Support attribute deprecated for template alias

[Quuxplusone]

Bugzilla Link	PR17862
Status	NEW
Importance	P enhancement
Reported by	agenda2010@grueninger.de
Reported on	2013-11-09 03:53:08 -0800
Last modified on	2021-10-18 14:38:03 -0700
Version	trunk
Hardware	PC Linux
CC	dgregor@apple.com, llvm-bugs@lists.llvm.org, pipping@exherbo.org, vsna7ma4he@jorrit.de
Fixed by commit(s)
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I want to rename a templated class. To make the transition easier for the
users, I'd like to keep the old class for one more version and mark it
deprecated with the extensions from GCC / Clang (attribute deprecated). To
avoid keeping an exact copy of the deprecated class, the use of template alias
would be handy. Unfortunately Clang (3.3 and recent trunk) does not emit a
warning (GCC does) for the following code in line 14:

template <class T>
struct NewClassName
{
    // ...
};

template <class T> using OldClassNameUsing __attribute__ ((deprecated))
  = NewClassName<T>;

typedef NewClassName<int> OldClassNameTypedef __attribute__ ((deprecated));

int main()
{
  OldClassNameUsing<int> objectUsing;
  OldClassNameTypedef objectTypedef;

  return 0;
}

Clang used:
clang version 3.4 (194323)
Target: x86_64-unknown-linux-gnu
Thread model: posix

[Quuxplusone]

To state the problem more precisely, the deprecation message is only missing
when templates are not specified.

template <class T>
struct NewClassName
{
    // ...
};

template <class T> using OldClassNameUsing __attribute__ ((deprecated))
  = NewClassName<T>;

using OldClassNameWithoutTemplateUsing __attribute__ ((deprecated))
  = NewClassName<int>;

typedef NewClassName<int> OldClassNameTypedef __attribute__ ((deprecated));

int main()
{
  OldClassNameUsing<int> objectUsing;  // no deprecation warning
  OldClassNameWithoutTemplateUsing objectWithoutTemplateUsing; // emits warning
  OldClassNameTypedef objectTypedef;   // emits warning

  return 0;
}

[Quuxplusone]

This has bitten us again: https://gitlab.dune-project.org/core/dune-
common/merge_requests/464#note_41116

All versions of clang up to 5.0 seem to be affected according to godbolt:
https://godbolt.org/g/Cpsbd7

godbolt does not support clang 6 at the moment, so I can't easily test that.

All forms of deprecating seem to be affected: [[deprecated(msg)]],
__attribute__((__deprecated__(msg))), __attribute__((deprecated(msg)))

[Quuxplusone]

Still an issue with Clang 13.