LibWeb: Make ExceptionOr work with non-JS::Value types

Fixes #6075.

2 files changed, 43 insertions, 8 deletions

diff --git a/Userland/Libraries/LibWeb/Bindings/ExceptionOrUtils.h b/Userland/Libraries/LibWeb/Bindings/ExceptionOrUtils.h
index 463e3bc495..081c04f21e 100644
--- a/Userland/Libraries/LibWeb/Bindings/ExceptionOrUtils.h
+++ b/Userland/Libraries/LibWeb/Bindings/ExceptionOrUtils.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/Optional.h>
 #include <AK/StdLibExtras.h>
 #include <LibJS/Runtime/VM.h>
 #include <LibWeb/Bindings/DOMExceptionWrapper.h>
@@ -49,16 +50,48 @@ ALWAYS_INLINE bool throw_dom_exception(JS::VM& vm, JS::GlobalObject& global_obje
     return false;
 }
 
-template<typename F, typename T = decltype(declval<F>()()), typename Ret = Conditional<!IsExceptionOr<T> && !IsVoid<T>, T, JS::Value>>
-Ret throw_dom_exception_if_needed(auto&& vm, auto&& global_object, F&& fn)
+namespace Detail {
+
+template<typename T>
+struct ExtractExceptionOrValueType {
+    using Type = T;
+};
+
+template<typename T>
+struct ExtractExceptionOrValueType<DOM::ExceptionOr<T>> {
+    using Type = T;
+};
+
+template<>
+struct ExtractExceptionOrValueType<void> {
+    using Type = JS::Value;
+};
+
+template<>
+struct ExtractExceptionOrValueType<DOM::ExceptionOr<void>> {
+    using Type = JS::Value;
+};
+
+}
+
+template<typename T>
+using ExtractExceptionOrValueType = typename Detail::ExtractExceptionOrValueType<T>::Type;
+
+// Return type depends on the return type of 'fn' (when invoked with no args):
+// void or ExceptionOr<void>: Optional<JS::Value>, always returns JS::js_undefined()
+// ExceptionOr<T>: Optional<T>
+// T: Optional<T>
+template<typename F, typename T = decltype(declval<F>()()), typename Ret = Conditional<!IsExceptionOr<T> && !IsVoid<T>, T, ExtractExceptionOrValueType<T>>>
+Optional<Ret> throw_dom_exception_if_needed(auto&& vm, auto&& global_object, F&& fn)
 {
     if constexpr (IsExceptionOr<T>) {
         auto&& result = fn();
         if (throw_dom_exception(vm, global_object, result))
-            return JS::Value();
+            return {};
         if constexpr (requires(T v) { v.value(); })
             return result.value();
-        return JS::Value();
+        else
+            return JS::js_undefined();
     } else if constexpr (IsVoid<T>) {
         fn();
         return JS::js_undefined();
@@ -68,10 +101,10 @@ Ret throw_dom_exception_if_needed(auto&& vm, auto&& global_object, F&& fn)
 }
 
 template<typename T>
-bool should_return_empty(T&& value)
+bool should_return_empty(const Optional<T>& value)
 {
     if constexpr (IsSame<JS::Value, T>)
-        return value.is_empty();
+        return value.has_value() && value.value().is_empty();
     return false;
 }
 
diff --git a/Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp b/Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp
index 611fe08135..555e0d4c58 100644
--- a/Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp
+++ b/Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp
@@ -1514,9 +1514,11 @@ JS_DEFINE_NATIVE_FUNCTION(@prototype_class@::@function.name:snakecase@)
         function_generator.set(".arguments", arguments_builder.string_view());
 
         function_generator.append(R"~~~(
-    auto retval = throw_dom_exception_if_needed(vm, global_object, [&] { return impl->@function.cpp_name@(@.arguments@); });
-    if (should_return_empty(retval))
+    auto result = throw_dom_exception_if_needed(vm, global_object, [&] { return impl->@function.cpp_name@(@.arguments@); });
+    if (should_return_empty(result))
         return JS::Value();
+
+    [[maybe_unused]] auto retval = result.release_value();
 )~~~");
 
         generate_return_statement(function.return_type);