[Bug c++/80783] New: Inconsistent constexpr diagnostic for some cstdlib functions

[victor.nawothnig at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80783

            Bug ID: 80783
           Summary: Inconsistent constexpr diagnostic for some cstdlib
                    functions
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: victor.nawothnig at gmail dot com
  Target Milestone: ---

A set of functions from cstdlib suffers from incorrect constexpr diagnostic
"call to non-constexpr function"

#include <cstdlib>
constexpr size_t f() { exit(0); return 1; }
/* Bad: Silently compiles */

When f() is being evaluated in a constant expression, the diagnostic properly
triggers again, this reduces the impact of this bug:

#include <cstdlib>
constexpr size_t f() { exit(0); return 1; }
int arr[f()];
/* Good: triggers diagnostic */

Interestingly the error seems to be tied to specific function signatures,
rather than the standard library headers. Given the following (without
including any headers):

extern "C" void free(int);
constexpr void f() { free(0); }
/* Good: triggers diagnostic, free has a non-standard signature */

extern "C" void free(void *);
constexpr void f() { free(0); }
/* Bad: silently compiles, free has a standard signature */

It also is limited to only a particular set of cstdlib functionsincluding but
not limited to: free, malloc, exit, atoi.  I have not found any other affected
headers yet, but I did not do an exhaustive search.