[Bug c/103502] New: -Wstrict-aliasing=3 doesn't warn on what is documented as UB

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103502

            Bug ID: 103502
           Summary: -Wstrict-aliasing=3 doesn't warn on what is documented
                    as UB
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stsp at users dot sourceforge.net
  Target Milestone: ---

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
---
Similarly, access by taking the address, casting the resulting pointer and
dereferencing the result has undefined behavior, even if the cast uses a union
type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}
---

Here is the test-case:
---
union a_union {
  int i;
  double d;
};

static int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

int main()
{
    return f();
}
---

It only warns at -Wstrict-aliasing=2 or 1,
but doesn't on 3. 3 is documented as the
most precise option. So obviously it should
warn on what the official gcc manual declares
as an UB.

Note: I very much wish such construct to
not be UB. Because of the lack of a warning
on -Wstrict-aliasing=3 I was freely using
such construct for type-punning for years...
until now I've found it invalid in gcc manual.
If it is actually valid, please fix the docs,
not gcc! :)