Re: [PATCH] c-family: Update unaligned adress of packed member check

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jan 16, 2019 at 04:11:44AM -0800, H.J. Lu wrote:
> > Why?  What is so special about C and (implicit?) casts where the rhs isn't
> > ADDR_EXPR?  Aren't all casts (explicit or implicit) from one pointer type
> > to another pointer and satisfy the rules something that should be warned
> 
> -Wincompatible-pointer-types is C only.   In C++,  incompatible pointer types
> aren't allowed at all.

How so?  You can certainly:
struct B { int i; };
struct C { struct B b; } __attribute__ ((packed));

extern struct C *p;
long* g8 (void) { return (long *)p; }

and similarly for C.  So, why is explicit cast something that shouldn't
be warned about in this case and implicit cast should get a warning,
especially when it already does get one (and one even enabled by default,
-Wincompatible-pointer-types)?
Either such casts are problematic and then it should treat explicit and
implicit casts the same, or they aren't, and then
-Wincompatible-pointer-types is all you need.

	Jakub