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

[aaron.ballman]

Summary

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.

[erichkeane]

We've discussed this offline, and I think this is the right thing to do, but I'll let others comment before approving.

[royjacobson]

+1 from me, at least as long as we don't define the feature test macro.

[cor3ntin]

@aaron.ballman is the plan to backport that to clang 16? I think it probably should.
We ought to try to improve the situation for clang 17

[royjacobson]

In D144572#4145614, @cor3ntin wrote:

@aaron.ballman is the plan to backport that to clang 16? I think it probably should.
We ought to try to improve the situation for clang 17

cxx_status.html is always updated from trunk, no? I don't think we need to bother with backporting if that's the case

[aaron.ballman]

In D144572#4145634, @royjacobson wrote:

In D144572#4145614, @cor3ntin wrote:

@aaron.ballman is the plan to backport that to clang 16? I think it probably should.
We ought to try to improve the situation for clang 17

cxx_status.html is always updated from trunk, no? I don't think we need to bother with backporting if that's the case

Correct, that only reflects what's happening on trunk, so the only thing we could backport from this is the extra test coverage (doesn't seem worth porting, IMO).

clang/test/CXX/expr/expr.const/p6-2a.cpp

 
 struct Temporary {
   int &&temp;
   constexpr ~Temporary() {
     int n = temp; // expected-note {{outside the expression that created the 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

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

+// 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
+

clang/www/cxx_status.html

</tr>
     <tr>
       <td><tt>char8_t</tt></td>
       <td><a href="https://wg21.link/p0482r6">P0482R6</a></td>
       <td class="full" align="center">Clang 7 <a href="#p0482">(11)</a></td>
     </tr>
     <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>
         <td class="full" align="center">Clang 14</td>
       </tr>
     <tr>
       <td><tt>std::is_constant_evaluated</tt></td>
       <td><a href="https://wg21.link/p0595r2">P0595R2</a></td>