[clang][docs] Reflect the implementation status for P2280R4

[frederick-vs-ja]

P2280R4 is officially a defect report per N4916. Although Clang 20 only implemented it for C++23 and later. I think we should mention that it is not backported yet.

[llvmbot]

@llvm/pr-subscribers-clang

Author: A. Jiang (frederick-vs-ja)

Changes

P2280R4 is officially a defect report. Although Clang 20 only implementated it for C++23 and later, and #95474 accidently removed the DR status in cxx_status.html. I think we should mention that it is a DR but not backported yet.

---

Full diff: https://github.com/llvm/llvm-project/pull/127166.diff

1 Files Affected:

• (modified) clang/www/cxx_status.html (+8-2)

diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
index 0fc3b1d314698..70490b2e4331f 100755
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -439,8 +439,8 @@ <h2 id="cxx23">C++23 implementation status</h2>
     </tr>
     <tr>
       <td>Using unknown pointers and references in constant expressions</td>
-      <td><a href="https://wg21.link/P2280R4">P2280R4</a></td>
-      <td class="unreleased" align="center">Clang 20</td>
+      <td><a href="https://wg21.link/P2280R4">P2280R4</a> (<a href="#dr">DR</a>)</td>
+      <td class="unreleased" align="center">Clang 20 <a href="#p2280">(12)</a></td>
     </tr>
     <tr>
       <td>static <code>operator()</code></td>
@@ -510,6 +510,12 @@ <h2 id="cxx23">C++23 implementation status</h2>
       <td class="full" align="center">Yes</td>
     </tr>
 </table>
+
+<p>
+<span id="p2280">(12): In Clang 20, this change is not yet retroactively
+applied to pre-C++23 modes.
+</span>
+</p>
 </details>
 
 

[Endilll]

I'd like to see a test that checks that it's available in C++23 but not C++20 (with a fixme on this). You can also point me out to an existing one.

[frederick-vs-ja]

@Endilll These checks (modified by #95474) and the newly added clang/test/SemaCXX/constant-expression-p2280r4.cpp are relevant.

llvm-project/clang/test/SemaCXX/constant-expression-cxx11.cpp

Lines 2431 to 2433 in d3d2ea6

	template<typename T> void f2(const T &t) { // cxx11_20-note 2{{declared here}}
	constexpr int k = t.size(); // cxx11_20-error 2{{constexpr variable 'k' must be initialized by a constant expression}} cxx11_20-note 2{{function parameter 't' with unknown value cannot be used in a constant expression}}
	}

llvm-project/clang/test/SemaCXX/constant-expression-cxx11.cpp

Lines 2437 to 2441 in d3d2ea6

	void g(array<3> a) {
	f1(a);
	f2(a); // cxx11_20-note {{in instantiation of function template}}
	f3(a);
	}

Ideally, the C++23 behavior should also be present in C++11~20 modes. Did you want to add FIXME to these checks?

[Endilll]

Ideally, the C++23 behavior should also be present in C++11~20 modes. Did you want to add FIXME to these checks?

Yes, this would be an improvement.

[frederick-vs-ja]

Oh, there seem to be a few remaining bugs. Now I've added some FIXME comments to the most related test files. @Endilll

[Endilll]

On top of this FIXME, add RUN lines for C++11 mode, then add expected directives for whatever comes out of it. So that if someone accidentally makes it work, it doesn't go unnoticed.

[cor3ntin]

We should instead do that in #129646 (and merge that first)