[cfe-commits] r165769 - /cfe/trunk/include/clang/StaticAnalyzer/Core/BugReporter/PathDiagnostic.h

[Richard Smith]

On Thu, Oct 11, 2012 at 7:10 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Thu, Oct 11, 2012 at 4:31 PM, Sean Silva <silvas at purdue.edu> wrote:
> > Author: silvas
> > Date: Thu Oct 11 18:31:18 2012
> > New Revision: 165769
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=165769&view=rev
> > Log:
> > Add missing classof().
> >
> > Somewhat troublingly, without this implemented, the check inside
> > isa_impl<> would silently use the parent's `classof()` when determining
> > whether it was okay to downcast from the parent to the child!
> >
> > Bug analysis:
> >
> > A build failure after removing the parent's `classof()` initially
> > alerted me to the bug, after which a little bit of thinking and reading
> > of the code identified the root cause.
> >
> > The compiler could be made to prevent this bug from happening if there
> > were a way to ensure that in the code
> >
> >     template <typename To, typename From, typename Enabler = void>
> >     struct isa_impl {
> >       static inline bool doit(const From &Val) {
> >         return To::classof(&Val);
> >       }
> >     };
> >
> > that `To::classof` is actually inside the class `To`, and not in a base
> > class. I am not aware of a way to check this in C++. If there is a means
> > to perform that check, please bring it up on the list and this will be
> > fixed.
>
> Stupid attempt:
>
> struct A { void f(); };
> struct B : A {
> #ifdef B_HAS_F
>   void f();
> #endif
> };
> decltype(&B::f) x;
> extern void (B::*x)(); // error unless B_HAS_F
>
> Unfortunately, not a complete solution, because you don't know the
> precise signature of the classof you're looking for.


I was pondering that (and you can solve the immediate problem pretty
straightforwardly), but I don't think it can be made to work if B::f is
overloaded.
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