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

[msebor at gcc dot gnu.org]

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

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
There is a warning like that in the middle-end: -Wsuggest-attribute=pure. 
Unfortunately, it's only good for functions that are actually emitted (i.e.,
not for C++ inline functions).

$ cat t.C && gcc -O2 -S -Wall -Wsuggest-attribute=pure t.C
typedef int X;

struct indirect_cmp {
  bool operator()(const X* l, const X* r);
};

bool indirect_cmp::operator()(const X* l, const X* r) { return *l < *r; }
t.C: In member function ‘bool indirect_cmp::operator()(const X*, const X*)’:
t.C:7:6: warning: function might be candidate for attribute ‘pure’
[-Wsuggest-attribute=pure]
 bool indirect_cmp::operator()(const X* l, const X* r) { return *l < *r; }
      ^~~~~~~~~~~~