[Clang] Implement LWG3823 for __is_aggregate

[royjacobson]

Summary

LWG3823 says that arrays of incomplete types are aggregates. Fix the clang builtin to match that.

Closes https://github.com/llvm/llvm-project/issues/59002

[cjdb]

LGTM. Thanks for fixing this!

[shafik]

clang/test/SemaCXX/type-traits.cpp

556

Should this be true now or am I misunderstanding.

[cjdb]

clang/test/SemaCXX/type-traits.cpp

556

This should stay false, because the change is only concerned with arrays of incomplete types.

[royjacobson]

clang/test/SemaCXX/type-traits.cpp

556

Should this be true now or am I misunderstanding.

If you refer to the 'or cv' part, I think you're mixing up the completeness precondition with the builtin's actual value.

[royjacobson]

rebase to retry CI

clang/docs/ReleaseNotes.rst

- Fix a number of recursively-instantiated constraint issues, which would possibly
   `Issue 50891 <https://github.com/llvm/llvm-project/issues/50891>`_
 - Clang 14 predeclared some builtin POSIX library functions in ``gnu2x`` mode,
   and Clang 15 accidentally stopped predeclaring those functions in that
   language mode. Clang 16 now predeclares those functions again. This fixes
   `Issue 56607 <https://github.com/llvm/llvm-project/issues/56607>`_.
 - GNU attributes being applied prior to standard attributes would be handled
   improperly, which was corrected to match the behaviour exhibited by GCC.
   `Issue 58229 <https://github.com/llvm/llvm-project/issues/58229>`_
+- The builtin type trait ``__is_aggregate`` now returns ``true`` for arrays of incomplete
+  types in accordance with the suggested fix for `LWG3823 https://cplusplus.github.io/LWG/issue3823`_
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now check compile-time determinable string literals as format strings.
   Fixes `Issue 55805: <https://github.com/llvm/llvm-project/issues/55805>`_.
 - ``-Wformat`` now recognizes ``%b`` for the ``printf``/``scanf`` family of
   functions and ``%B`` for the ``printf`` family of functions. Fixes
   `Issue 56885: <https://github.com/llvm/llvm-project/issues/56885>`_.

clang/lib/Sema/SemaExprCXX.cpp

static bool CheckUnaryTypeTraitTypeCompleteness(Sema &S, TypeTrait UTT,
   //   If T is a class type, T shall be a complete type.
   case UTT_IsFinal:
   case UTT_IsSealed:
     if (ArgTy->getAsCXXRecordDecl())
       return !S.RequireCompleteType(
           Loc, ArgTy, diag::err_incomplete_type_used_in_type_trait_expr);
     return true;
 
+  // LWG3823: T shall be an array type, a complete type, or cv void.
+  case UTT_IsAggregate:
+    if (ArgTy->isArrayType() || ArgTy->isVoidType())
+      return true;
+
+    return !S.RequireCompleteType(
+        Loc, ArgTy, diag::err_incomplete_type_used_in_type_trait_expr);
+
   // C++1z [meta.unary.prop]:
   //   remove_all_extents_t<T> shall be a complete type or cv void.
-  case UTT_IsAggregate:
   case UTT_IsTrivial:
   case UTT_IsTriviallyCopyable:
   case UTT_IsStandardLayout:
   case UTT_IsPOD:
   case UTT_IsLiteral:
   // By analogy, is_trivially_relocatable imposes the same constraints.
   case UTT_IsTriviallyRelocatable:
   // Per the GCC type traits documentation, T shall be a complete type, cv void,

clang/test/SemaCXX/type-traits.cpp

 };
 
 void is_aggregate()
 {
   constexpr bool TrueAfterCpp11 = __cplusplus > 201103L;
   constexpr bool TrueAfterCpp14 = __cplusplus > 201402L;
 
   __is_aggregate(AnIncompleteType); // expected-error {{incomplete type}}
-  __is_aggregate(AnIncompleteType[]); // expected-error {{incomplete type}}
-  __is_aggregate(AnIncompleteType[1]); // expected-error {{incomplete type}}
-  __is_aggregate(AnIncompleteTypeAr); // expected-error {{incomplete type}}
-  __is_aggregate(AnIncompleteTypeArNB); // expected-error {{incomplete type}}
-  __is_aggregate(AnIncompleteTypeArMB); // expected-error {{incomplete type}}
   __is_aggregate(IncompleteUnion); // expected-error {{incomplete type}}
 
+  // Valid since LWG3823
+  static_assert(__is_aggregate(AnIncompleteType[]), "");
+  static_assert(__is_aggregate(AnIncompleteType[1]), "");
+  static_assert(__is_aggregate(AnIncompleteTypeAr), "");
+  static_assert(__is_aggregate(AnIncompleteTypeArNB), "");
+  static_assert(__is_aggregate(AnIncompleteTypeArMB), "");
+
   static_assert(!__is_aggregate(NonPOD), "");
   static_assert(__is_aggregate(NonPODAr), "");
   static_assert(__is_aggregate(NonPODArNB), "");
   static_assert(__is_aggregate(NonPODArMB), "");
 
   static_assert(!__is_aggregate(Enum), "");
   static_assert(__is_aggregate(POD), "");
   static_assert(__is_aggregate(Empty), "");
   static_assert(__is_aggregate(EmptyAr), "");
   static_assert(__is_aggregate(EmptyArNB), "");
   static_assert(__is_aggregate(EmptyArMB), "");
   static_assert(!__is_aggregate(void), "");
   static_assert(!__is_aggregate(const volatile void), "");
   static_assert(!__is_aggregate(int), "");
   static_assert(__is_aggregate(IntAr), "");
   static_assert(__is_aggregate(IntArNB), "");
   static_assert(__is_aggregate(EmptyUnion), "");
   static_assert(__is_aggregate(Union), "");
   static_assert(__is_aggregate(Statics), "");
   static_assert(__is_aggregate(HasFunc), "");