[PATCH] D33826: [clang-tidy] avoid pointer cast to more strict alignment check

John McCall via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Aug 30 11:35:23 PDT 2017

rjmccall added a comment.

In https://reviews.llvm.org/D33826#856652, @lebedev.ri wrote:

> In https://reviews.llvm.org/D33826#856619, @Eugene.Zelenko wrote:
> > In https://reviews.llvm.org/D33826#856610, @lebedev.ri wrote:
> >
> > > Any status update here? :)
> > >  I generally do see a benefit in this check, because `-Wcast-align` (at least currently?) does not warn on `reinterpret_cast<>()`.
> >
> >
> > I think will be good idea to extend -Wcast-align.
> Hm, are you *sure* `reinterpret_cast<>()` is not allowed to increase alignment?
>  There was a commit that intentionally removed that warning: (by @rjmccall, i believe)
>  https://github.com/llvm-mirror/clang/commit/35a38d95da89d48778019c37b5f8c9a20f7e309c

One of the fundamental design principles to keep in mind when implementing warnings like -Wcast-align is that we're trying to warn users about having written something dangerous, not scold them for writing code a particular way.  Users do need to do these casts sometimes, and there has to be some way of doing them without a warning.  So the question of when to warn has to consider whether the code is explicitly acknowledging the danger.  It would, for example, be a mistake to warn in C about double-casts through (void*), because that is not something that people do accidentally; it is very likely that it is an attempt to suppress the warning.  In C++, it seemed to me that a reinterpret_cast is by its nature explicitly acknowledging the danger, so it is never really appropriate to warn.



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