QDataStream: allow streaming of enums backed by long and unsigned long

On Unix 64-bit platforms (LP64), compilers will use long and unsigned
long as the underlying type of 64-bit enumerations without explicit
type, such as:
  enum E { V = 0x1'0000'0000 };

User code may also explicitly choose std::int64_t or std::uint64_t,
which on those platforms map to long and unsigned long, respectively.
Likewise std::ptrdiff_t and std::size_t, which are the same.

This means we do allow streaming of enums with explicit long and
unsigned long types, such as:
  enum ELong : long { A, B, C };
which change sizes between platforms.

[ChangeLog][QtCore][QDataStream] QDataStream now supports streaming
enumerations whose underlying type is long or unsigned long (like
std::int64_t, std::uint64_t, ptrdiff_t, size_t on 64-bit Unix
platforms).

Fixes: QTBUG-86424
Change-Id: I34ef9422a252ce3df468fffd4ba074cddea20005
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/serialization/qdatastream.h

@@ -525,12 +525,21 @@ inline QDataStream &operator>>(QDataStream &s, QFlags<Enum> &e)
 template <typename T>
 typename std::enable_if_t<std::is_enum<T>::value, QDataStream &>
 operator<<(QDataStream &s, const T &t)
-{ return s << static_cast<typename std::underlying_type<T>::type>(t); }
+{
+    // std::underlying_type_t<T> may be long or ulong, for which QDataStream
+    // provides no streaming operators. For those, cast to qint64 or quint64.
+    return s << typename QIntegerForSizeof<T>::Unsigned(t);
+}
 
 template <typename T>
 typename std::enable_if_t<std::is_enum<T>::value, QDataStream &>
 operator>>(QDataStream &s, T &t)
-{ return s >> reinterpret_cast<typename std::underlying_type<T>::type &>(t); }
+{
+    typename QIntegerForSizeof<T>::Unsigned i;
+    s >> i;
+    t = T(i);
+    return s;
+}
 
 #ifndef Q_QDOC
 

tests/auto/corelib/serialization/qdatastream/tst_qdatastream.cpp

@@ -3697,6 +3697,41 @@ void tst_QDataStream::enumTest()
     }
     ba.clear();
 
+    enum class E5 : qint64
+    {
+        A,
+        B,
+        C
+    };
+    {
+        QDataStream stream(&ba, QIODevice::WriteOnly);
+        stream << E5::C;
+        QCOMPARE(ba.size(), int(sizeof(E5)));
+    }
+    {
+        QDataStream stream(ba);
+        E5 e;
+        stream >> e;
+        QCOMPARE(e, E5::C);
+    }
+    ba.clear();
+
+    // unlike regular streaming operators, we accept long and ulong, because
+    // usually the only reason people see them is because they have
+    // std::(u)int64_t underlying types.
+    enum class ELong : long { A, B, C };
+    {
+        QDataStream stream(&ba, QIODevice::WriteOnly);
+        stream << ELong::A;
+        QCOMPARE(ba.size(), sizeof(long));
+    }
+    {
+        QDataStream stream(ba);
+        ELong e;
+        stream >> e;
+        QCOMPARE(e, ELong::A);
+    }
+    ba.clear();
 }
 
 void tst_QDataStream::floatingPointPrecision()