[Bug c++/80711] New: warn on non-const accessor member functions

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80711

            Bug ID: 80711
           Summary: warn on non-const accessor member functions
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

C++ accessor (and similar) member functions that return a value computed from
one or more private data members without modifying the owning object can and
should be declared const.  Doing so not only improves the const-correctness of
code that relies on objects of the type, it also aids its analyzability.  It's
easy (and not uncommon) to forget to declare accessors const.  GCC could help
detect the missing const by issuing a warning on such accessors, similarly to
how it helps detect candidates for attribute const and pure.

This is an enhancement to request to add such a warning.  The test case below
illustrates where it would be issued and shows the similar -Wsuggest-attribute
warning that the new one could be modeled on.

$ cat t.C && gcc -O2 -S -Wall -Wextra -Wsuggest-attribute=pure t.C
class Int
{
public:
  int get () { return val; }   // suggested warning: function can be declared
const

private:
  int val;
};

int get_int (Int &i)
{
  return i.get ();
}
t.C: In function ‘int get_int(Int&)’:
t.C:10:5: warning: function might be candidate for attribute ‘pure’
[-Wsuggest-attribute=pure]
 int get_int (Int &i)
     ^~~~~~~