Warning on move and dereference of unique_ptr in the same expression

[Allan Sandfeld Jensen <linux at carewolf dot com>]

Hello gcc

I have now twice hit obscure bugs in Chromium that crashed on some compilers 
but not on others, and didn't produce any warnings on any compiler. I would 
like to know if this code is as undefined as I think it is, and if it would 
make sense to have gcc warn about it.

Both cases basically has this form:

std::unique_ptr<A> a;

a->b->callMethod(something, bind(callback, std::move(a)));

This crashed with MSVC and gcc 5, but not with newer gcc or with clang.

When it crashes it is because the arguments and the move therein have been 
evaluated before a->b is resolved.

I assume this is undefined behavior? So why isn't the warning for using and 
modifying in the same expression triggered?

Best regards
'Allan