Q_ARRAY_LITERAL: fix the checks on the payload's datatype

The check was a misnomer -- non-POD types can go in unions
since C++11. And we may want them, e.g. types without a trivial
default constructor. What we really want is to check for a
literal type (so that the array payload can be built entirely
at compile time, and put in .rodata). So, amend the check.

Also, make the dummy array constexpr, to be sure that we are
indeed building the payload using constexpr constructors.
That would make the first check redundant, but the fact that
we're still using a macro for constexpr makes me think that
not all compilers support it, so I'm leaving the first check
in...

Change-Id: I9f1473aa74dff5b6b6535ae4cd8325451c0b18e6
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/tools/qarraydata.h

@@ -324,10 +324,10 @@ struct QArrayDataPointerRef
     /**/
 
 #define Q_ARRAY_LITERAL_IMPL(Type, ...)                                         \
-    union { Type type_must_be_POD; } dummy; Q_UNUSED(dummy)                     \
+    Q_STATIC_ASSERT(std::is_literal_type<Type>::value);                         \
                                                                                 \
     /* Portable compile-time array size computation */                          \
-    Type data[] = { __VA_ARGS__ }; Q_UNUSED(data)                               \
+    Q_CONSTEXPR Type data[] = { __VA_ARGS__ }; Q_UNUSED(data);                  \
     enum { Size = sizeof(data) / sizeof(data[0]) };                             \
                                                                                 \
     static const QStaticArrayData<Type, Size> literal = {                       \