[PATCH] Enhanced ignored-qualifiers checks for restrict

[Richard Smith]

First off, more warnings for dubious uses of `restrict` seem valuable to me. The two warnings you suggest seem like good ideas. Some thoughts:

I don't think cast expressions are the right thing to target here. The same issue applies to compound literals, new-expressions, and so on. `restrict` only has meaning when used in "a declaration of an ordinary identifier that provides a means of designating an object P", so perhaps we should warn on it (at least at the top level) in all cases where it's not part of such a declaration (and can't be indirectly part of a declaration, such as if it's in a typedef or template argument).

Taking the address of a `restrict`-qualified pointer has weird semantics, and perhaps deserves a warning. For instance:

  int *restrict a = &...;
  int **p = &a;
  *a = 1;
  int k = **p;

... is fine, even though `a` is accessed through a non-`restrict`-qualified pointer, because `*q` is based on `a`.

Similarly, declaring an object of a type that has a non-top-level `restrict` qualifier also has weird semantics, and we should perhaps warn on that.

Perhaps we should also warn on copying a `restrict`-qualified pointer to a non-`restrict`-qualified pointer, since that also seems like a very common error.

http://reviews.llvm.org/D5872