[Bug c++/69818] New: feature request: generate a warning for C++ functional cast expression on pointer or reference

["cvs at cs dot utoronto.ca" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69818

            Bug ID: 69818
           Summary: feature request: generate a warning for C++ functional
                    cast expression on pointer or reference
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cvs at cs dot utoronto.ca
  Target Milestone: ---

We have -Wold-style-cast for C-style cast expressions, but there is no warning
for C++ functional cast expressions even though they are essentially equal in
their power and potential for problems.  Consider this contrived example:

  short x;
  typedef long& R;
  long& y = R(x);
  typedef long* P;
  long* z = P(&x);

Hopefully this pattern is not common with typedefs like this, but I have
encountered bugs in generic code where the typedefs are instead template
parameters bound to a pointer or reference type.

I am not proposing a simple warn on all uses warning like -Wold-style-cast, but
instead would like to suggest one of the following:

A) warn on all functional cast expressions where the destination is a pointer
or reference type, or
B) warn on all functional cast expressions where the destination is a pointer
or reference type and the equivalent static_cast<>() expression would produce
an error.

Personally, I think option B would provide the greatest bug finding benefit
with few false positives.  Option B might be simplified to simply warn in all
cases where the equivalent static_cast<>() expression would produce an error.