https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89960
Bug ID: 89960
Summary: Implicit derived to base conversion considered type
punning.
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: cassio.neri at gmail dot com
Target Milestone: ---
Consider:
struct base {
int i;
void f(){}
};
template <void(base::*F)()>
struct derived : base {
void g1() {
return (this->*F)();
}
void g2() {
base* p = this;
return (p->*F)();
}
};
void h() {
derived<&base::f> x;
x.g1();
x.g2();
}
Compiling with -O2 -Wstrict-aliasing gives a warning
warning: dereferencing type-punned pointer will break strict-aliasing rules
[-Wstrict-aliasing]
return (this->*F)();
~~~~~~~~~~^~
It looks like the implicit conversion from derived to base is considered
type-punning.
Remarks: The warning goes away if either:
1) -O2 is not used.
2) -Wstrict-aliasing is not used.
3) base has no non-static data members.
4) F is not a template parameter.
5) x.g1()) is not called. (In contrast, x.g2() compiles fine and this is a
workaround for the issue.)
6) if another compiler is used (other vendor's but also gcc 4.6.4 or earlier)
