Error when JSON/YAML parsing would set property default in a Dynamic

Resolves apple#561

pkl-core/src/main/java/org/pkl/core/stdlib/json/ParserNodes.java

@@ -172,6 +172,11 @@ public void endObject(@Nullable EconomicMap<Object, ObjectMember> members) {
 
     @Override
     public void startObjectValue(@Nullable EconomicMap<Object, ObjectMember> members, String name) {
+      if (!useMapping && "default".equals(name)) {
+        // https://github.com/apple/pkl/issues/561
+        throw new ParseException(
+            "Cannot parse object key `default` into a `Dynamic` value", getLocation());
+      }
       currPath.push(Identifier.get(name));
     }
 

pkl-core/src/main/java/org/pkl/core/stdlib/yaml/ParserNodes.java

@@ -469,6 +469,12 @@ private void addMembers(MappingNode node, VmObject object) {
           var memberName =
               convertedKey instanceof String string && !useMapping ? Identifier.get(string) : null;
 
+          // https://github.com/apple/pkl/issues/561
+          if (memberName != null && "default".equals(convertedKey)) {
+            throw new YamlEngineException(
+                "Cannot parse object key `default` into a Dynamic value. Node: " + node);
+          }
+
           var member =
               new ObjectMember(
                   sourceSection,

pkl-core/src/test/files/LanguageSnippetTests/input/api/jsonParser1.pkl

@@ -66,3 +66,11 @@ res16 = parser.parse("""
 
 // invalid syntax
 res17 = test.catch(() -> parser.parse("!@#$%"))
+
+res18 = test.catch(() -> parser.parse("""
+  {"default": null}
+  """))
+
+res19 = (parser) { useMapping = true }.parse("""
+  {"default": null}
+  """)

pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlParser2.pkl

@@ -91,3 +91,11 @@ res15 = parser.parseAll("""
   hobby: surfing
   ...
   """)
+
+res16 = test.catch(() -> parser.parse("""
+  {"default": null}
+  """))
+
+res17 = (parser) { useMapping = true }.parse("""
+  default: null
+  """)

pkl-core/src/test/files/LanguageSnippetTests/output/api/jsonParser1.pcf

@@ -48,3 +48,7 @@ res16 {
   }
 }
 res17 = "Error parsing JSON document."
+res18 = "Error parsing JSON document."
+res19 {
+  ["default"] = null
+}

pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlParser2.pcf

@@ -47,3 +47,7 @@ res15 = List(new {
 }, new {
   hobby = "surfing"
 })
+res16 = "Error parsing YAML document."
+res17 {
+  ["default"] = null
+}