Produce more helpful output when module output is overwritten (#716)

Fix a bug where overwriting `output` causes a PklBugException.

This now produces a more helpful message pointing to the actual problem.

Co-authored-by: translatenix <119817707+translatenix@users.noreply.github.com>

pkl-core/src/main/java/org/pkl/core/EvaluatorImpl.java

@@ -137,7 +137,7 @@ public class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = (VmTyped) VmUtils.readMember(module, Identifier.OUTPUT);
+          var output = readModuleOutput(module);
           return VmUtils.readTextProperty(output);
         });
   }
@@ -147,7 +147,7 @@ public class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = (VmTyped) VmUtils.readMember(module, Identifier.OUTPUT);
+          var output = readModuleOutput(module);
           var value = VmUtils.readMember(output, Identifier.VALUE);
           if (value instanceof VmValue vmValue) {
             vmValue.force(false);
@@ -162,7 +162,7 @@ public class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = (VmTyped) VmUtils.readMember(module, Identifier.OUTPUT);
+          var output = readModuleOutput(module);
           var filesOrNull = VmUtils.readMember(output, Identifier.FILES);
           if (filesOrNull instanceof VmNull) {
             return Map.of();
@@ -243,7 +243,7 @@ public class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = (VmTyped) VmUtils.readMember(module, Identifier.OUTPUT);
+          var output = readModuleOutput(module);
           var value = VmUtils.readMember(output, Identifier.VALUE);
           var valueClassInfo = VmUtils.getClass(value).getPClassInfo();
           if (valueClassInfo.equals(classInfo)) {
@@ -365,13 +365,40 @@ public class EvaluatorImpl implements Evaluator {
             "evaluationTimedOut", (timeout.getSeconds() + timeout.getNano() / 1_000_000_000d)));
   }
 
+  private VmTyped readModuleOutput(VmTyped module) {
+    var value = VmUtils.readMember(module, Identifier.OUTPUT);
+    if (value instanceof VmTyped typedOutput
+        && typedOutput.getVmClass().getPClassInfo() == PClassInfo.ModuleOutput) {
+      return typedOutput;
+    }
+
+    var moduleUri = module.getModuleInfo().getModuleKey().getUri();
+    var builder =
+        new VmExceptionBuilder()
+            .evalError(
+                "invalidModuleOutput",
+                "output",
+                PClassInfo.ModuleOutput.getDisplayName(),
+                VmUtils.getClass(value).getPClassInfo().getDisplayName(),
+                moduleUri);
+    var outputMember = module.getMember(Identifier.OUTPUT);
+    assert outputMember != null;
+    var uriOfValueMember = outputMember.getSourceSection().getSource().getURI();
+    // If `output` was explicitly re-assigned, show that in the stack trace.
+    if (!uriOfValueMember.equals(PClassInfo.pklBaseUri)) {
+      builder.withSourceSection(outputMember.getBodySection()).withMemberName("output");
+    }
+    throw builder.build();
+  }
+
   private VmException moduleOutputValueTypeMismatch(
       VmTyped module, PClassInfo<?> expectedClassInfo, Object value, VmTyped output) {
     var moduleUri = module.getModuleInfo().getModuleKey().getUri();
     var builder =
         new VmExceptionBuilder()
             .evalError(
-                "invalidModuleOutputValue",
+                "invalidModuleOutput",
+                "output.value",
                 expectedClassInfo.getDisplayName(),
                 VmUtils.getClass(value).getPClassInfo().getDisplayName(),
                 moduleUri);

pkl-core/src/main/java/org/pkl/core/PClassInfo.java

@@ -61,6 +61,8 @@ public final class PClassInfo<T> implements Serializable {
   public static final PClassInfo<LinkedHashMap> Mapping =
       pklBaseClassInfo("Mapping", LinkedHashMap.class);
   public static final PClassInfo<PModule> Module = pklBaseClassInfo("Module", PModule.class);
+  public static final PClassInfo<PObject> ModuleOutput =
+      pklBaseClassInfo("ModuleOutput", PObject.class);
   public static final PClassInfo<PClass> Class = pklBaseClassInfo("Class", PClass.class);
   public static final PClassInfo<TypeAlias> TypeAlias =
       pklBaseClassInfo("TypeAlias", TypeAlias.class);
@@ -216,6 +218,7 @@ public final class PClassInfo<T> implements Serializable {
           entry(Listing.className, Listing),
           entry(Mapping.className, Mapping),
           entry(Module.className, Module),
+          entry(ModuleOutput.className, ModuleOutput),
           entry(Class.className, Class),
           entry(TypeAlias.className, TypeAlias),
           entry(Regex.className, Regex),

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

@@ -833,8 +833,8 @@ A type union cannot have more than one default type.
 notAUnion=\
 Only type unions can have a default marker (*).
 
-invalidModuleOutputValue=\
+invalidModuleOutput=\
-Expected `output.value` of module `{2}` to be of type `{0}`, but got type `{1}`.
+Expected `{0}` of module `{3}` to be of type `{1}`, but got type `{2}`.
 
 cannotResolveDependencyWithoutHierarchicalUris=\
 Cannot import dependency because project URI `{0}` does not have a hierarchical path.

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput1.pkl

@@ -0,0 +1 @@
+output: String = "abc"

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput2.pkl

@@ -0,0 +1,3 @@
+class Test {}
+
+output: Test = new {}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput3.pkl

@@ -0,0 +1 @@
+output = null

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput1.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Expected `output` of module `file:///$snippetsDir/input/errors/invalidOutput1.pkl` to be of type `ModuleOutput`, but got type `String`.
+
+x | output: String = "abc"
+                     ^^^^^
+at output (file:///$snippetsDir/input/errors/invalidOutput1.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput2.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Expected `output` of module `file:///$snippetsDir/input/errors/invalidOutput2.pkl` to be of type `ModuleOutput`, but got type `invalidOutput2#Test`.
+
+x | output: Test = new {}
+                   ^^^^^^
+at output (file:///$snippetsDir/input/errors/invalidOutput2.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput3.err

@@ -0,0 +1,10 @@
+–– Pkl Error ––
+Expected value of type `ModuleOutput`, but got `null`.
+
+xx | hidden output: ModuleOutput = new {
+                    ^^^^^^^^^^^^
+at pkl.base#Module.output (pkl:base)
+
+x | output = null
+             ^^^^
+at invalidOutput3#output (file:///$snippetsDir/input/errors/invalidOutput3.pkl)