double operator < is not a strict weak ordering due to NaN

...so recent LLVM 19 trunk libc++ in debug mode complained during
CppunitTest_chart2_export2 about

> ~/llvm/inst/bin/../include/c++/v1/__debug_utils/strict_weak_ordering_check.h:59: assertion __comp(*(__first + __a), *(__first + __b)) failed: Your comparator is not a valid strict-weak ordering

at

> 5   libsystem_c.dylib                   0x0000000183279a40 abort + 180
> 6   libc++.1.0.dylib                    0x00000001030f9d98 _ZNSt3__123__cxx_atomic_notify_oneEPVKv + 0
> 7   libchartcorelo.dylib                0x00000002f817f0ec _ZNSt3__135__check_strict_weak_ordering_sortedB8de190000INS_11__wrap_iterIPNS_6vectorIdNS_9allocatorIdEEEEEEN5chart12_GLOBAL__N_116lcl_LessXOfPointEEEvT_SB_RT0_ + 960
> 8   libchartcorelo.dylib                0x00000002f817e6cc _ZNSt3__118__stable_sort_implB8de190000INS_17_ClassicAlgPolicyENS_11__wrap_iterIPNS_6vectorIdNS_9allocatorIdEEEEEEN5chart12_GLOBAL__N_116lcl_LessXOfPointEEEvT0_SC_RT1_ + 268
> 9   libchartcorelo.dylib                0x00000002f8172a90 _ZNSt3__111stable_sortB8de190000INS_11__wrap_iterIPNS_6vectorIdNS_9allocatorIdEEEEEEN5chart12_GLOBAL__N_116lcl_LessXOfPointEEEvT_SB_T0_ + 68
> 10  libchartcorelo.dylib                0x00000002f8172820 _ZN5chart11VDataSeries15doSortByXValuesEv + 508
> 11  libchartcorelo.dylib                0x00000002f8064c44 _ZN5chart9AreaChart12createShapesEv + 1528
> 12  libchartcorelo.dylib                0x00000002f80f2ae0 _ZN5chart9ChartView28impl_createDiagramAndContentERKNS_18CreateShapeParam2DERKN3com3sun4star3awt4SizeE + 4440
> 13  libchartcorelo.dylib                0x00000002f80f77ac _ZN5chart9ChartView14createShapes2DERKN3com3sun4star3awt4SizeE + 2728
> 14  libchartcorelo.dylib                0x00000002f80f58ec _ZN5chart9ChartView12createShapesEv + 692
> 15  libchartcorelo.dylib                0x00000002f80f4598 _ZN5chart9ChartView15impl_updateViewEb + 288

But the introduced use of `std::strong_order(first[0], second[0]) < 0` then
triggered a false

> lo/core/chart2/source/view/main/VDataSeries.cxx:105:61: error: NullToMemberPointer ValueDependentIsNotNull ZeroLiteral -> nullptr [loplugin:nullptr]
>   105 |             return std::strong_order(first[0], second[0]) < 0;
>       |                                                             ^

so needed some hack in loplugin:nullptr.

And old versions of libc++, still used at least on Android, do not have any
implementations of std::strong_order.  So detect those cases in configure.ac
(checking for std::strong_order for double, which is what is actually being used
in the code) and fall back to operator <=> for now, even if that will not
provide a strict weak ordering and will thus continue to violate the
requirements of std::sort.

And then our venerable clang-format 5.0.0 would have broken the token `<=>` into
`<= >`, so exclude include/o3tl/compare.hxx from its mis-treatment.

Change-Id: I7a64a630eb5f560dce59f3ff9d51ca3d1adc70be
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/163075
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

4 files changed, 26 insertions, 1 deletions

diff --git a/compilerplugins/clang/check.cxx b/compilerplugins/clang/check.cxx
index 4ff081b6923e..60e476cc37b7 100644
--- a/compilerplugins/clang/check.cxx
+++ b/compilerplugins/clang/check.cxx
@@ -133,6 +133,16 @@ TypeCheck TypeCheck::Pointer() const {
     return TypeCheck();
 }
 
+TypeCheck TypeCheck::MemberPointerOf() const {
+    if (!type_.isNull()) {
+        auto const t = type_->getAs<clang::MemberPointerType>();
+        if (t != nullptr) {
+            return TypeCheck(t->getClass());
+        }
+    }
+    return TypeCheck();
+}
+
 TerminalCheck TypeCheck::Enum() const {
     if (!type_.isNull()) {
         auto const t = type_->getAs<clang::EnumType>();
diff --git a/compilerplugins/clang/check.hxx b/compilerplugins/clang/check.hxx
index af6cd355a416..65ee16d77f2c 100644
--- a/compilerplugins/clang/check.hxx
+++ b/compilerplugins/clang/check.hxx
@@ -62,6 +62,8 @@ public:
 
     TypeCheck Pointer() const;
 
+    TypeCheck MemberPointerOf() const;
+
     TerminalCheck Enum() const;
 
     TypeCheck LvalueReference() const;
diff --git a/compilerplugins/clang/nullptr.cxx b/compilerplugins/clang/nullptr.cxx
index 0213b6b05c88..04fb5bf0f9fd 100644
--- a/compilerplugins/clang/nullptr.cxx
+++ b/compilerplugins/clang/nullptr.cxx
@@ -131,11 +131,18 @@ bool Nullptr::VisitImplicitCastExpr(CastExpr const * expr) {
     case Expr::NPCK_CXX11_nullptr:
         break;
     default:
-        if (loplugin::TypeCheck(expr->getType()).Typedef("locale_t")
+        auto const tc = loplugin::TypeCheck(expr->getType());
+        if (tc.Typedef("locale_t")
             .GlobalNamespace())
         {
             break; // POSIX locale_t is left unspecified
         }
+        // Hack to handle libc++ and stdlibc++ `std::strong_ordering x; x < 0` etc.:
+        if (tc.MemberPointerOf().ClassOrStruct("_CmpUnspecifiedParam").StdNamespace()
+            || tc.Pointer().ClassOrStruct("__unspec").Namespace("__cmp_cat").StdNamespace())
+        {
+            break;
+        }
         handleNull(expr->getSubExpr(), expr->getCastKindName(), k);
         break;
     }
diff --git a/compilerplugins/clang/test/nullptr.cxx b/compilerplugins/clang/test/nullptr.cxx
index bf7376cb6562..392a930bf9b8 100644
--- a/compilerplugins/clang/test/nullptr.cxx
+++ b/compilerplugins/clang/test/nullptr.cxx
@@ -7,6 +7,10 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+#include "sal/config.h"
+
+#include <compare>
+
 struct S
 {
     void* p;
@@ -18,6 +22,8 @@ int main()
         0 // expected-error {{NullToPointer ValueDependentIsNotNull ZeroLiteral -> nullptr [loplugin:nullptr]}}
     };
     (void)s;
+
+    (void)(std::strong_order(0, 1) < 0);
 }
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */