[Bug c++/92539] New: [8/9/10 Regression] -Warray-bounds false positive (loop unroll?)

[nok.raven at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92539

            Bug ID: 92539
           Summary: [8/9/10 Regression] -Warray-bounds false positive
                    (loop unroll?)
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nok.raven at gmail dot com
  Target Milestone: ---

static bool
ischar(int ch)
{
    return (0 == (ch & ~0xff) || ~0 == (ch | 0xff)) != 0;
}

static bool eat(char const*& first, char const* last)
{
    if (first != last && ischar(*first)) {
        ++first;
        return true;
    }
    return false;
}

static bool eat_two(char const*& first, char const* last)
{
    auto save = first;
    if (eat(first, last) && eat(first, last))
        return true;
    first = save;
    return false;
}

static bool foo(char const*& first, char const* last)
{
    auto local_iterator = first;
    int i = 0;
    for (; i < 3; ++i)
        if (!eat_two(local_iterator, last))
            return false;
    first = local_iterator;
    return true;
}

static bool test(char const* in, bool full_match = true)
{
    auto last = in;
    while (*last)
        ++last;
    return foo(in, last) && (!full_match || (in == last));
}

int main()
{
    return test("aa");
}

$ g++ -std=c++17 -O3 -Wall

<source>: In function 'int main()':
<source>:9:23: warning: array subscript 4 is outside array bounds of 'const
char [3]' [-Warray-bounds]
    9 |     if (first != last && ischar(*first)) {
      |         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
<source>:9:23: warning: array subscript 5 is outside array bounds of 'const
char [3]' [-Warray-bounds]
    9 |     if (first != last && ischar(*first)) {
      |         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
<source>:41:42: warning: array subscript 6 is outside array bounds of 'const
char [3]' [-Warray-bounds]
   41 |     return foo(in, last) && (!full_match || (in == last));
      |                             ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~

https://godbolt.org/z/86XVfe

The code is reduced Spirit.X3 test.

The -fsanitize=address and -fsanitize=undefined do not show any errors.