Cannot assign property with fixed/const in spread (#428)

When using spread syntax with typed object, properties that are fixed
or const cannot be assigned or amended.
This should be checked in the checkTypedProperty() function.

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorSpreadNode.java

@@ -343,19 +343,39 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
   }
 
   protected void checkTypedProperty(VmClass clazz, ObjectMember member) {
-    if (member.isLocal() || clazz.hasProperty(member.getName())) return;
-    CompilerDirectives.transferToInterpreter();
-    var exception =
-        exceptionBuilder()
-            .cannotFindProperty(clazz.getPrototype(), member.getName(), false, false)
-            .build();
-    if (member.getHeaderSection().isAvailable()) {
-      exception
-          .getInsertedStackFrames()
-          .put(
-              getRootNode().getCallTarget(),
-              VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
+    if (member.isLocal()) return;
+
+    var memberName = member.getName();
+    if (!clazz.hasProperty(memberName)) {
+      CompilerDirectives.transferToInterpreter();
+      var exception =
+          exceptionBuilder()
+              .cannotFindProperty(clazz.getPrototype(), memberName, false, false)
+              .build();
+      if (member.getHeaderSection().isAvailable()) {
+        exception
+            .getInsertedStackFrames()
+            .put(
+                getRootNode().getCallTarget(),
+                VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
+      }
+      throw exception;
+    }
+
+    var classProperty = clazz.getProperty(memberName);
+    if (classProperty != null && classProperty.isConstOrFixed()) {
+      CompilerDirectives.transferToInterpreter();
+      var errMsg =
+          classProperty.isConst() ? "cannotAssignConstProperty" : "cannotAssignFixedProperty";
+      var exception = exceptionBuilder().evalError(errMsg, memberName).build();
+      if (member.getHeaderSection().isAvailable()) {
+        exception
+            .getInsertedStackFrames()
+            .put(
+                getRootNode().getCallTarget(),
+                VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
+      }
+      throw exception;
     }
-    throw exception;
   }
 }

pkl-core/src/test/files/LanguageSnippetTests/input/errors/cannotAssignFixedProperty3.pkl

@@ -0,0 +1,5 @@
+amends "../basic/fixedProperty2.pkl"
+
+p = new {
+  ...new Dynamic { name = "Osprey" }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/cannotAssignFixedProperty3.err

@@ -0,0 +1,14 @@
+–– Pkl Error ––
+Cannot assign to fixed property `name`.
+
+x | ...new Dynamic { name = "Osprey" }
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at cannotAssignFixedProperty3#p (file:///$snippetsDir/input/errors/cannotAssignFixedProperty3.pkl)
+
+x | ...new Dynamic { name = "Osprey" }
+                     ^^^^
+at cannotAssignFixedProperty3#p.name (file:///$snippetsDir/input/errors/cannotAssignFixedProperty3.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)