From f284a0560a397e478147b26f23fd6efe117da6f1 Mon Sep 17 00:00:00 2001
From: Jen Basch <jbasch@apple.com>
Date: Thu, 3 Sep 2026 16:03:34 -0700
Subject: [PATCH] Error when inferring a self type in a method argument (#1847)

---
 .../expression/member/AbstractInferParentNode.java |  2 +-
 .../InferParentWithinMethodArgumentNode.java       | 14 ++++++++++++++
 .../main/java/org/pkl/core/ast/type/TypeNode.java  | 14 ++++++++++++++
 .../input/methods/inferParameterType.pkl           |  8 ++++++++
 .../output/methods/inferParameterType.pcf          |  3 +++
 5 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/pkl-core/src/main/java/org/pkl/core/ast/expression/member/AbstractInferParentNode.java b/pkl-core/src/main/java/org/pkl/core/ast/expression/member/AbstractInferParentNode.java
index bf16fd239..23076fb66 100644
--- a/pkl-core/src/main/java/org/pkl/core/ast/expression/member/AbstractInferParentNode.java
+++ b/pkl-core/src/main/java/org/pkl/core/ast/expression/member/AbstractInferParentNode.java
@@ -36,7 +36,7 @@ public abstract class AbstractInferParentNode extends ExpressionNode {
     this.language = language;
   }
 
-  protected final Object getDefaultValue(
+  protected Object getDefaultValue(
       VirtualFrame frame,
       @Nullable TypeNode typeNode,
       SourceSection headerSection,
diff --git a/pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinMethodArgumentNode.java b/pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinMethodArgumentNode.java
index 8b3469106..9d2989aa8 100644
--- a/pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinMethodArgumentNode.java
+++ b/pkl-core/src/main/java/org/pkl/core/ast/expression/member/InferParentWithinMethodArgumentNode.java
@@ -85,4 +85,18 @@ public abstract class InferParentWithinMethodArgumentNode
     var typeNode = getTypeNode(frame, method);
     return getDefaultValue(frame, typeNode, method.getHeaderSection(), method.getQualifiedName());
   }
+
+  @Override
+  protected Object getDefaultValue(
+      VirtualFrame frame,
+      @Nullable TypeNode typeNode,
+      SourceSection headerSection,
+      String qualifiedName) {
+    if (typeNode != null && typeNode.isSelfType()) {
+      CompilerDirectives.transferToInterpreter();
+      throw exceptionBuilder().evalError("cannotInferParent").build();
+    }
+
+    return super.getDefaultValue(frame, typeNode, headerSection, qualifiedName);
+  }
 }
diff --git a/pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java b/pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java
index 7c9859fc9..951cc343c 100644
--- a/pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java
+++ b/pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java
@@ -155,6 +155,20 @@ public abstract class TypeNode extends PklNode {
     return ret.get();
   }
 
+  public final boolean isSelfType() {
+    var ret = new MutableBoolean(false);
+    acceptTypeNode(
+        true,
+        typeNode -> {
+          if (typeNode instanceof NonFinalSelfTypeNode || typeNode instanceof FinalSelfTypeNode) {
+            ret.set(true);
+            return false;
+          }
+          return true;
+        });
+    return ret.get();
+  }
+
   /** Visit child type nodes of this type. */
   protected abstract boolean acceptTypeNode(boolean visitTypeArguments, TypeNodeConsumer consumer);
 
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/methods/inferParameterType.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/methods/inferParameterType.pkl
index 3e059dd1d..8356412f7 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/input/methods/inferParameterType.pkl
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/methods/inferParameterType.pkl
@@ -16,7 +16,12 @@ class Qux extends Foo {
   function bar(baz: Foo): Int = thisBar(new { x = baz.x })
 }
 
+class Quux {
+  function selfType(arg: this): String = arg.getClass().toString()
+}
+
 function bar(baz: Foo): Int = baz.x
+function selfType(arg: this): String = arg.getClass().toString()
 
 const function outerMethod(a: Int, b: Qux) = a + b.x * 2
 
@@ -41,6 +46,9 @@ intrinsicConstructor = test.catch(() -> List(new {}))
 genericMethod = test.catch(() -> Pair(new {}, new {}))
 fnApply = test.catch(() -> quux.apply(new {}))
 tooManyParams = test.catch(() -> bar(new {}, new {}))
+finalSelfType = test.catch(() -> new Quux {}.selfType(new {}))
+nonFinalSelfType1 = test.catch(() -> selfType(new {}))
+nonFinalSelfType2 = test.catch(() -> new Qux {}.selfType(new {}))
 
 class Bar1 { x: Int = 1 }
 class Bar2 { x: Int = 2 }
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/methods/inferParameterType.pcf b/pkl-core/src/test/files/LanguageSnippetTests/output/methods/inferParameterType.pcf
index ff693a58b..272c5447b 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/output/methods/inferParameterType.pcf
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/methods/inferParameterType.pcf
@@ -14,5 +14,8 @@ intrinsicConstructor = "Cannot tell which parent to amend."
 genericMethod = "Cannot tell which parent to amend."
 fnApply = "Cannot tell which parent to amend."
 tooManyParams = "Expected 1 function argument but got 2."
+finalSelfType = "Cannot tell which parent to amend."
+nonFinalSelfType1 = "Cannot tell which parent to amend."
+nonFinalSelfType2 = "Cannot tell which parent to amend."
 polyCall1 = 1
 polyCall2 = 20