Fix QVariant/QMetaType::compare APIs

std::optional<int> is the wrong datatype to use for compare.

First and foremost, it can't be used in the idiomatic form of

  auto r = a.compare(b);
  if (r < 0) ~~~ // a is less than b
  if (r > 0) ~~~ // a is greater than b

which we *already* feature in Qt (QString, QByteArray).

Also, std::optional<int> (explicitly) converts to bool, which is
a trap, because the result of the comparison can be accidentally
tested as a bool:

  if (a.compare(b)) ~~~ // oops! does NOT mean a<b

Not to mention extending this to algorithms:

  auto lessThan = [](QVariant a, QVariant b) { return a.compare(b); }; // oops!
  std::ranges::sort(vectorOfVariants, lessThan);

which thankfully doesn't compile as is -- std::optional has
an *explicit* operator bool, and the Compare concept requires an
implicit conversion. However, the error the user is going to face
will be "cannot convert to bool because the operator is explicit",
which is deceiving because the fix is NOT supposed to be:

  auto lessThan = [](QVariant a, QVariant b) { return (bool)a.compare(b); }; // big oops!

Instead: backport to Qt the required subset of C++20's <compare>
API, and use that. This commits just adds the necessary parts
for compare() (i.e. partial ordering), the rest of <compare>
(classes, functions, conversions) can be added to 6.1.

Change-Id: I2b5522da47854da39f79993e1207fad033786f00
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 3e59c97c3453926fc66479d9ceca03901df55f90)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index 58f1772e91..0273962636 100644
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -2576,17 +2576,17 @@ void tst_QMetaType::compareCustomEqualOnlyType()
 
     // check QMetaType::compare works/doesn't crash for equals only comparators
     auto cmp = type.compare(variant50.constData(), variant50.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     bool equals = type.equals(variant50.constData(), variant50.constData());
     QVERIFY(equals);
 
     cmp = type.compare(variant100.constData(), variant100x.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     equals = type.equals(variant100.constData(), variant100x.constData());
     QVERIFY(equals);
 
     cmp = type.compare(variant50.constData(), variant100.constData());
-    QVERIFY(!cmp);
+    QCOMPARE(cmp, QPartialOrdering::Unordered);
     equals = type.equals(variant50.constData(), variant100.constData());
     QVERIFY(!equals);