Adding a const & here for consistency reasons

The other vector types have T const & scalar for their binary operator+

glm/detail/type_vec4.inl

@@ -627,7 +627,7 @@ namespace detail
 	// -- Binary arithmetic operators --
 
 	template<typename T, qualifier Q>
-	GLM_FUNC_QUALIFIER vec<4, T, Q> operator+(vec<4, T, Q> const& v, T scalar)
+	GLM_FUNC_QUALIFIER vec<4, T, Q> operator+(vec<4, T, Q> const& v, T const & scalar)
 	{
 		return vec<4, T, Q>(v) += scalar;
 	}