Enforce stricter rules on reference access (#1718)

This changes the following:

1. If given a union type, the member must exist on each member of that type
2. If accessing a member off `Reference<D, Null>`, give a `Reference<D, Null>`

Also:
* Improve error messages thrown during member access.
* Add test around accessing members off of a function type

pkl-core/src/main/java/org/pkl/core/ast/expression/binary/SubscriptNode.java

@@ -23,7 +23,11 @@ import com.oracle.truffle.api.dsl.Specialization;
 import com.oracle.truffle.api.nodes.IndirectCallNode;
 import com.oracle.truffle.api.nodes.NodeInfo;
 import com.oracle.truffle.api.source.SourceSection;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.runtime.*;
+import org.pkl.core.runtime.VmReference.VmReferenceAccessError;
+import org.pkl.core.runtime.VmReference.VmReferenceAccessErrorType;
+import org.pkl.core.util.ErrorMessages;
 
 @NodeInfo(shortName = "[]")
 public abstract class SubscriptNode extends BinaryExpressionNode {
@@ -100,18 +104,32 @@ public abstract class SubscriptNode extends BinaryExpressionNode {
     return readMember(dynamic, key, callNode);
   }
 
+  private @Nullable String getReferenceHint(
+      VmReference reference, VmReferenceAccessError err, Object key) {
+    var myType = reference.getReferentType();
+    if (err.getErrorType() != VmReferenceAccessErrorType.CANNOT_FIND_MEMBER) {
+      return null;
+    }
+    return err.getType().equals(myType)
+        ? ErrorMessages.create("operatorNotDefined2", getShortName(), myType, VmUtils.getClass(key))
+        : ErrorMessages.create(
+            "operatorNotDefined3", getShortName(), myType, err.getType(), VmUtils.getClass(key));
+  }
+
   @Specialization
   protected VmReference eval(VmReference reference, Object key) {
-    var result = reference.withSubscriptAccess(key);
-    if (result != null) return result;
-
-    CompilerDirectives.transferToInterpreter();
-    throw exceptionBuilder()
-        .evalError(
-            "operatorNotDefined2", getShortName(), reference.exportType(), VmUtils.getClass(key))
-        .withProgramValue("Left operand", reference)
-        .withProgramValue("Right operand", key)
-        .build();
+    try {
+      return reference.withSubscriptAccess(key);
+    } catch (VmReferenceAccessError err) {
+      CompilerDirectives.transferToInterpreter();
+      throw exceptionBuilder()
+          .evalError(
+              "operatorNotDefined2", getShortName(), reference.exportType(), VmUtils.getClass(key))
+          .withProgramValue("Left operand", reference)
+          .withProgramValue("Right operand", key)
+          .withHint(getReferenceHint(reference, err, key))
+          .build();
+    }
   }
 
   @Specialization

pkl-core/src/main/java/org/pkl/core/ast/expression/member/ReadPropertyNode.java

@@ -24,10 +24,13 @@ import com.oracle.truffle.api.nodes.IndirectCallNode;
 import com.oracle.truffle.api.nodes.NodeInfo;
 import com.oracle.truffle.api.source.SourceSection;
 import org.jspecify.annotations.Nullable;
+import org.pkl.core.PType;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.MemberLookupMode;
 import org.pkl.core.ast.member.ClassProperty;
 import org.pkl.core.runtime.*;
+import org.pkl.core.runtime.VmReference.VmReferenceAccessError;
+import org.pkl.core.util.ErrorMessages;
 
 @NodeInfo(shortName = ".")
 @ImportStatic(BaseModule.class)
@@ -61,16 +64,38 @@ public abstract class ReadPropertyNode extends ExpressionNode {
     this(sourceSection, propertyName, MemberLookupMode.EXPLICIT_RECEIVER, false);
   }
 
+  protected String getReferenceErrorHint(VmReference reference, VmReferenceAccessError err) {
+    var myType = reference.getReferentType();
+    return switch (err.getErrorType()) {
+      case CANNOT_FIND_MEMBER ->
+          err.getType().equals(myType)
+              ? ErrorMessages.create("cannotFindPropertyInType", propertyName, myType)
+              : ErrorMessages.create(
+                  "cannotFindPropertyInType2", propertyName, myType, err.getType());
+      case EXTERNAL_MEMBER ->
+          ErrorMessages.create("cannotReferenceExternalProperty", propertyName, err.getType());
+      case DEFAULT_MEMBER ->
+          ErrorMessages.create(
+              "cannotReferenceDefaultProperty",
+              ((PType.Class) err.getType()).getPClass().getSimpleName());
+      case EXTERNAL_CLASS ->
+          ErrorMessages.create(
+              "cannotReferencePropertyInExternalClass", propertyName, err.getType());
+    };
+  }
+
   @Specialization
   protected VmReference evalReference(VmReference receiver) {
     assert lookupMode == MemberLookupMode.EXPLICIT_RECEIVER;
-    var result = receiver.withPropertyAccess(propertyName);
-    if (result != null) return result;
-
-    CompilerDirectives.transferToInterpreter();
-    throw exceptionBuilder()
-        .evalError("cannotFindPropertyInReference", propertyName, receiver.exportType())
-        .build();
+    try {
+      return receiver.withPropertyAccess(propertyName);
+    } catch (VmReferenceAccessError err) {
+      CompilerDirectives.transferToInterpreter();
+      throw exceptionBuilder()
+          .evalError("cannotFindPropertyInObjectNoHint", propertyName, receiver.exportType())
+          .withHint(getReferenceErrorHint(receiver, err))
+          .build();
+    }
   }
 
   // This method effectively covers `VmObject receiver` but is implemented in a more

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

@@ -42,7 +42,8 @@ public final class VmReference extends VmValue {
   private final Object data;
   private final ImRrbt<VmTyped> path;
   // candidate types can only be: PType.Class, PType.Alias (only preservedAliasTypes),
-  // PType.StringLiteral, PType.UNKNOWN, or PType.Union (containing only the previous; flattened)
+  // PType.StringLiteral, PType.UNKNOWN, PType.Function, PType.TypeVariable, or PType.Union
+  // (containing only the previous; flattened)
   private final PType referentType;
 
   private boolean forced = false;
@@ -131,9 +132,12 @@ public final class VmReference extends VmValue {
         }
         result.add(new PType.Class(clazz.getPClass(), typeArgs));
       }
-    } else if (type instanceof PType.Nullable nullable) {
+    }
+    // normalize `T?` to `T | Null`
+    else if (type instanceof PType.Nullable nullable) {
       normalizeTypes(nullable.getBaseType(), moduleClass, result);
       result.add(new PType.Class(BaseModule.getNullClass().export()));
+      // erase `T(someConstraint)` to `T`
     } else if (type instanceof PType.Constrained constrained) {
       normalizeTypes(constrained.getBaseType(), moduleClass, result);
     } else if (type instanceof PType.Alias alias) {
@@ -148,6 +152,11 @@ public final class VmReference extends VmValue {
       }
     } else if (type == PType.MODULE) {
       result.add(new PType.Class(moduleClass));
+    } else {
+      // remaining types: PType.Function, PType.TypeVariable. no normalizing needed; TypeVariable
+      // gets replaced upon instantiation, and Function can bubble up to users as a reference error
+      // if accessed.
+      result.add(type);
     }
   }
 
@@ -156,29 +165,26 @@ public final class VmReference extends VmValue {
     return Collections.singleton(t);
   }
 
-  public @Nullable VmReference withPropertyAccess(Identifier property) {
+  public VmReference withPropertyAccess(Identifier property) {
     var propString = property.toString();
     return withAccess(
         (t, candidates) -> getCandidatePropertyType(t, propString, candidates),
         () -> newAccess(property.toString(), null));
   }
 
-  public @Nullable VmReference withSubscriptAccess(Object key) {
+  public VmReference withSubscriptAccess(Object key) {
     return withAccess(
         (t, candidates) -> getCandidateSubscriptType(t, key, candidates),
         () -> newAccess(null, key));
   }
 
   @TruffleBoundary
-  private @Nullable VmReference withAccess(
+  private VmReference withAccess(
       BiConsumer<PType, Set<PType>> checkCandidate, Supplier<VmTyped> makeAccess) {
     Set<PType> candidates = new HashSet<>();
     for (var t : iterateTypes(referentType)) {
       checkCandidate.accept(t, candidates);
     }
-    if (candidates.isEmpty()) {
-      return null; // no valid access found
-    }
     return new VmReference(domain, data, path.append(makeAccess.get()), minimizeTypes(candidates));
   }
 
@@ -189,33 +195,54 @@ public final class VmReference extends VmValue {
       return;
     }
     // restriction: only class types can have their properties referenced
-    if (!(type instanceof PType.Class clazz)) return;
-    // restriction: cannot reference properties of external classes
-    if (clazz.getPClass().isExternal()) return;
+    if (!(type instanceof PType.Class clazz)) {
+      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
+    }
     if (clazz.getPClass().getInfo() == PClassInfo.Dynamic) {
-      // restriction: cannot reference Dynamic.default
-      if (!property.equals("default")) result.add(PType.UNKNOWN);
+      if (property.equals("default")) {
+        // restriction: cannot reference Dynamic.default
+        throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.DEFAULT_MEMBER);
+      }
+      result.add(PType.UNKNOWN);
       return;
     }
     // restriction: cannot reference Listing/Mapping.default
     if (clazz.getPClass().getInfo() == PClassInfo.Listing
         || clazz.getPClass().getInfo() == PClassInfo.Mapping) {
-      return;
+      var errorType =
+          property.equals("default")
+              ? VmReferenceAccessErrorType.DEFAULT_MEMBER
+              : VmReferenceAccessErrorType.CANNOT_FIND_MEMBER;
+      throw new VmReferenceAccessError(type, errorType);
     }
+    var baseModule = BaseModule.getModuleClass().export();
     // restriction: cannot reference Module.output.
     //   generalized: properties originally defined in external classes; the only extant example.
     // This is implemented specifically because this is the only case where an external class
     //   containing a property can be subclassed.
     // And this can't check prop.getOwner().isExternal() because fully overriding the property with
     //   a new type annotation means the owner isn't Module.
-    if (clazz.getPClass().isSubclassOf(BaseModule.getModuleClass().export())
-        && property.equals("output")) return;
+    if (clazz.getPClass().isSubclassOf(baseModule) && property.equals("output")) {
+      throw new VmReferenceAccessError(
+          new PType.Class(baseModule), VmReferenceAccessErrorType.EXTERNAL_CLASS);
+    }
 
-    var prop = clazz.getPClass().getAllProperties().get(property);
-    // restriction: cannot reference external properties
-    if (prop == null || prop.isExternal()) {
+    // dot access on `Reference<D, Null>` gives `Reference<D, Null>`
+    if (clazz.getPClass().getInfo() == PClassInfo.Null) {
+      result.add(clazz);
       return;
     }
+
+    var prop = clazz.getPClass().getAllProperties().get(property);
+    if (prop == null) {
+      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
+    }
+
+    // restriction: cannot reference external properties
+    if (prop.isExternal()) {
+      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.EXTERNAL_MEMBER);
+    }
+
     normalizeTypes(prop.getType(), clazz.getPClass().getModuleClass(), result);
   }
 
@@ -235,7 +262,7 @@ public final class VmReference extends VmValue {
       return;
     }
     if (!(type instanceof PType.Class clazz)) {
-      return;
+      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
     }
     if (clazz.getPClass().getInfo() == PClassInfo.Dynamic) {
       result.add(PType.UNKNOWN);
@@ -243,9 +270,10 @@ public final class VmReference extends VmValue {
     }
     if (clazz.getPClass().getInfo() == PClassInfo.Listing
         || clazz.getPClass().getInfo() == PClassInfo.List) {
-      if (key instanceof Long) {
-        normalizeTypes(clazz.getTypeArguments().get(0), clazz.getPClass().getModuleClass(), result);
+      if (!(key instanceof Long)) {
+        throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
       }
+      normalizeTypes(clazz.getTypeArguments().get(0), clazz.getPClass().getModuleClass(), result);
       return;
     }
     if (clazz.getPClass().getInfo() == PClassInfo.Mapping
@@ -262,6 +290,14 @@ public final class VmReference extends VmValue {
         }
       }
     }
+
+    // subscript access on `Reference<D, Null>` gives `Reference<D, Null>`
+    if (clazz.getPClass().getInfo() == PClassInfo.Null) {
+      result.add(clazz);
+      return;
+    }
+
+    throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
   }
 
   /**
@@ -490,4 +526,29 @@ public final class VmReference extends VmValue {
     var callNode = DirectCallNode.create(toStringMethod.getCallTarget());
     return (String) callNode.call(this, getVmClass().getPrototype());
   }
+
+  public enum VmReferenceAccessErrorType {
+    CANNOT_FIND_MEMBER,
+    EXTERNAL_MEMBER,
+    DEFAULT_MEMBER,
+    EXTERNAL_CLASS
+  }
+
+  public static final class VmReferenceAccessError extends RuntimeException {
+    private final PType type;
+    private final VmReferenceAccessErrorType errorType;
+
+    public VmReferenceAccessError(PType type, VmReferenceAccessErrorType errorType) {
+      this.type = type;
+      this.errorType = errorType;
+    }
+
+    public PType getType() {
+      return type;
+    }
+
+    public VmReferenceAccessErrorType getErrorType() {
+      return errorType;
+    }
+  }
 }

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

@@ -319,6 +319,10 @@ Operator `{0}` is not defined for operand type `{1}`.
 operatorNotDefined2=\
 Operator `{0}` is not defined for operand types `{1}` and `{2}`.
 
+operatorNotDefined3=\
+Operator `{0}` is not defined for operand types `{1}` and `{3}`.\n\
+\t└─ Operator `{0}` is not defined for operand types `{2}` and `{3}`.
+
 operatorNotDefinedLeft=\
 Operator `{0}` is not defined for left operand type `{1}`.
 
@@ -388,9 +392,25 @@ Cannot find property `{0}` in object of type `{1}`.\n\
 Did you mean any of the following?\n\
 {2}
 
-cannotFindPropertyInReference=\
+cannotFindPropertyInObjectNoHint=\
 Cannot find property `{0}` in object of type `{1}`.
 
+cannotFindPropertyInType=\
+Cannot find property `{0}` in type `{1}`.
+
+cannotFindPropertyInType2=\
+Cannot find property `{0}` in type `{1}`.\n\
+\t└─ Cannot find property `{0}` in type `{2}`.
+
+cannotReferenceExternalProperty=\
+Cannot reference external property `{0}` in class `{1}`.
+
+cannotReferenceDefaultProperty=\
+Cannot reference property `default` in class `{0}`.
+
+cannotReferencePropertyInExternalClass=\
+Cannot reference property `{0}` because it is defined inside external class `{1}`.
+
 cannotFindMethodInScope=\
 Cannot find method `{0}`.
 

pkl-core/src/test/files/LanguageSnippetTests/input/api/reference.pkl

@@ -36,6 +36,7 @@ class AProperties {
   someListing: Listing<Int>
   someList: List<Int>
   nonInt: String
+  nullProp: Null
 }
 
 class B extends Resource {
@@ -64,6 +65,7 @@ class K {
   bId: String | *Ref<String>?
   aProperties: AProperties | *Ref<AProperties>?
   bProperties: BProperties | *Ref<BProperties>?
+  nullProp: Ref<Null>?
   aValues: Listing<Int | Ref<Int?>>?
   bValues: Listing<String | Ref<String?>>?
   splitUnion: Ref<Listing<String> | Listing<Int>>?
@@ -91,6 +93,7 @@ k: K = new {
   bId = bRef.id
   aProperties = aRef.outputs
   bProperties = bRef.outputs
+  nullProp = aRef.outputs.nullProp.prop1.prop2["foo"].prop3
   aValues {
     aRef.outputs.foo
     aRef.outputs.someMapping["key"]

pkl-core/src/test/files/LanguageSnippetTests/input/basic/reference.pkl

@@ -17,39 +17,40 @@ local class Holder {
   optional: Mapping<String, Int?>
   listing: Listing<Int>
   union: Listing<String> | Listing<Int>
+  constrainedType: String(isEmpty)
 }
 
-local const h: Holder = new {
-  num = 1
-  text = "t"
-  lit = "literal"
-  small = 8
-  optional { ["k"] = 5 }
-  listing { 1; 2 }
-  union = new Listing<Int> { 1 }
+local class OtherHolder {
+  num: Int?
+  constrainedType: String(isEmpty)
 }
 
-local hRef1: Ref<Holder> = ref.Reference(d, Holder, h)
-local hRef2: Ref<Holder> = ref.Reference(d, Holder, h)
+local holderRef: Ref<Holder> = ref.Reference(d, Holder, null)
+local holderRef2: Ref<Holder> = ref.Reference(d, Holder, null)
+local otherHolderRef: Ref<OtherHolder> = ref.Reference(d, OtherHolder, null)
 
 facts {
   ["equality"] {
-    hRef1 == hRef2
-    hRef1.num == hRef2.num
-    hRef1.lit == hRef2.lit
-    hRef1.small == hRef2.small
-    hRef1.optional["k"] == hRef2.optional["k"]
-    hRef1.listing[0] == hRef2.listing[0]
-    hRef1.union == hRef2.union
+    holderRef == holderRef2
+    holderRef.num == holderRef2.num
+    holderRef.lit == holderRef2.lit
+    holderRef.small == holderRef2.small
+    holderRef.optional["k"] == holderRef2.optional["k"]
+    holderRef.listing[0] == holderRef2.listing[0]
+    holderRef.union == holderRef2.union
+  }
+
+  ["equality -- constraint gets erased"] {
+    holderRef.constrainedType == otherHolderRef.constrainedType
   }
 
   ["inequality"] {
-    hRef1.num != hRef2.text
+    holderRef.num != holderRef2.text
   }
 
   ["set deduplication"] {
-    Set(hRef1.num, hRef2.num).length == 1
-    Set(hRef1.union, hRef2.union).length == 1
-    Set(hRef1.num, hRef2.text).length == 2
+    Set(holderRef.num, holderRef2.num).length == 1
+    Set(holderRef.union, holderRef2.union).length == 1
+    Set(holderRef.num, holderRef2.text).length == 2
   }
 }

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference11.pkl

@@ -6,7 +6,7 @@ class D extends ref.Domain {
 typealias Ref<T> = ref.Reference<D, T>
 local d: D = new {}
 
-test = ref.Reference(d, String, "") as Ref<Alias1?>
+test = ref.Reference(d, String, "") as Ref<Alias1>
 
-typealias Alias1 = Int | Alias2?
+typealias Alias1 = Int | Alias2
 typealias Alias2 = String(length < 5)

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference24.pkl

@@ -0,0 +1,17 @@
+import "pkl:ref"
+
+class D extends ref.Domain {
+  function renderReference(_): String = throw("not implemented")
+}
+
+class Bird {
+  name: String
+}
+
+class Holder {
+  $: Bird | Int
+}
+
+myRef: ref.Reference<D, Bird | Int> = ref.Reference(new D {}, Holder, null).$
+
+res = myRef.name

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference25.pkl

@@ -0,0 +1,13 @@
+import "pkl:ref"
+
+class D extends ref.Domain {
+  function renderReference(_): String = throw("not implemented")
+}
+
+class Holder {
+  $: Duration | Listing<String>
+}
+
+local myRef = ref.Reference(new D {}, Holder, null).$
+
+res = myRef[0]

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference26.pkl

@@ -0,0 +1,9 @@
+import "pkl:ref"
+
+class D extends ref.Domain {
+  function renderReference(_): String = throw("not implemented")
+}
+
+local myRef = ref.Reference(new D {}, Listing, null)
+
+res = myRef.name

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference27.pkl

@@ -0,0 +1,13 @@
+import "pkl:ref"
+
+class D extends ref.Domain {
+  function renderReference(_): String = throw("not implemented")
+}
+
+class Holder {
+  func: (Int) -> Boolean
+}
+
+local myRef = ref.Reference(new D {}, Holder, null).func
+
+res = myRef.name

pkl-core/src/test/files/LanguageSnippetTests/input/errors/reference4.pkl

@@ -6,7 +6,7 @@ class D extends ref.Domain {
 local d: D = new {}
 typealias Ref<T> = ref.Reference<D, T>
 
-test = ref.Reference(d, String, "") as Ref<Alias1?>
+test = ref.Reference(d, String, "") as Ref<Alias1>
 
-typealias Alias1 = Int | Alias2?
+typealias Alias1 = Int | Alias2
 typealias Alias2 = Boolean

pkl-core/src/test/files/LanguageSnippetTests/output/api/reference.pcf

@@ -20,6 +20,7 @@ k {
   bId = "${b.id}"
   aProperties = "${a.outputs}"
   bProperties = "${b.outputs}"
+  nullProp = "${a.outputs.nullProp.prop1.prop2[foo].prop3}"
   aValues {
     "${a.outputs.foo}"
     "${a.outputs.someMapping[key]}"
@@ -43,12 +44,13 @@ j {
   bId = null
   aProperties = "${a.outputs}"
   bProperties = null
+  nullProp = null
   aValues = null
   bValues = null
   splitUnion = "${a.outputs.someListing}"
 }
 refInterpolation = "${a.outputs.someListing[1]}"
-kInterpolation = "new K { aId = Reference(new D {}, String, new A { name = \"a\"; id = \"some-a-value\" }).id; bId = Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).id; aProperties = Reference(new D {}, reference#AProperties, new A { name = \"a\"; id = \"some-a-value\" }).outputs; bProperties = Reference(new D {}, reference#BProperties, new B { name = \"b\"; id = \"some-b-value\" }).outputs; aValues { Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.foo; Reference(new D {}, Int | Null, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someMapping[\"key\"]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someMap[new MapKey { k = 123 }]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someListing[0]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someList[9223372036854775807]; Reference(new D {}, Int, new B { name = \"b\"; id = \"some-b-value\" }).outputs.nonString }; bValues { Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.foo; Reference(new D {}, Null | String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someMapping[\"key\"]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someMap[new MapKey { k = 123 }]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someListing[0]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someList[9223372036854775807]; Reference(new D {}, String, new A { name = \"a\"; id = \"some-a-value\" }).outputs.nonInt }; splitUnion = null }"
+kInterpolation = "new K { aId = Reference(new D {}, String, new A { name = \"a\"; id = \"some-a-value\" }).id; bId = Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).id; aProperties = Reference(new D {}, reference#AProperties, new A { name = \"a\"; id = \"some-a-value\" }).outputs; bProperties = Reference(new D {}, reference#BProperties, new B { name = \"b\"; id = \"some-b-value\" }).outputs; nullProp = Reference(new D {}, Null, new A { name = \"a\"; id = \"some-a-value\" }).outputs.nullProp.prop1.prop2[\"foo\"].prop3; aValues { Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.foo; Reference(new D {}, Int | Null, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someMapping[\"key\"]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someMap[new MapKey { k = 123 }]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someListing[0]; Reference(new D {}, Int, new A { name = \"a\"; id = \"some-a-value\" }).outputs.someList[9223372036854775807]; Reference(new D {}, Int, new B { name = \"b\"; id = \"some-b-value\" }).outputs.nonString }; bValues { Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.foo; Reference(new D {}, Null | String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someMapping[\"key\"]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someMap[new MapKey { k = 123 }]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someListing[0]; Reference(new D {}, String, new B { name = \"b\"; id = \"some-b-value\" }).outputs.someList[9223372036854775807]; Reference(new D {}, String, new A { name = \"a\"; id = \"some-a-value\" }).outputs.nonInt }; splitUnion = null }"
 aValuesJoined = """
   ${a.outputs.foo}
   ${a.outputs.someMapping[key]}

pkl-core/src/test/files/LanguageSnippetTests/output/basic/reference.pcf

@@ -8,6 +8,9 @@ facts {
     true
     true
   }
+  ["equality -- constraint gets erased"] {
+    true
+  }
   ["inequality"] {
     true
   }

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference11.err

@@ -5,7 +5,7 @@ xx | typealias Alias2 = String(length < 5)
                         ^^^^^^^^^^^^^^^^^^
 at reference11#Alias2 (file:///$snippetsDir/input/errors/reference11.pkl)
 
-xx | typealias Alias1 = Int | Alias2?
+xx | typealias Alias1 = Int | Alias2
                               ^^^^^^
 at reference11#Alias1 (file:///$snippetsDir/input/errors/reference11.pkl)
 
@@ -13,8 +13,8 @@ x | typealias Ref<T> = ref.Reference<D, T>
                        ^^^^^^^^^^^^^^^^^^^
 at reference11#Ref (file:///$snippetsDir/input/errors/reference11.pkl)
 
-x | test = ref.Reference(d, String, "") as Ref<Alias1?>
-                                           ^^^^^^^^^^^^
+x | test = ref.Reference(d, String, "") as Ref<Alias1>
+                                           ^^^^^^^^^^^
 at reference11#test (file:///$snippetsDir/input/errors/reference11.pkl)
 
 xxx | renderer.renderDocument(value)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference13.err

@@ -5,6 +5,8 @@ x | test = ref.Reference(d, Mapping, "").default
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference13#test (file:///$snippetsDir/input/errors/reference13.pkl)
 
+Cannot reference property `default` in class `Mapping`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference14.err

@@ -5,6 +5,8 @@ x | test = ref.Reference(d, Listing, "").default
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference14#test (file:///$snippetsDir/input/errors/reference14.pkl)
 
+Cannot reference property `default` in class `Listing`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference15.err

@@ -5,6 +5,8 @@ x | test = ref.Reference(d, Dynamic, "").default
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference15#test (file:///$snippetsDir/input/errors/reference15.pkl)
 
+Cannot reference property `default` in class `Dynamic`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference16.err

@@ -5,6 +5,8 @@ xx | test = ref.Reference(d, ReferencedModule.getClass(), "").output
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference16#test (file:///$snippetsDir/input/errors/reference16.pkl)
 
+Cannot reference property `output` because it is defined inside external class `Module`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference17.err

@@ -5,6 +5,8 @@ xx | test = ref.Reference(d, ReferencedModuleWithOutputOverride.getClass(), "").
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference17#test (file:///$snippetsDir/input/errors/reference17.pkl)
 
+Cannot reference property `output` because it is defined inside external class `Module`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference18.err

@@ -5,6 +5,8 @@ xx | test = ref.Reference(d, ModuleSubclass, "").output
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference18#test (file:///$snippetsDir/input/errors/reference18.pkl)
 
+Cannot reference property `output` because it is defined inside external class `Module`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference24.err

@@ -0,0 +1,17 @@
+–– Pkl Error ––
+Cannot find property `name` in object of type `pkl.ref#Reference<reference24#D, Int | reference24#Bird>`.
+
+xx | res = myRef.name
+           ^^^^^^^^^^
+at reference24#res (file:///$snippetsDir/input/errors/reference24.pkl)
+
+Cannot find property `name` in type `Int | reference24#Bird`.
+	└─ Cannot find property `name` in type `Int`.
+
+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/reference25.err

@@ -0,0 +1,19 @@
+–– Pkl Error ––
+Operator `[]` is not defined for operand types `pkl.ref#Reference<reference25#D, Duration | Listing<String>>` and `Int`.
+Left operand : Reference(new D {}, Duration | Listing<String>, null).$
+Right operand: 0
+
+xx | res = myRef[0]
+           ^^^^^^^^
+at reference25#res (file:///$snippetsDir/input/errors/reference25.pkl)
+
+Operator `[]` is not defined for operand types `Duration | Listing<String>` and `Int`.
+	└─ Operator `[]` is not defined for operand types `Duration` and `Int`.
+
+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/reference26.err

@@ -0,0 +1,16 @@
+–– Pkl Error ––
+Cannot find property `name` in object of type `pkl.ref#Reference<reference26#D, Listing<unknown>>`.
+
+x | res = myRef.name
+          ^^^^^^^^^^
+at reference26#res (file:///$snippetsDir/input/errors/reference26.pkl)
+
+Cannot find property `name` in type `Listing<unknown>`.
+
+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/reference27.err

@@ -0,0 +1,16 @@
+–– Pkl Error ––
+Cannot find property `name` in object of type `pkl.ref#Reference<reference27#D, (Int) -> Boolean>`.
+
+xx | res = myRef.name
+           ^^^^^^^^^^
+at reference27#res (file:///$snippetsDir/input/errors/reference27.pkl)
+
+Cannot find property `name` in type `(Int) -> Boolean`.
+
+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/reference4.err

@@ -1,5 +1,5 @@
 –– Pkl Error ––
-Expected value of type `pkl.ref#Reference<reference4#D, reference4#Alias1?>`, but got type `pkl.ref#Reference<reference4#D, String>`.
+Expected value of type `pkl.ref#Reference<reference4#D, reference4#Alias1>`, but got type `pkl.ref#Reference<reference4#D, String>`.
 Value: Reference(new D {}, String, "")
 
 x | typealias Ref<T> = ref.Reference<D, T>

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference6.err

@@ -5,6 +5,8 @@ xx | test = ref.Reference(d, String, new A {}).c
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference6#test (file:///$snippetsDir/input/errors/reference6.pkl)
 
+Cannot find property `c` in type `String`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/reference7.err

@@ -7,6 +7,8 @@ x | test = ref.Reference(d, List, List())["hi"]
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at reference7#test (file:///$snippetsDir/input/errors/reference7.pkl)
 
+Operator `[]` is not defined for operand types `List<unknown>` and `String`.
+
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 at pkl.base#Module.output.text (pkl:base)