[clang] e328d68 - [C++20] Stop claiming full support for consteval (for the moment!)

[Aaron Ballman via cfe-commits]

Author: Aaron Ballman
Date: 2023-02-23T07:41:35-05:00
New Revision: e328d68d5ba1fe365c36655b8ae287928da6d3ed

URL: https://github.com/llvm/llvm-project/commit/e328d68d5ba1fe365c36655b8ae287928da6d3ed
DIFF: https://github.com/llvm/llvm-project/commit/e328d68d5ba1fe365c36655b8ae287928da6d3ed.diff

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

During Clang 15, 3d2629dd3aab17098813c68b5b76bb864bc5e285 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.

Differential Revision: https://reviews.llvm.org/D144572

Added: 
    clang/test/CXX/expr/expr.const/p8-2a.cpp

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

Removed: 
    


################################################################################
diff  --git a/clang/test/CXX/expr/expr.const/p6-2a.cpp b/clang/test/CXX/expr/expr.const/p6-2a.cpp
index 312c28354188d..2ef067c549003 100644
--- a/clang/test/CXX/expr/expr.const/p6-2a.cpp
+++ b/clang/test/CXX/expr/expr.const/p6-2a.cpp
@@ -41,3 +41,16 @@ struct Temporary {
   }
 };
 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

diff  --git a/clang/test/CXX/expr/expr.const/p8-2a.cpp b/clang/test/CXX/expr/expr.const/p8-2a.cpp
new file mode 100644
index 0000000000000..b8bee64b0aae2
--- /dev/null
+++ b/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
+

diff  --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
index f93a788aa0d7a..c397b2ba25da1 100755
--- a/clang/www/cxx_status.html
+++ b/clang/www/cxx_status.html
@@ -1120,7 +1120,14 @@ <h2 id="cxx20">C++20 implementation status</h2>
     <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>