[Bug c++/78130] New: Strict overflow warning appears to be invalid

[rianquinn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78130

            Bug ID: 78130
           Summary: Strict overflow warning appears to be invalid
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rianquinn at gmail dot com
  Target Milestone: ---

Currently bounds checks inside Microsoft's GSL are causing a strict overflow
warning to trigger that appears to be invalid (i.e. the bounds checks should be
fine). The issue has been documented here:
https://github.com/Microsoft/GSL/pull/405

The bounds check looks like this (both count and size() are std::ptrdiff_t):
Expects(count >= 0 && count <= size());

The code can be seen here:
https://github.com/Microsoft/GSL/blob/master/gsl/span#L475

The warning is the following:
warning: assuming signed overflow does not occur when assuming
that (X - c) > X is always false [-Wstrict-overflow]

And the warning occurs on both GCC 5 and 6. Simplifying the code still
generates the warning:
Expects(count >= 0); // fine
Expects(count <= size()); // Generates warning

Doing the following fixes the issue, but is obviously less than ideal:
Expects(count >= 0 && (count < size() || count == size()));