QVarLengthArray: re-publish Prealloc as a nested PreallocatedSize

This gives users of the class easy access to the Prealloc template
argument, without having to write a pattern-matcher like

   template <typename T>
   constexpr qsizetype preallocated_size_v;
   template <typename T, qsizetype N>
   constexpr qsizetype preallocated_size_v<QVarLengthArray<T,N>> = N;

first.

[ChangeLog][QtCore][QVarLengthArray] Added PreallocatedSize nested
constant, equal to the Prealloc template argument.

Change-Id: I928eaa5e62967445cdd7b0c2759567483fdb8997
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -278,6 +278,8 @@ class QVarLengthArray
     template <typename InputIterator>
     using if_input_iterator = QtPrivate::IfIsInputIterator<InputIterator>;
 public:
+    static constexpr qsizetype PreallocatedSize = Prealloc;
+
     using size_type = typename Base::size_type;
     using value_type = typename Base::value_type;
     using pointer = typename Base::pointer;

src/corelib/tools/qvarlengtharray.qdoc

@@ -462,6 +462,15 @@
     \a defaultValue.
 */
 
+/*
+    \var QVarLengthArray::PreallocatedSize
+    \since 6.8
+
+    The same value as the \c{Prealloc} template argument. Provided for easier
+    access compared to manually extracting the value from the template
+    argument.
+*/
+
 /*!
     \typedef QVarLengthArray::size_type
     \since 4.7

tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp

@@ -66,6 +66,7 @@ private slots:
     void sizeConstructor_QString() { sizeConstructor<QString>(); }
     void sizeConstructor_NonCopyable() { sizeConstructor<NonCopyable>(); }
     void append();
+    void preallocatedSize();
 #if QT_DEPRECATED_SINCE(6, 3)
     void prepend();
 #endif
@@ -194,6 +195,15 @@ void tst_QVarLengthArray::append()
     v2.append(5);
 }
 
+void tst_QVarLengthArray::preallocatedSize()
+{
+    // The default is 256:
+    static_assert(QVarLengthArray<int>::PreallocatedSize == 256);
+    // Otherwise, whatever was given as template argument:
+    static_assert(QVarLengthArray<int, 42>::PreallocatedSize == 42);
+    static_assert(QVarLengthArray<int, 1'000'000>::PreallocatedSize == 1'000'000);
+}
+
 #if QT_DEPRECATED_SINCE(6, 3)
 QT_WARNING_PUSH
 QT_WARNING_DISABLE_DEPRECATED