[Bug c/115237] New: -Wsuggest-attribute=pure false positive for obviously non-pure function

[eggert at cs dot ucla.edu via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115237

            Bug ID: 115237
           Summary: -Wsuggest-attribute=pure false positive for obviously
                    non-pure function
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

This is gcc (GCC) 14.1.1 20240507 (Red Hat 14.1.1-1) on x86-64. I ran into this
problem when compiling tzcode; this is a stripped-down version of the problem.
Compile the following with 'gcc -O2 -Wsuggest-attribute=pure -S t.i':

  _Noreturn void error ();

  int
  unite (int a, int b)
  {
    int sum;
    if (a & b)
      error ();
    return a | b;
  }

GCC complains:

  t.i: In function ‘unite’:
  t.i:4:3: warning: function might be candidate for attribute ‘pure’ if it is
known to return normally [-Wsuggest-attribute=pure]
      4 |   unite (int a, int b)
        |   ^~~~~

However, an important goal of the function is to report an error and exit if
a&b is nonzero. The function must not be pure, because pure functions must
return; they cannot exit or longjmp out or whatever. So this warning is clearly
incorrect.

I suggest suppressing the warning if the function that might call a noreturn
function.