Update __cpp_concepts macro

[erichkeane]

After discussion with a few others, and seeing the state of our concepts support, I believe it is worth trying to see if we can update this for Clang19. The forcing function is that libstdc++'s <expected> header is guarded by this macro, so we need to update it to support that.

Closes #57524.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Erich Keane (erichkeane)

Changes

After discussion with a few others, and seeing the state of our concepts support, I believe it is worth trying to see if we can update this for Clang19. The forcing function is that libstdc++'s <expected> header is guarded by this macro, so we need to update it to support that.

---

Full diff: https://github.com/llvm/llvm-project/pull/87998.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+5)
• (modified) clang/lib/Frontend/InitPreprocessor.cpp (+1-4)
• (modified) clang/test/Lexer/cxx-features.cpp (+1-1)
• (modified) clang/www/cxx_status.html (+2-10)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 28e8ddb3c41c3e..4f02459ca1f09f 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -110,6 +110,11 @@ C++20 Feature Support
   templates (`P1814R0 <https://wg21.link/p1814r0>`_).
   (#GH54051).
 
+- __cpp_concepts macro now updated to `202002L`, as we are confident that,
+  modulo a handful of bugs and core issues, that our concepts implementation is
+  sufficient. This should enable libstdc++'s `<expected>` to work correctly in
+  clang.
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 48ad92063bd461..84069e96f41464 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -720,10 +720,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   if (LangOpts.CPlusPlus20) {
     Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
 
-    // P0848 is implemented, but we're still waiting for other concepts
-    // issues to be addressed before bumping __cpp_concepts up to 202002L.
-    // Refer to the discussion of this at https://reviews.llvm.org/D128619.
-    Builder.defineMacro("__cpp_concepts", "201907L");
+    Builder.defineMacro("__cpp_concepts", "202002");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
     Builder.defineMacro("__cpp_consteval", "202211L");
     Builder.defineMacro("__cpp_constexpr_dynamic_alloc", "201907L");
diff --git a/clang/test/Lexer/cxx-features.cpp b/clang/test/Lexer/cxx-features.cpp
index 9496746c6fd663..1de32498cd345b 100644
--- a/clang/test/Lexer/cxx-features.cpp
+++ b/clang/test/Lexer/cxx-features.cpp
@@ -85,7 +85,7 @@
 #error "wrong value for __cpp_char8_t"
 #endif
 
-#if check(concepts, 0, 0, 0, 0, 201907, 201907, 201907)
+#if check(concepts, 0, 0, 0, 0, 202002, 202002, 202002)
 #error "wrong value for __cpp_concepts"
 #endif
 
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
index c1d95dadbb27e2..130148c7420fa1 100755
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -544,16 +544,8 @@ <h2 id="cxx20">C++20 implementation status</h2>
       </tr>
       <tr> <!-- from Cologne -->
         <td><a href="https://wg21.link/p0848r3">P0848R3</a></td>
-        <td rowspan="1" class="partial" align="center">
-          <details>
-            <summary>Clang 16 (Partial)</summary>
-            Because of other concepts implementation deficits, the __cpp_concepts macro is not yet set to 202002L.
-            Also, the related defect reports <a href="https://wg21.link/cwg1496">DR1496</a> and
-            <a href="https://wg21.link/cwg1734">DR1734</a> are not yet implemented. Accordingly, deleted
-            special member functions are treated as eligible even though they shouldn't be.
-          </details>
-        </td>
-        </tr>
+        <td rowspan="1" class="full" align="center">Clang 19</td>
+      </tr>
       <tr>
         <td><a href="https://wg21.link/p1616r1">P1616R1</a></td>
         <td rowspan="2" class="full" align="center">Clang 10</td>

[cor3ntin]

Is that not true any more?

[erichkeane]

Those aren't implemented, and that IS true, but it is kind of orthogonal to concepts. Those issues pre-date concepts, and are only somewhat more visible thanks to concepts, but are not 'worse' nor 'caused' by it.

[cor3ntin]

We have fairly large hands concepts C++20 concepts - but i agree doing that early in the 19 cycle makes sense

[AaronBallman]

LGTM, feel free to take or leave my suggestion on the release note.

[AaronBallman]

Suggested change

	- __cpp_concepts macro now updated to `202002L`, as we are confident that,
	modulo a handful of bugs and core issues, that our concepts implementation is
	sufficient. This should enable libstdc++'s `<expected>` to work correctly in
	clang.
	- We have sufficient confidence in the concepts implementation to update the
	``__cpp_concepts`` macro to `202002L`. This enables ``<expected>`` from
	libstdc++ to work correctly with Clang.

??