[clang] Implement P3144R2 "Deleting a Pointer to an Incomplete Type..." (#97733)

This patch implements (not yet published)
[P3144R2](https://wiki.edg.com/pub/Wg21stlouis2024/StrawPolls/p3144r2.pdf)
"Deleting a Pointer to an Incomplete Type Should be Ill-formed". Wording
changes (not yet merged into the working draft) read:
> 7.6.2.9 [expr.delete] Delete
> If the object being deleted has incomplete class type at the point of
deletion <del>and the complete class has a
non-trivial destructor or a deallocation function, the behavior is
undefined</del>, <ins>the program is ill-formed</ins>.

We preserve status quo of emitting a warning when deleting a pointer to
incomplete type up to, and including, C++23, but make it ill-formed
since C++26. Same goes for deleting pointers to `void`, which has been
allowed as an extension.

1 files changed, 9 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 69074f9..fcf2189 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -3719,8 +3719,11 @@ Sema::ActOnCXXDelete(SourceLocation StartLoc, bool UseGlobal,
       // The C++ standard bans deleting a pointer to a non-object type, which
       // effectively bans deletion of "void*". However, most compilers support
       // this, so we treat it as a warning unless we're in a SFINAE context.
-      Diag(StartLoc, diag::ext_delete_void_ptr_operand)
-        << Type << Ex.get()->getSourceRange();
+      // But we still prohibit this since C++26.
+      Diag(StartLoc, LangOpts.CPlusPlus26 ? diag::err_delete_incomplete
+                                          : diag::ext_delete_void_ptr_operand)
+          << (LangOpts.CPlusPlus26 ? Pointee : Type)
+          << Ex.get()->getSourceRange();
     } else if (Pointee->isFunctionType() || Pointee->isVoidType() ||
                Pointee->isSizelessType()) {
       return ExprError(Diag(StartLoc, diag::err_delete_operand)
@@ -3729,7 +3732,10 @@ Sema::ActOnCXXDelete(SourceLocation StartLoc, bool UseGlobal,
       // FIXME: This can result in errors if the definition was imported from a
       // module but is hidden.
       if (!RequireCompleteType(StartLoc, Pointee,
-                               diag::warn_delete_incomplete, Ex.get())) {
+                               LangOpts.CPlusPlus26
+                                   ? diag::err_delete_incomplete
+                                   : diag::warn_delete_incomplete,
+                               Ex.get())) {
         if (const RecordType *RT = PointeeElem->getAs<RecordType>())
           PointeeRD = cast<CXXRecordDecl>(RT->getDecl());
       }