From e586885c590e06227352a44ec31ba8ffead7d02c Mon Sep 17 00:00:00 2001
From: Benjamin Navarro <navarro.benjamin13@gmail.com>
Date: Fri, 18 Nov 2022 15:35:53 +0100
Subject: [PATCH] fix operator+ being picked for 3rd party types (#299)

---
 include/units.h    |  2 +-
 unitTests/main.cpp | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/units.h b/include/units.h
index 3a956f5d..6e321bd7 100644
--- a/include/units.h
+++ b/include/units.h
@@ -2503,7 +2503,7 @@ namespace units
 	//	LINEAR ARITHMETIC
 	//------------------------------
 
-	template<class UnitTypeLhs, class UnitTypeRhs, std::enable_if_t<!traits::is_same_scale<UnitTypeLhs, UnitTypeRhs>::value, int> = 0>
+	template<class UnitTypeLhs, class UnitTypeRhs, std::enable_if_t<traits::is_unit_t<UnitTypeLhs>::value && traits::is_unit_t<UnitTypeRhs>::value && !traits::is_same_scale<UnitTypeLhs, UnitTypeRhs>::value, int> = 0>
 	constexpr inline int operator+(const UnitTypeLhs& /* lhs */, const UnitTypeRhs& /* rhs */) noexcept
 	{
 		static_assert(traits::is_same_scale<UnitTypeLhs, UnitTypeRhs>::value, "Cannot add units with different linear/non-linear scales.");
diff --git a/unitTests/main.cpp b/unitTests/main.cpp
index 96befa34..0a9dc5be 100644
--- a/unitTests/main.cpp
+++ b/unitTests/main.cpp
@@ -927,6 +927,22 @@ TEST_F(UnitContainer, make_unit)
 	EXPECT_EQ(meter_t(5), dist);
 }
 
+namespace test {
+
+// Test types for https://github.com/nholthaus/units/issues/299
+struct Foo {
+	template<typename Unit>
+	Foo operator+(const unit_t<Unit>&) const {
+		return Foo{};
+	}
+};
+
+struct Bar : Foo {
+	// operator+ is implicitly inherited
+};
+
+}
+
 TEST_F(UnitContainer, unitTypeAddition)
 {
 	// units
@@ -963,6 +979,9 @@ TEST_F(UnitContainer, unitTypeAddition)
 
 	d = 1.0 + scalar_t(1.0);
 	EXPECT_NEAR(2.0, d, 5.0e-6);
+
+	test::Bar bar;
+	bar + a_m;
 }
 
 TEST_F(UnitContainer, unitTypeUnaryAddition)