Add qAsConst

...to turn mutable lvalues into const lvalues.

Like the proposed std::as_const, it should not apply to
rvalues to avoid lifetime issues in code like

     for (auto x : qAsConst(someFunc()))
         // dangling

At a more basic level, qAsConst isn't useful for rvalues,
because one can always store them in an lvalue first, with
no loss in performance (the object is created by the
compiler silently anyway). So the correct way to write the
above is:

    const auto funcResult = someFunc();
    for (auto e : funcResult)

To fail compilation when passing rvalues, I used the
const-&& pattern also employed by std::cref(), and the
proposed std::as_const.

Intended as internal API, but not put into the QtPrivate
namespace to make it simpler to use.

We could wait for std::as_const, but that is far, far
away (just entered the current C++17 draft as of this
writing), and the Qt containers with their tendency to
detach are a problem _now_.

Change-Id: I8824a59d2274de5c5cd642f117212322e4648025
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/corelib/global/qglobal.h

@@ -1046,8 +1046,17 @@ template <typename T> struct QEnableIf<true, T> { typedef T Type; };
 
 template <bool B, typename T, typename F> struct QConditional { typedef T Type; };
 template <typename T, typename F> struct QConditional<false, T, F> { typedef F Type; };
+
+template <typename T> struct QAddConst { typedef const T Type; };
 }
 
+// this adds const to non-const objects (like std::as_const)
+template <typename T>
+Q_DECL_CONSTEXPR typename QtPrivate::QAddConst<T>::Type &qAsConst(T &t) Q_DECL_NOTHROW { return t; }
+// prevent rvalue arguments:
+template <typename T>
+void qAsConst(const T &&) Q_DECL_EQ_DELETE;
+
 QT_END_NAMESPACE
 
 // We need to keep QTypeInfo, QSysInfo, QFlags, qDebug & family in qglobal.h for compatibility with Qt 4.