LibWeb: Move ExceptionOr from DOM/ to WebIDL/

This is a concept fully defined in the Web IDL spec and doesn't belong
in the DOM directory/namespace - not even DOMException, despite the name
:^)

Userland/Libraries/LibWeb/WebIDL/ExceptionOr.h

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2021-2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/Optional.h>
+#include <AK/RefPtr.h>
+#include <LibWeb/DOM/DOMException.h>
+
+namespace Web::WebIDL {
+
+#define ENUMERATE_SIMPLE_WEBIDL_EXCEPTION_TYPES(E) \
+    E(EvalError)                                   \
+    E(RangeError)                                  \
+    E(ReferenceError)                              \
+    E(TypeError)                                   \
+    E(URIError)
+
+#define E(x) x,
+enum class SimpleExceptionType {
+    ENUMERATE_SIMPLE_WEBIDL_EXCEPTION_TYPES(E)
+};
+#undef E
+
+struct SimpleException {
+    SimpleExceptionType type;
+    String message;
+};
+
+template<typename ValueType>
+class ExceptionOr {
+public:
+    ExceptionOr() requires(IsSame<ValueType, Empty>)
+        : m_result(Empty {})
+    {
+    }
+
+    ExceptionOr(ValueType const& result)
+        : m_result(result)
+    {
+    }
+
+    ExceptionOr(ValueType&& result)
+        : m_result(move(result))
+    {
+    }
+
+    ExceptionOr(JS::NonnullGCPtr<DOM::DOMException> exception)
+        : m_exception(move(exception))
+    {
+    }
+
+    ExceptionOr(SimpleException exception)
+        : m_exception(move(exception))
+    {
+    }
+
+    ExceptionOr(Variant<SimpleException, JS::NonnullGCPtr<DOM::DOMException>> exception)
+        : m_exception(move(exception).template downcast<Empty, SimpleException, JS::NonnullGCPtr<DOM::DOMException>>())
+    {
+    }
+
+    ExceptionOr(ExceptionOr&& other) = default;
+    ExceptionOr(ExceptionOr const& other) = default;
+    ~ExceptionOr() = default;
+
+    ValueType& value() requires(!IsSame<ValueType, Empty>)
+    {
+        return m_result.value();
+    }
+
+    ValueType release_value()
+    {
+        return m_result.release_value();
+    }
+
+    Variant<SimpleException, JS::NonnullGCPtr<DOM::DOMException>> exception() const
+    {
+        return m_exception.template downcast<SimpleException, JS::NonnullGCPtr<DOM::DOMException>>();
+    }
+
+    bool is_exception() const
+    {
+        return !m_exception.template has<Empty>();
+    }
+
+    // These are for compatibility with the TRY() macro in AK.
+    [[nodiscard]] bool is_error() const { return is_exception(); }
+    Variant<SimpleException, JS::NonnullGCPtr<DOM::DOMException>> release_error() { return exception(); }
+
+private:
+    Optional<ValueType> m_result;
+
+    // https://webidl.spec.whatwg.org/#idl-exceptions
+    Variant<Empty, SimpleException, JS::NonnullGCPtr<DOM::DOMException>> m_exception {};
+};
+
+template<>
+class ExceptionOr<void> : public ExceptionOr<Empty> {
+public:
+    using ExceptionOr<Empty>::ExceptionOr;
+};
+
+}