Compile time check the fileListingHelper specializations

Since we can now rely on deleted functions (supported by MSVC 2013),
we can avoid the scenario we had with the last snapshot update where
the file picker internal API changed slightly (see 1a744ca4

 for more
details).

Change-Id: Iab26a7834a367f24afe5fbf37fae64639aa9c12e
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>

src/core/type_conversion.h

@@ -155,7 +155,14 @@ inline base::FilePath toFilePath(const QString &str)
 }
 
 template <typename T>
-inline T fileListingHelper(const QString &) {qFatal("Specialization missing for %s.", Q_FUNC_INFO); return T(); }
+inline T fileListingHelper(const QString &)
+// Clang is still picky about this though it should be supported eventually.
+// See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#941
+#ifndef Q_CC_CLANG
+= delete;
+#else
+{ return T(); }
+#endif
 
 template <>
 inline content::FileChooserFileInfo fileListingHelper<content::FileChooserFileInfo>(const QString &file)