Utils: Add std::expected implementation

Adds a std::expected implementation that is compatible with >= C++11.

FilePath::fileContents and FilePath::writeFileContents as well as
FilePath::copyFile are changed to return std::expected.

A couple of macros have been added to aid in using the expected types.

An auto test was added showing how to use the library.

Change-Id: Ibe3aecfc1029a0cf13b45bf5184ff03a04a2393b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/utils/expected.h

@@ -0,0 +1,47 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0+ OR GPL-3.0 WITH Qt-GPL-exception-1.0
+
+#pragma once
+
+#include "qtcassert.h"
+
+#include "../3rdparty/tl_expected/include/tl/expected.hpp"
+
+namespace Utils {
+
+template<class T, class E>
+using expected = tl::expected<T, E>;
+
+template<class T>
+using expected_str = tl::expected<T, QString>;
+
+template<class E>
+using unexpected = tl::unexpected<E>;
+using unexpect_t = tl::unexpect_t;
+
+static constexpr unexpect_t unexpect{};
+
+template<class E>
+constexpr unexpected<std::decay_t<E>> make_unexpected(E &&e)
+{
+    return tl::make_unexpected(e);
+}
+
+} // namespace Utils
+
+//! If 'expected' has an error the error will be printed and the 'action' will be executed.
+#define QTC_ASSERT_EXPECTED(expected, action) \
+    { \
+        if (Q_LIKELY(expected)) { \
+        } else { \
+            ::Utils::writeAssertLocation(QString("%1:%2: %3") \
+                                             .arg(__FILE__) \
+                                             .arg(__LINE__) \
+                                             .arg(expected.error()) \
+                                             .toUtf8() \
+                                             .data()); \
+            action; \
+        } \
+        do { \
+        } while (0); \
+    }