[PATCH] Deprecated attribute on namespaces

[Richard Smith]

On Sat, Nov 8, 2014 at 4:05 PM, Aaron Ballman <aaron at aaronballman.com>
wrote:

> C++1z now allows attributes on namespace and enumerators. We already
> supported deprecated enumerators, but we do not support namespaces.
> This patch addresses that.
>
> A few notes:
>
> It was decided that the deprecated attribute only applies to things
> with names (this makes reporting diagnostics a bit more sane), so this
> patch disallows deprecating an anonymous namespace.
>
> Since the declaration context can now cause deprecations, this
> requires updating some other test cases.


This wasn't the intention when this was discussed with EWG; the idea was
that using a deprecated name would result in a warning, but merely naming
something that's in a deprecated context would not. So:

namespace [[deprecated]] N {
  int x;
  int y = x; // no warning
  int f();
}
using N::x; // warning
int z1 = x; // no warning
using namespace N; // warning
int z2 = y; // no warning

// no warnings from this function
int N::f() {
  return y;
}

This allows, for instance:

namespace OldNamespace {
  // old stuff
}
namespace NewNamespace {
  using OldNamespace::a;
  using OldNamespace::b;
  namespace C = OldNamespace::Inner;
}
namespace [[deprecated("use NewNamespace instead")]] OldNamespace {}

On the Clang side, the idea would be to check declarations that get used as
part of a NestedNameSpecifier to see if they're deprecated, rather than
recursing through parents to see if each used declaration is in a
deprecated context.

Several of them now receive
> two notes -- one for the declaration of a variable of enumerator type,
> another for the use of the enumerator.
>
> There are still improvements to be made for situations where a class
> is effectively providing a namespace (static members, inner types,
> etc).
>
> ~Aaron
>
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