Fix cyclic dependencies bug

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -288,6 +288,10 @@ public AstBuilder(
     isMethodReturnTypeChecked = !isStdLibModule || IoUtils.isTestMode();
   }
 
+  public ModuleInfo getModuleInfo() {
+    return moduleInfo;
+  }
+
   public static AstBuilder create(
       Source source,
       VmLanguage language,
@@ -311,7 +315,14 @@ public static AstBuilder create(
       var moduleName = IoUtils.inferModuleName(moduleKey);
       moduleInfo =
           new ModuleInfo(
-              sourceSection, headerSection, null, moduleName, moduleKey, resolvedModuleKey, false);
+              sourceSection,
+              headerSection,
+              null,
+              moduleName,
+              moduleKey,
+              resolvedModuleKey,
+              false,
+              null);
     } else {
       var declaredModuleName = moduleDecl.getName();
       var moduleName =
@@ -320,6 +331,20 @@ public static AstBuilder create(
               : IoUtils.inferModuleName(moduleKey);
       var clause = moduleDecl.getExtendsOrAmendsDecl();
       var isAmend = clause != null && clause.getType() == ExtendsOrAmendsClause.Type.AMENDS;
+
+      // if this is an amending module, resolve the module being amended
+      ModuleKey amendedModuleKey = null;
+      if (isAmend) {
+        try {
+          var amendedModuleUri = URI.create(clause.getUrl().getString());
+          if (!amendedModuleUri.isAbsolute()) {
+            amendedModuleUri = resolvedModuleKey.getUri().resolve(amendedModuleUri);
+          }
+          amendedModuleKey = moduleResolver.resolve(amendedModuleUri, null);
+        } catch (Exception ignored) {
+        }
+      }
+
       moduleInfo =
           new ModuleInfo(
               sourceSection,
@@ -328,7 +353,8 @@ public static AstBuilder create(
               moduleName,
               moduleKey,
               resolvedModuleKey,
-              isAmend);
+              isAmend,
+              amendedModuleKey);
     }
 
     return new AstBuilder(source, language, moduleInfo, moduleResolver);

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

@@ -99,7 +99,10 @@ public VmClass executeGeneric(VirtualFrame frame) {
       // nodes
       // via static final fields without having to fear recursive field initialization.
       prototype = module;
-      prototype.setExtraStorage(moduleInfo);
+      // Only set ModuleInfo if not already set (to handle cyclic dependencies)
+      if (!prototype.hasExtraStorage()) {
+        prototype.setExtraStorage(moduleInfo);
+      }
       prototype.addProperties(prototypeMembers);
     } else {
       prototype =

pkl-core/src/main/java/org/pkl/core/ast/type/ResolveDeclaredTypeNode.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 +19,7 @@
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.runtime.Identifier;
+import org.pkl.core.runtime.VmLanguage;
 import org.pkl.core.runtime.VmObjectLike;
 import org.pkl.core.runtime.VmTyped;
 import org.pkl.core.util.Nullable;
@@ -73,11 +74,36 @@ protected VmTyped getImport(
     var result = module.getCachedValue(importName);
     if (result == null) {
       result = callNode.call(member.getCallTarget(), module, module, importName);
+
+      var importedModule = (VmTyped) result;
+      if (importedModule.isNotInitialized()
+          && importedModule.isModuleObject()
+          && importedModule.getModuleInfo().isAmend()) {
+        // this is an amending module. Try to find the prototype
+        var prototypeModule = findPrototypeModule(importedModule);
+        if (prototypeModule != null) {
+          result = prototypeModule;
+        }
+      }
+
       module.setCachedValue(importName, result);
     }
     return (VmTyped) result;
   }
 
+  private @Nullable VmTyped findPrototypeModule(VmTyped amendingModule) {
+    try {
+      var moduleInfo = amendingModule.getModuleInfo();
+      var amendedModuleKey = moduleInfo.getAmendedModuleKey();
+
+      if (amendedModuleKey != null) {
+        return VmLanguage.get(null).loadModule(amendedModuleKey);
+      }
+    } catch (Exception ignored) {
+    }
+    return null;
+  }
+
   protected @Nullable Object getType(
       VmTyped module, Identifier typeName, SourceSection typeNameSection) {
     var member = module.getMember(typeName);

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

@@ -37,6 +37,7 @@ public final class ModuleInfo {
   private final ModuleKey moduleKey;
   private final ResolvedModuleKey resolvedModuleKey;
   private final boolean isAmend;
+  private final @Nullable ModuleKey amendedModuleKey;
 
   @LateInit private List<VmTyped> annotations;
 
@@ -54,6 +55,26 @@ public ModuleInfo(
       ModuleKey moduleKey,
       ResolvedModuleKey resolvedModuleKey,
       boolean isAmend) {
+    this(
+        sourceSection,
+        headerSection,
+        docComment,
+        moduleName,
+        moduleKey,
+        resolvedModuleKey,
+        isAmend,
+        null);
+  }
+
+  public ModuleInfo(
+      SourceSection sourceSection,
+      SourceSection headerSection,
+      SourceSection @Nullable [] docComment,
+      String moduleName,
+      ModuleKey moduleKey,
+      ResolvedModuleKey resolvedModuleKey,
+      boolean isAmend,
+      @Nullable ModuleKey amendedModuleKey) {
 
     this.sourceSection = sourceSection;
     this.headerSection = headerSection;
@@ -62,6 +83,7 @@ public ModuleInfo(
     this.moduleKey = moduleKey;
     this.resolvedModuleKey = resolvedModuleKey;
     this.isAmend = isAmend;
+    this.amendedModuleKey = amendedModuleKey;
   }
 
   public void initAnnotations(List<VmTyped> annotations) {
@@ -179,4 +201,11 @@ public ModuleSchema getModuleSchema(VmTyped module) {
   public boolean isAmend() {
     return isAmend;
   }
+
+  /**
+   * Returns the ModuleKey of the module being amended, or null if this is not an amending module.
+   */
+  public @Nullable ModuleKey getAmendedModuleKey() {
+    return amendedModuleKey;
+  }
 }

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

@@ -108,6 +108,10 @@ void initializeModule(
     var builder =
         AstBuilder.create(
             source, this, moduleContext, moduleKey, resolvedModuleKey, moduleResolver);
+
+    // set ModuleInfo early to handle cyclic dependencies
+    emptyModule.setExtraStorage(builder.getModuleInfo());
+
     var moduleNode = builder.visitModule(moduleContext);
     moduleNode.getCallTarget().call(emptyModule, emptyModule);
     MinPklVersionChecker.check(emptyModule, importNode);

pkl-core/src/main/java/org/pkl/core/runtime/VmTyped.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.
@@ -60,6 +60,10 @@ public VmClass getVmClass() {
     return clazz;
   }
 
+  public boolean isNotInitialized() {
+    return clazz == null;
+  }
+
   public @Nullable VmTyped getParent() {
     return (VmTyped) parent;
   }

pkl-core/src/test/files/LanguageSnippetTests/input-helper/cycles/Foo.pkl

@@ -0,0 +1,5 @@
+import "Qux.pkl"
+
+prop: Qux?
+
+typealias Bar = "bar"

pkl-core/src/test/files/LanguageSnippetTests/input-helper/cycles/Qux.pkl

@@ -0,0 +1,3 @@
+import "amendsFoo.pkl"
+
+res: amendsFoo.Bar

pkl-core/src/test/files/LanguageSnippetTests/input-helper/cycles/amendsFoo.pkl

@@ -0,0 +1 @@
+amends "Foo.pkl"

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

@@ -0,0 +1,3 @@
+import "../../input-helper/cycles/amendsFoo.pkl"
+
+prop: amendsFoo

pkl-core/src/test/files/LanguageSnippetTests/input/modules/cycles.pkl

@@ -0,0 +1,3 @@
+import "../../input-helper/cycles/amendsFoo.pkl"
+
+foo: amendsFoo.Bar = "bar"

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

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Module `Foo` cannot be extended or used as type because it amends another module.
+
+x | prop: amendsFoo
+          ^^^^^^^^^
+at cyclicalAmendsType (file:///$snippetsDir/input/errors/cyclicalAmendsType.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/modules/cycles.pcf

@@ -0,0 +1 @@
+foo = "bar"