[PATCH] D146090: [Clang] Updating handling of defaulted comparison operators to reflect changes from P2448R2

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

Please also update the P2448 <https://reviews.llvm.org/P2448> row in cxx_status.html and add release notes.



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Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:9410-9421
+def ext_incorrect_defaulted_comparison_constexpr : Extension<
   "defaulted definition of %select{%sub{select_defaulted_comparison_kind}1|"
   "three-way comparison operator}0 "
-  "cannot be declared %select{constexpr|consteval}2 because "
+  "declared %select{constexpr|consteval}2 but "
   "%select{it|the corresponding implicit 'operator=='}0 "
-  "invokes a non-constexpr comparison function">;
+  "invokes a non-constexpr comparison function are a C++2b extension">, InGroup<CXX2bDefCompCallsNonConstexpr>;
+def warn_cxx2b_incorrect_defaulted_comparison_constexpr : Warning<
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The grammar of these diagnostics looks a bit peculiar. Suggested an alternative, but it's still a bit awkward ("...but for which..."), maybe you can find something better.


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Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:9416
+  "invokes a non-constexpr comparison function are a C++2b extension">, InGroup<CXX2bDefCompCallsNonConstexpr>;
+def warn_cxx2b_incorrect_defaulted_comparison_constexpr : Warning<
+  "defaulted definition of %select{%sub{select_defaulted_comparison_kind}1|"
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We usually spell these `CXXabCompat` diagnostics `warn_cxxab_compat_...`


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Comment at: clang/lib/Sema/SemaDeclCXX.cpp:8799-8806
   // C++2a [dcl.fct.def.default]p3 [P2002R0]:
   //   An explicitly-defaulted function that is not defined as deleted may be
   //   declared constexpr or consteval only if it is constexpr-compatible.
   // C++2a [class.compare.default]p3 [P2002R0]:
   //   A defaulted comparison function is constexpr-compatible if it satisfies
   //   the requirements for a constexpr function [...]
   // The only relevant requirements are that the parameter and return types are
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Would be good to mention the change in C++23 here.


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Comment at: clang/lib/Sema/SemaDeclCXX.cpp:8808-8809
   if (FD->isConstexpr()) {
     if (CheckConstexprReturnType(*this, FD, CheckConstexprKind::Diagnose) &&
         CheckConstexprParameterTypes(*this, FD, CheckConstexprKind::Diagnose) &&
         !Info.Constexpr) {
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We'll also be getting diagnostics here if the parameter and return types are not literal types. You can pass `...::CheckValid` instead of `...::Diagnose` to find out if there would be an error, but we should probably instead make the `CheckConstexpr...` functions produce warnings (`ExtWarn` prior to C++23) rather than errors in general. That's a much larger change, though -- there's probably half a dozen diagnostics in the `CheckConstexpr*` functions that will need to be updated to properly support P2448R2. If you just want to implement this one part of P2448 for now and leave the rest for a later change, that seems fine to me.


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Comment at: clang/lib/Sema/SemaDeclCXX.cpp:8813
+	  (getLangOpts().CPlusPlus2b ? diag::warn_cxx2b_incorrect_defaulted_comparison_constexpr :
+                                        diag::ext_incorrect_defaulted_comparison_constexpr ))
           << FD->isImplicit() << (int)DCK << FD->isConsteval();
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Don't need the parens around this. This layout is a little unusual, what does clang-format do here?


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Comment at: clang/test/CXX/class/class.compare/class.compare.default/p3.cpp:132
   A b;
-  friend constexpr bool operator==(const E&, const E&) = default; // expected-error {{cannot be declared constexpr because it invokes a non-constexpr comparison function}}
+  friend constexpr bool operator==(const E&, const E&) = default;
   friend constexpr bool operator!=(const E&, const E&) = default;
----------------
Can we test these both ways, like you did in p4?


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  https://reviews.llvm.org/D146090/new/

https://reviews.llvm.org/D146090