceptionBuilder().undefinedPropertyValue(propertyName, VmUtils.getReceiver(frame));
+
+    // attempt to give a hint when a union type is missing a default
+    if (typeNode != null) {
+      String aliasName = 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) {
+        var unionText = union.getSourceSection().getCharacters().toString();
+        if (aliasName != null) {
+          builder.withHint(
+              String.format(
+                  "Union type `%s` (from typealias `%s`) has no default member.",
+                  unionText, aliasName));
+        } else {
+          builder.withHint(String.format("Union type `%s` has no default member.", unionText));
+        }
+      }
+    }
+
+    throw builder.build();
   }
 }

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

@@ -2598,6 +2598,10 @@ 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/test/files/LanguageSnippetTests/input/errors/amendUnionAliasAliasWithoutDefault.pkl

@@ -0,0 +1,13 @@
+local typealias B = Listing<String> | String
+local typealias C = B(true)
+
+local class A {
+  a: Listing<String> | String
+  b: B
+  c: C
+}
+
+result =
+  new A {
+    c {}
+  }.c

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

@@ -0,0 +1,13 @@
+local typealias B = Listing<String> | String
+local typealias C = B
+
+local class A {
+  a: Listing<String> | String
+  b: B
+  c: C
+}
+
+result =
+  new A {
+    b {}
+  }.b

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

@@ -0,0 +1,13 @@
+local typealias B = Listing<String> | String
+local typealias C = B
+
+local class A {
+  a: Listing<String> | String
+  b: B
+  c: C
+}
+
+result =
+  new A {
+    a {}
+  }.a

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

@@ -0,0 +1,20 @@
+–– Pkl Error ––
+Tried to read property `c` but its value is undefined.
+
+x | c: C
+    ^
+at amendUnionAliasAliasWithoutDefault#A.c (file:///$snippetsDir/input/errors/amendUnionAliasAliasWithoutDefault.pkl)
+
+Union type `Listing<String> | String` (from typealias `B`) has no default member.
+
+xx | new A {
+     ^^^^^^^
+at amendUnionAliasAliasWithoutDefault#result (file:///$snippetsDir/input/errors/amendUnionAliasAliasWithoutDefault.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/amendUnionAliasWithoutDefault.err

@@ -0,0 +1,20 @@
+–– Pkl Error ––
+Tried to read property `b` but its value is undefined.
+
+x | b: B
+    ^
+at amendUnionAliasWithoutDefault#A.b (file:///$snippetsDir/input/errors/amendUnionAliasWithoutDefault.pkl)
+
+Union type `Listing<String> | String` (from typealias `B`) has no default member.
+
+xx | new A {
+     ^^^^^^^
+at amendUnionAliasWithoutDefault#result (file:///$snippetsDir/input/errors/amendUnionAliasWithoutDefault.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/amendUnionWithoutDefault.err

@@ -0,0 +1,20 @@
+–– Pkl Error ––
+Tried to read property `a` but its value is undefined.
+
+x | a: Listing<String> | String
+    ^
+at amendUnionWithoutDefault#A.a (file:///$snippetsDir/input/errors/amendUnionWithoutDefault.pkl)
+
+Union type `Listing<String> | String` has no default member.
+
+xx | new A {
+     ^^^^^^^
+at amendUnionWithoutDefault#result (file:///$snippetsDir/input/errors/amendUnionWithoutDefault.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/noDefault2.err

@@ -5,7 +5,7 @@ x | foo: Foo|"baz"
     ^^^
 at noDefault2#foo (file:///$snippetsDir/input/errors/noDefault2.pkl)
 
-The above error occurred when rendering path `foo` of module `file:///$snippetsDir/input/errors/noDefault2.pkl`.
+Union type `Foo|"baz"` has no default member.
 
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^