Clarify assert on use of const char* as TestData type in tests.

Currently all C-style strings used as data types in QTest::addColumn
will assert at runtime with, e.g.: "expected data of type
'const char*', got 'QString' for element 0 of data with tab 'blah'".
This patch makes it clear that C-style strings are disallowed.

Change-Id: Iadbb8f10e16607de95c7adb43d9ff3310aac6738
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/testlib/qtestcase.h

@@ -47,6 +47,7 @@
 #include <QtCore/qstring.h>
 #include <QtCore/qnamespace.h>
 #include <QtCore/qmetatype.h>
+#include <QtCore/qtypetraits.h>
 
 #include <string.h>
 
@@ -218,6 +219,8 @@ namespace QTest
     template <typename T>
     inline void addColumn(const char *name, T * = 0)
     {
+        typedef QtPrivate::is_same<T, const char*> QIsSameTConstChar;
+        Q_STATIC_ASSERT_X(!QIsSameTConstChar::value, "const char* is not allowed as a test data format.");
         addColumnInternal(qMetaTypeId<T>(), name);
     }
     Q_TESTLIB_EXPORT QTestData &newRow(const char *dataTag);