[Bug c++/78873] New: Virtual call after conversion to base class pointer is not devirtualized

["ambrop7 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78873

            Bug ID: 78873
           Summary: Virtual call after conversion to base class pointer is
                    not devirtualized
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ambrop7 at gmail dot com
  Target Milestone: ---

The following test case shows the issue. The generated code for
TestDevirtualization<Impl> could perform a direct call to Impl::Foo but it
performs a call via the vtable.

#include <stdio.h>

struct Iface {
    virtual void Foo() = 0;
};

struct Impl : public Iface {
    __attribute__ ((noinline))
    void Foo() override final
    {
        printf("Impl::Foo\n");
    }
};

template <typename Type>
__attribute__ ((noinline))
void TestDevirtualuzation(Type *obj)
{
    static_cast<Iface *>(obj)->Foo();
}

int main() {
    Impl impl;
    TestDevirtualuzation(&impl);
    return 0;
}

I have heard arguments that optimizing this would not be legal, with the
following test case which supposedly might be valid:

struct Liar : Iface {
  void Foo() {}
};

Impl impl;
TestDevirtualuzation(reinterpret_cast<Liar*>(&impl));

But I think it is not valid; the result of the reinterpret_cast does not point
to a Liar object, so the static_cast done in TestDevirtualuzation *should* be
invalid. I couldn't find a clear statement in the standard about this though.