Specify interface requirements as an C++ concept

PR #19440.

test/CMakeLists.txt

@@ -8,6 +8,7 @@ include_directories("../src")
 set(testFiles
     testalgorithm.cpp
     testbittorrenttrackerentry.cpp
+    testconceptsstringable.cpp
     testglobal.cpp
     testorderedset.cpp
     testpath.cpp

test/testconceptsstringable.cpp

@@ -0,0 +1,73 @@
+/*
+ * Bittorrent Client using Qt and libtorrent.
+ * Copyright (C) 2023  Mike Tzou (Chocobo1)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ *
+ * In addition, as a special exception, the copyright holders give permission to
+ * link this program with the OpenSSL project's "OpenSSL" library (or with
+ * modified versions of it that use the same license as the "OpenSSL" library),
+ * and distribute the linked executables. You must obey the GNU General Public
+ * License in all respects for all of the code used other than "OpenSSL".  If you
+ * modify file(s), you may extend this exception to your version of the file(s),
+ * but you are not obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ */
+
+#include <QObject>
+#include <QString>
+#include <QTest>
+
+#include "base/concepts/stringable.h"
+
+class TestConceptsStringable final : public QObject
+{
+    Q_OBJECT
+    Q_DISABLE_COPY_MOVE(TestConceptsStringable)
+
+public:
+    TestConceptsStringable() = default;
+
+private slots:
+    void testStringable() const
+    {
+        struct A
+        {
+        };
+        static_assert(!Stringable<A>);
+
+        struct B
+        {
+            B(QString) {}
+        };
+        static_assert(!Stringable<B>);
+
+        struct C
+        {
+            QString toString() const { return {}; }
+        };
+        static_assert(!Stringable<C>);
+
+        struct D
+        {
+            D(QString) {}
+            QString toString() const { return {}; }
+        };
+        static_assert(Stringable<D>);
+    }
+};
+
+QTEST_APPLESS_MAIN(TestConceptsStringable)
+#include "testconceptsstringable.moc"

test/testutilsversion.cpp

@@ -51,9 +51,9 @@ private slots:
         // Utils::Version<2, 0>();
 
         using TwoDigits = Utils::Version<2, 1>;
-        TwoDigits(0);
-        TwoDigits(50);
-        TwoDigits(0, 1);
+        QCOMPARE(TwoDigits(0), TwoDigits(u"0"_s));
+        QCOMPARE(TwoDigits(50), TwoDigits(u"50"_s));
+        QCOMPARE(TwoDigits(0, 1), TwoDigits(u"0.1"_s));
 
         using ThreeDigits = Utils::Version<3, 3>;
         // should not compile:
@@ -61,7 +61,7 @@ private slots:
         // ThreeDigits(1, 2);
         // ThreeDigits(1.0, 2, 3);
         // ThreeDigits(1, 2, 3, 4);
-        ThreeDigits(1, 2, 3);
+        QCOMPARE(ThreeDigits(1, 2, 3), ThreeDigits(u"1.2.3"_s));
     }
 
     void testIsValid() const