qcompare.h: standardize on Coverity-friendly ctors

Coverity complained that the Qt::strong_ordering(std::strong_ordering)
ctor may leave m_order uninitialized, which is true if you assume that
'stdorder' could be anything else but {less, greater, equal}, which,
however, should not happen™.

Standardize on the pattern that QPartialOrdering(Qt::partial_ordering)
was using: init m_order to equivalent, and then check for the other
possible states.

I would have preferred adding 'else Q_UNREACHABLE()', but these are
constexpr functions, so we'd need the GCC 8 protection, and then the
else would have a body longer than one line, and I don't know whether
violating the coding style and adding {} only on the else would fly
with reviewers, so that's done in a follow-up.

Amends several changes (this code has seen a lot of churn over time).

Coverity-Id: 475148
Change-Id: I3d88cdaaffbdfb8720161470b5f89046a3a15088
Reviewed-by: Ivan Solovev <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>
(cherry picked from commit f6d878d5ce4d6e41c088698bd337cf7f4438433e)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>
(cherry picked from commit da619f9e0b855d1c4b72820ed304ae8229f76829)

1 files changed, 4 insertions, 10 deletions

diff --git a/src/corelib/global/qcompare.h b/src/corelib/global/qcompare.h
index 9e1afe7527e..f2c87c0551c 100644
--- a/src/corelib/global/qcompare.h
+++ b/src/corelib/global/qcompare.h
@@ -143,11 +143,10 @@ public:
 
 #ifdef __cpp_lib_three_way_comparison
     constexpr Q_IMPLICIT partial_ordering(std::partial_ordering stdorder) noexcept
+        : m_order{} // == equivalent
     {
         if (stdorder == std::partial_ordering::less)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Less);
-        else if (stdorder == std::partial_ordering::equivalent)
-            m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Equivalent);
         else if (stdorder == std::partial_ordering::greater)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Greater);
         else if (stdorder == std::partial_ordering::unordered)
@@ -337,11 +336,10 @@ public:
 
 #ifdef __cpp_lib_three_way_comparison
     constexpr Q_IMPLICIT weak_ordering(std::weak_ordering stdorder) noexcept
+        : m_order{} // == equivalent
     {
         if (stdorder == std::weak_ordering::less)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Less);
-        else if (stdorder == std::weak_ordering::equivalent)
-            m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Equivalent);
         else if (stdorder == std::weak_ordering::greater)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Greater);
     }
@@ -533,13 +531,10 @@ public:
 
 #ifdef __cpp_lib_three_way_comparison
     constexpr Q_IMPLICIT strong_ordering(std::strong_ordering stdorder) noexcept
+        : m_order{} // == equivalent
     {
         if (stdorder == std::strong_ordering::less)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Less);
-        else if (stdorder == std::strong_ordering::equivalent)
-            m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Equivalent);
-        else if (stdorder == std::strong_ordering::equal)
-            m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Equal);
         else if (stdorder == std::strong_ordering::greater)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Greater);
     }
@@ -802,11 +797,10 @@ public:
 
 #ifdef __cpp_lib_three_way_comparison
     constexpr Q_IMPLICIT QPartialOrdering(std::partial_ordering stdorder) noexcept
+        : m_order{} // == equivalent
     {
         if (stdorder == std::partial_ordering::less)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Less);
-        else if (stdorder == std::partial_ordering::equivalent)
-            m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Equivalent);
         else if (stdorder == std::partial_ordering::greater)
             m_order = static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Ordering::Greater);
         else if (stdorder == std::partial_ordering::unordered)