Bug 1793959 - Add ThrowMsgKind::DecoratorInvalidReturnType; r=mgaudet

We'll need to be able to throw a TypeError if the decorator returns something
other than a callable or undefined.

Differential Revision: https://phabricator.services.mozilla.com/D162140

js/public/friend/ErrorNumbers.msg

@@ -799,4 +799,7 @@ MSG_DEF(JSMSG_SHADOW_REALM_EXPORT_NOT_STRING,    0, JSEXN_TYPEERR, "exportName m
 MSG_DEF(JSMSG_SHADOW_REALM_IMPORTVALUE_FAILED,    0, JSEXN_TYPEERR, "import value failed")
 MSG_DEF(JSMSG_SHADOW_REALM_VALUE_NOT_EXPORTED,    0, JSEXN_TYPEERR, "value not exported")
 
+// Decorators
+MSG_DEF(JSMSG_DECORATOR_INVALID_RETURN_TYPE, 0, JSEXN_TYPEERR, "Invalid value returned from decorator: decorators can only return undefined or a callable")
+
 //clang-format on

js/src/vm/ThrowMsgKind.cpp

@@ -28,6 +28,8 @@ JSErrNum js::ThrowMsgKindToErrNum(ThrowMsgKind kind) {
       return JSMSG_SET_MISSING_PRIVATE;
     case ThrowMsgKind::AssignToPrivateMethod:
       return JSMSG_ASSIGN_TO_PRIVATE_METHOD;
+    case ThrowMsgKind::DecoratorInvalidReturnType:
+      return JSMSG_DECORATOR_INVALID_RETURN_TYPE;
   }
 
   MOZ_CRASH("Unexpected message kind");

js/src/vm/ThrowMsgKind.h

@@ -23,6 +23,8 @@ enum class ThrowMsgKind : uint8_t {
   MissingPrivateOnGet,
   MissingPrivateOnSet,
   AssignToPrivateMethod,
+  // Decorators:
+  DecoratorInvalidReturnType,
 };
 
 JSErrNum ThrowMsgKindToErrNum(ThrowMsgKind kind);