[PATCH] D144572: [C++20] Stop claiming full support for consteval (for the moment!)

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman created this revision.
aaron.ballman added reviewers: cor3ntin, erichkeane, royjacobson, clang-language-wg.
Herald added a project: All.
aaron.ballman requested review of this revision.
Herald added a project: clang.

During Clang 15, 3d2629dd3aab17098813c68b5b76bb864bc5e285 <https://reviews.llvm.org/rG3d2629dd3aab17098813c68b5b76bb864bc5e285> claimed we achieved full support for `consteval` in C++20. However, further testing shows that Clang doesn't correctly handle all of the examples from https://wg21.link/P1073R3 and has several other known issues that are preventing us from defining the `__cpp_consteval` macro.

I think we should only claim Partial support for the moment. Once we correct the major outstanding issues, then I think we should change the status back to full support and define `__cpp_consteval` at the same time (even if it's only to the `201811L` value instead of the latest value from C++2b). This helps users understand the support situation more clearly.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D144572

Files:
  clang/test/CXX/expr/expr.const/p6-2a.cpp
  clang/test/CXX/expr/expr.const/p8-2a.cpp
  clang/www/cxx_status.html


Index: clang/www/cxx_status.html
===================================================================
--- clang/www/cxx_status.html
+++ clang/www/cxx_status.html
@@ -1120,7 +1120,14 @@
     <tr>
       <td rowspan=2>Immediate functions (<tt>consteval</tt>)</td>
       <td><a href="https://wg21.link/p1073r3">P1073R3</a></td>
-      <td class="full" align="center">Clang 15</td>
+      <td class="partial" align="center">
+        <details><summary>Clang 15 (Partial)</summary>
+          Clang still incorrectly defers some consteval executions to runtime,
+          resulting in CodeGen crashes. Additionally, Clang does not properly
+          handle default arguments in consteval functions under all
+          circumstances.
+        </details>
+      </td>
     </tr>
       <tr> <!-- from Prague -->
         <td><a href="https://wg21.link/p1937r2">P1937R2</a></td>
Index: clang/test/CXX/expr/expr.const/p8-2a.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/expr/expr.const/p8-2a.cpp
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+// expected-no-diagnostics
+
+namespace P1073R3 {
+struct N {
+  constexpr N() {}
+  N(N const&) = delete;
+};
+
+template<typename T> constexpr void bad_assert_copyable() { T t; T t2 = t; }
+using ineffective = decltype(bad_assert_copyable<N>());
+
+// bad_assert_copyable<N> is not needed for constant evaluation
+// (and thus not instantiated)
+template<typename T> consteval void assert_copyable() { T t; T t2 = t; }
+using check = decltype(assert_copyable<N>());
+// FIXME: this should give an error because assert_copyable<N> is instantiated
+// (because it is needed for constant evaluation), but the attempt to copy t is
+// ill-formed.
+} // namespace P1073R3
+
Index: clang/test/CXX/expr/expr.const/p6-2a.cpp
===================================================================
--- clang/test/CXX/expr/expr.const/p6-2a.cpp
+++ clang/test/CXX/expr/expr.const/p6-2a.cpp
@@ -41,3 +41,16 @@
   }
 };
 constexpr Temporary t = {3}; // expected-error {{must have constant destruction}} expected-note {{created here}} expected-note {{in call}}
+
+namespace P1073R3 {
+consteval int f() { return 42; } // expected-note 3 {{declared here}}
+consteval auto g() { return f; }
+// FIXME: there should be no diagnostics associated with either h() or r.
+consteval int h(int (*p)() = g()) { return p(); } // expected-error {{call to consteval function 'P1073R3::g' is not a constant expression}} \
+                                                     expected-note {{declared here}} \
+                                                     expected-note {{pointer to a consteval declaration is not a constant expression}}
+constexpr int r = h();   // expected-note {{in the default initalizer of 'p'}}
+constexpr auto e = g();  // expected-error {{call to consteval function 'P1073R3::g' is not a constant expression}} \
+                            expected-error {{constexpr variable 'e' must be initialized by a constant expression}} \
+                            expected-note 2 {{pointer to a consteval declaration is not a constant expression}}
+} // namespace P1073R3


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