From ab3587d2f66f3b238b139f741284b4b4c90902f0 Mon Sep 17 00:00:00 2001
From: Nana Sakisaka <1901813+saki7@users.noreply.github.com>
Date: Fri, 23 May 2025 00:10:55 +0900
Subject: [PATCH] Correctly promote `partial | promote(partial)`

---
 include/yk/compare/comparator.hpp | 10 ++++++++--
 test/compare.cpp                  | 20 +++++++++++++++++---
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/include/yk/compare/comparator.hpp b/include/yk/compare/comparator.hpp
index e5e5e043..d866ee36 100644
--- a/include/yk/compare/comparator.hpp
+++ b/include/yk/compare/comparator.hpp
@@ -280,8 +280,14 @@ struct promote_comparator : comparator_interface {
     const auto res = std::invoke(comp1, x, y);
 
     if constexpr (std::same_as<From, To>) {
-      if (res == From::equivalent) return std::invoke(comp2, x, y);
-      return res;
+      if constexpr (std::same_as<From, std::partial_ordering>) {
+        if (res == From::equivalent || res == From::unordered) return std::invoke(comp2, x, y);
+        return res;
+
+      } else {
+        if (res == From::equivalent) return std::invoke(comp2, x, y);
+        return res;
+      }
 
     } else {
       if (res == From::less) return To::less;
diff --git a/test/compare.cpp b/test/compare.cpp
index 45b98444..32f24f49 100644
--- a/test/compare.cpp
+++ b/test/compare.cpp
@@ -434,17 +434,31 @@ BOOST_AUTO_TEST_CASE(ClosureTypeTraits)
   }
 }
 
+struct PartialPartial
+{
+    double a, b;
+    std::partial_ordering operator<=>(const PartialPartial&) const noexcept = default;
+};
+
 BOOST_AUTO_TEST_CASE(promotion)
 {
   using namespace yk::comparators;
 
+  constexpr double NaN = std::numeric_limits<double>::quiet_NaN();
+
   // partial | promote(partial)
   {
     const auto partial_comparator = [](double a, double b) -> std::partial_ordering { return a <=> b; };
 
     auto comp = partial_comparator | promote(partial_comparator);
     BOOST_TEST((comp(3.14, 1.41) == std::partial_ordering::greater));
-    BOOST_TEST((comp(3.14, std::numeric_limits<double>::quiet_NaN()) == std::partial_ordering::unordered));
+    BOOST_TEST((comp(3.14, NaN) == std::partial_ordering::unordered));
+  }
+  {
+    auto comp = extract(&PartialPartial::a) | promote(extract(&PartialPartial::b));
+    BOOST_TEST((comp(PartialPartial{1.0, NaN}, PartialPartial{2.0, NaN}) == std::partial_ordering::less));
+    BOOST_TEST(((PartialPartial{NaN, 1.0} <=> PartialPartial{NaN, 2.0}) == std::partial_ordering::unordered));
+    BOOST_TEST((comp(PartialPartial{NaN, 1.0}, PartialPartial{NaN, 2.0}) == std::partial_ordering::less));
   }
 
   // partial | promote(weak)
@@ -454,7 +468,7 @@ BOOST_AUTO_TEST_CASE(promotion)
 
     auto comp = partial_comparator | promote(weak_comparator);
     BOOST_TEST((comp(3.14, 1.41) == std::weak_ordering::greater));
-    BOOST_TEST((comp(3.14, std::numeric_limits<double>::quiet_NaN()) == std::weak_ordering::less));
+    BOOST_TEST((comp(3.14, NaN) == std::weak_ordering::less));
   }
 
   // partial | promote(strong)
@@ -464,7 +478,7 @@ BOOST_AUTO_TEST_CASE(promotion)
 
     auto comp = partial_comparator | promote(strong_comparator);
     BOOST_TEST((comp(3.14, 1.41) == std::strong_ordering::greater));
-    BOOST_TEST((comp(3.14, std::numeric_limits<double>::quiet_NaN()) == std::strong_ordering::less));
+    BOOST_TEST((comp(3.14, NaN) == std::strong_ordering::less));
   }
 
   // weak | promote(weak)