Support QCborMap::operator[] taking a string literal

Use a template on the size of the char[], as suggested by Ville
Voutilainen.  This resolves ambiguity about whether such look-ups
should be done via QString or QCborValue (not that it would have made
any difference).

When we come to add mutating indexing of QCborValue, chained
dereferences like map[i][j][k] need to stay in operator[] const
throughout, to avoid detaching intermediates to create references into
them due to using the mutating operator[] on the earlier dereference's
return.  So const-qualify the QCborValue operator[] const variants at
the same time, to match those of QCborValue itself.

Change-Id: Ib1652ae9440fe3767a653afa2856b74040210e07
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qcborarray.h

@@ -185,7 +185,7 @@ public:
     QCborValue at(qsizetype i) const;
     QCborValue first() const { return at(0); }
     QCborValue last() const { return at(size() - 1); }
-    QCborValue operator[](qsizetype i) const { return at(i); }
+    const QCborValue operator[](qsizetype i) const { return at(i); }
     QCborValueRef first() { Q_ASSERT(!isEmpty()); return begin()[0]; }
     QCborValueRef last() { Q_ASSERT(!isEmpty()); return begin()[size() - 1]; }
     QCborValueRef operator[](qsizetype i)

src/corelib/serialization/qcbormap.h

@@ -195,14 +195,22 @@ public:
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
     QCborValue value(const QCborValue &key) const
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
-    QCborValue operator[](qint64 key) const
+#if !defined(QT_NO_CAST_FROM_ASCII) && !defined(QT_RESTRICTED_CAST_FROM_ASCII)
+    template<size_t N> QT_ASCII_CAST_WARN const QCborValue value(const char (&key)[N]) const
+    { return value(QString::fromUtf8(key, N - 1)); }
+#endif
+    const QCborValue operator[](qint64 key) const
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
-    QCborValue operator[](QLatin1String key) const
+    const QCborValue operator[](QLatin1String key) const
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
-    QCborValue operator[](const QString & key) const
+    const QCborValue operator[](const QString & key) const
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
-    QCborValue operator[](const QCborValue &key) const
+    const QCborValue operator[](const QCborValue &key) const
     { const_iterator it = find(key); return it == end() ? QCborValue() : it.value(); }
+#if !defined(QT_NO_CAST_FROM_ASCII) && !defined(QT_RESTRICTED_CAST_FROM_ASCII)
+    template<size_t N> QT_ASCII_CAST_WARN const QCborValue operator[](const char (&key)[N]) const
+    { return operator[](QString::fromUtf8(key, N - 1)); }
+#endif
     QCborValueRef operator[](qint64 key);
     QCborValueRef operator[](QLatin1String key);
     QCborValueRef operator[](const QString & key);

tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp

@@ -413,6 +413,9 @@ void tst_QCborValue::mapDefaultInitialization()
     QVERIFY(m.value(QLatin1String("Hello")).isUndefined());
     QVERIFY(m.value(QStringLiteral("Hello")).isUndefined());
     QVERIFY(m.value(QCborValue()).isUndefined());
+#if !defined(QT_NO_CAST_FROM_ASCII) && !defined(QT_RESTRICTED_CAST_FROM_ASCII)
+    QVERIFY(m.value("Hello").isUndefined());
+#endif
 
     QVERIFY(m == m);
     QVERIFY(m == QCborMap{});