[Bug c++/80157] New: Spurious "Wsign-conversion" warning with const, size_t, array.

["ian at geometrian dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80157

            Bug ID: 80157
           Summary: Spurious "Wsign-conversion" warning with const,
                    size_t, array.
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ian at geometrian dot com
  Target Milestone: ---

Minimal example (compile GCC >= ~6.1 with -Wsign-conversion):

    #include <cstdlib>
    bool f(size_t const xy[2], int z) {
        return xy[0] + xy[1] < static_cast<size_t>(z);
    }

Produces an erroneous warning:

    <source>: In function 'bool f(const size_t*, int)':
    <source>:3:25: warning: conversion to 'long unsigned int' from 'int' may
change the sign of the result [-Wsign-conversion]
      return xy[0] + xy[1] < static_cast<size_t>(z);
                             ^~~~~~~~~~~~~~~~~~~~~~
    Compiler exited with result code 0

Taking the const out, using unsigned, or passing x and y separately rather than
as an array resolves the warning.  Doesn't occur on Clang, MSVC, or earlier
GCC.

May be related to bug 66170.