[Interest] QML type hints and their problems

[Ulf Hermann]

a, Use type annotations on all JavaScript functions in QML files. 
Period. It helps tooling, gives you better diagnostics from qmllint and 
qmlsc, helps you understand your code better. There are no downsides.

b, In this particular case we could be more lenient, but what if that 
was actually an unrelated "contains" method? Mind that you can shadow 
those methods by inheriting or by mixing C++ and QML. If we get the 
wrong one, we can probably still JavaScript-coerce the argument to point 
and the return value to bool, but the result will be a mess and rather 
hard to debug.

c, The change that introduced the containment masks, 
https://codereview.qt-project.org/c/qt/qtdeclarative/+/211550 , only 
intended the contains() functions to be defined in C++. Later, starting 
with https://codereview.qt-project.org/c/qt/qtdeclarative/+/249400 , 
we've added type annotations for JavaScript functions. Using those you 
can now define typed functions in JavaScript and they are treated the 
same way as C++ functions. So, be happy that this works at all ;)