From 77079c8125d1dd9fce21be15909afaa3a380f07d Mon Sep 17 00:00:00 2001
From: rleigh-electradx <roger.leigh@electradx.com>
Date: Tue, 4 Aug 2026 19:04:19 +0100
Subject: [PATCH] Fix etl::common_type decaying both arguments to T2 (#1529)

The two-type etl::common_type specialization recursed with
decay<T2> twice when either argument was not already decayed,
so etl::common_type_t<long&, int&> yielded int instead of long.
Recurse with decay<T1> and decay<T2>, matching the cppreference
sample implementation, and add regression tests comparing
against std::common_type for reference-type arguments.

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>
---
 include/etl/type_traits.h |  2 +-
 test/test_type_traits.cpp | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/etl/type_traits.h b/include/etl/type_traits.h
index 13d21810..6ecf9451 100644
--- a/include/etl/type_traits.h
+++ b/include/etl/type_traits.h
@@ -3724,7 +3724,7 @@ namespace etl
   struct common_type<T1, T2>
     : etl::conditional< etl::is_same<T1, typename etl::decay<T1>::type>::value && etl::is_same<T2, typename etl::decay<T2>::type>::value,
                         private_common_type::common_type_2_impl<T1, T2>,
-                        common_type<typename etl::decay<T2>::type, typename etl::decay<T2>::type>>::type
+                        common_type<typename etl::decay<T1>::type, typename etl::decay<T2>::type>>::type
   {
   };
 
diff --git a/test/test_type_traits.cpp b/test/test_type_traits.cpp
index 6756d6d9..603c992b 100644
--- a/test/test_type_traits.cpp
+++ b/test/test_type_traits.cpp
@@ -2268,5 +2268,23 @@ namespace
       CHECK_TRUE(etl::is_unsigned<char32_t>::value);
 #endif
     }
+
+#if ETL_USING_CPP11
+    //*************************************************************************
+    TEST(test_common_type)
+    {
+      // Reference types must decay to the same result as std::common_type.
+      CHECK((etl::is_same<etl::common_type_t<long&, int&>, long>::value));
+      CHECK((etl::is_same<etl::common_type_t<long&, int&>, std::common_type<long&, int&>::type>::value));
+      CHECK((etl::is_same<etl::common_type_t<int&, long&>, long>::value));
+      CHECK((etl::is_same<etl::common_type_t<int&, long&>, std::common_type<int&, long&>::type>::value));
+      CHECK((etl::is_same<etl::common_type_t<const char&, char&>, char>::value));
+      CHECK((etl::is_same<etl::common_type_t<const char&, char&>, std::common_type<const char&, char&>::type>::value));
+
+      // Already decayed types.
+      CHECK((etl::is_same<etl::common_type_t<int, long, int>, long>::value));
+      CHECK((etl::is_same<etl::common_type_t<int, long, int>, std::common_type<int, long, int>::type>::value));
+    }
+#endif
   }
 } // namespace