[Clang][NFC] Mark P2552 as implemented. (#92007)

wg21.link/P2552 suggest that __has_cpp_attribute
should return a non-zero value for all attributes that the
implementation does something interesting with.

Clang does something meaninful with all attributes except for:

- no_unique_address which we do not support for msvc target
- carries_dependency which arguably does nothing interesting. P2552
shies away from specifying a behavior for that attribute (despite being
the only one for which a recommandation would have been interesting,
arguably)

As such, we have nothing to change for this paper. This paper is a DR
and clang always behaved reasonably.

Author: cor3ntin

clang/test/SemaCXX/cxx2c-attributes.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 %s -x c++ -std=c++11 -triple x86_64-pc-linux -fsyntax-only -verify -Wno-c++17-extensions
+// RUN: %clang_cc1 %s -x c++ -std=c++11 -triple x86_64-windows-msvc -fsyntax-only -verify=msvc -Wno-c++17-extensions
+// expected-no-diagnostics
+
+// Check we return non-zero values for supported attributes as per
+// wg21.link/P2552
+static_assert(__has_cpp_attribute(assume));
+
+// The standard does not prescribe a behavior for [[carries_dependency]]
+
+static_assert(__has_cpp_attribute(deprecated));
+static_assert(__has_cpp_attribute(fallthrough));
+static_assert(__has_cpp_attribute(likely));
+static_assert(__has_cpp_attribute(unlikely));
+static_assert(__has_cpp_attribute(maybe_unused));
+static_assert(__has_cpp_attribute(nodiscard));
+static_assert(__has_cpp_attribute(noreturn));
+
+// We do not support [[no_unique_address]] in MSVC emulation mode
+static_assert(__has_cpp_attribute(no_unique_address)); // msvc-error {{static assertion failed}}

clang/www/cxx_status.html

@@ -130,7 +130,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
  <tr>
   <td>On the ignorability of standard attributes</td>
   <td><a href="https://wg21.link/P2552R3">P2552R3</a> (<a href="#dr">DR</a>)</td>
-  <td class="none" align="center">No</td>
+  <td class="full" align="center">Yes</td>
  </tr>
  <tr>
   <td>Static storage for braced initializers</td>