Add an extra check for qssize_t's size and type

The definitions of size_t and ptrdiff_t ([support.types.layout] p2 and
p3 respectively) do not specify that they need to be as big as a
pointer. They just need to be big enough to hold the size of the largest
object and the biggest array subscript, respectively, the platform
supports (e.g., 16-bit DOS would have them as 16-bit in all memory
models, except huge).

But we depend on them actually being the size of a pointer in many
places, such as in QArrayData::offset, that stores the linear distance
from the end of the structure to the beginning of the data, wherever it
is in memory.

It's also a good idea to verify that qptrdiff and qssize_t are the same
type.

Change-Id: I9ad33fff8b634979bdbafffd14bbd1223afc58e8
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/global/qglobal.cpp

@@ -140,6 +140,11 @@ Q_STATIC_ASSERT_X(std::numeric_limits<float>::has_infinity &&
 Q_STATIC_ASSERT_X(std::numeric_limits<float>::radix == 2,
                   "Qt assumes binary IEEE 754 floating point");
 
+// not required by the definition of size_t, but we depend on this
+Q_STATIC_ASSERT_X(sizeof(size_t) == sizeof(void *), "size_t and a pointer don't have the same size");
+Q_STATIC_ASSERT(sizeof(size_t) == sizeof(qssize_t)); // implied by the definition
+Q_STATIC_ASSERT((std::is_same<qssize_t, qptrdiff>::value));
+
 /*!
     \class QFlag
     \inmodule QtCore

src/corelib/global/qglobal.h

@@ -436,6 +436,9 @@ namespace QtPrivate {
 
       sizeof(void *) == sizeof(quintptr)
       && sizeof(void *) == sizeof(qptrdiff)
+
+  size_t and qssize_t are not guaranteed to be the same size as a pointer, but
+  they usually are.
 */
 template <int> struct QIntegerForSize;
 template <>    struct QIntegerForSize<1> { typedef quint8  Unsigned; typedef qint8  Signed; };