LibWeb: Generate code for throwing DOMExceptions based on ExceptionOr<T>

There's a little bit of template magic involved here to make it work,
but this seems alright. Very cool! :^)

Co-authored-by: AnotherTest <ali.mpfard@gmail.com>

Userland/Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp

@@ -1091,6 +1091,7 @@ void generate_prototype_implementation(const IDL::Interface& interface)
 
     generator.append(R"~~~(
 #include <AK/Function.h>
+#include <AK/StdLibExtras.h>
 #include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/Function.h>
@@ -1100,6 +1101,7 @@ void generate_prototype_implementation(const IDL::Interface& interface)
 #include <LibWeb/Bindings/@wrapper_class@.h>
 #include <LibWeb/Bindings/CanvasRenderingContext2DWrapper.h>
 #include <LibWeb/Bindings/CommentWrapper.h>
+#include <LibWeb/Bindings/DOMExceptionWrapper.h>
 #include <LibWeb/Bindings/DOMImplementationWrapper.h>
 #include <LibWeb/Bindings/DocumentFragmentWrapper.h>
 #include <LibWeb/Bindings/DocumentTypeWrapper.h>
@@ -1115,6 +1117,7 @@ void generate_prototype_implementation(const IDL::Interface& interface)
 #include <LibWeb/Bindings/WindowObject.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/EventListener.h>
+#include <LibWeb/DOM/ExceptionOr.h>
 #include <LibWeb/DOM/Window.h>
 #include <LibWeb/HTML/EventHandler.h>
 #include <LibWeb/HTML/HTMLElement.h>
@@ -1407,6 +1410,55 @@ JS_DEFINE_NATIVE_SETTER(@prototype_class@::@attribute.setter_callback@)
         function_generator.set("function.name:snakecase", snake_name(function.name));
 
         function_generator.append(R"~~~(
+#ifndef EXCEPTION_OR_TEMPLATES
+#define EXCEPTION_OR_TEMPLATES
+
+template<typename>
+struct IsExceptionOr : AK::FalseType {
+};
+
+template<typename T>
+struct IsExceptionOr<ExceptionOr<T>> : AK::TrueType {
+};
+
+template<typename T>
+ALWAYS_INLINE bool throw_dom_exception(JS::VM& vm, JS::GlobalObject& global_object, ExceptionOr<T>& result)
+{
+    if (result.is_exception()) {
+        vm.throw_exception(global_object, DOMExceptionWrapper::create(global_object, const_cast<DOM::DOMException&>(result.exception())));
+        return true;
+    }
+    return false;
+}
+
+template<typename F, typename T = decltype(declval<F>()()), typename Ret = typename Conditional<!IsExceptionOr<T>::value && !IsVoid<T>::value, T, JS::Value>::Type>
+Ret throw_dom_exception_if_needed(auto&& vm, auto&& global_object, F&& fn) {
+    if constexpr (IsExceptionOr<T>::value) {
+        auto&& result = fn();
+        if (throw_dom_exception(vm, global_object, result))
+            return JS::Value();
+        if constexpr (requires(T v) { v.value(); })
+            return result.value();
+        return JS::Value();
+    } else if constexpr (IsVoid<T>::value) {
+        fn();
+        return JS::js_undefined();
+    } else {
+        return fn();
+    }
+}
+
+template <typename T>
+bool should_return_empty(T&& value)
+{
+    if constexpr (IsSame<JS::Value, T>::value)
+        return value.is_empty();
+    return false;
+}
+
+
+#endif
+
 JS_DEFINE_NATIVE_FUNCTION(@prototype_class@::@function.name:snakecase@)
 {
     auto* impl = impl_from(vm, global_object);
@@ -1420,15 +1472,11 @@ JS_DEFINE_NATIVE_FUNCTION(@prototype_class@::@function.name:snakecase@)
         generate_arguments(generator, function.parameters, arguments_builder);
         function_generator.set(".arguments", arguments_builder.string_view());
 
-        if (function.return_type.name != "undefined") {
-            function_generator.append(R"~~~(
-    auto retval = impl->@function.name:snakecase@(@.arguments@);
-)~~~");
-        } else {
-            function_generator.append(R"~~~(
-    impl->@function.name:snakecase@(@.arguments@);
+        function_generator.append(R"~~~(
+    auto retval = throw_dom_exception_if_needed(vm, global_object, [&] { return impl->@function.name:snakecase@(@.arguments@); });
+    if (should_return_empty(retval))
+        return JS::Value();
 )~~~");
-        }
 
         generate_return_statement(function.return_type);