Replace _append_variant() with variant_cat_t

include/trajopt/constraint/differential/DifferentialConstraint.h

@@ -6,13 +6,13 @@
 #include "trajopt/constraint/Constraint.h"
 #include "trajopt/constraint/differential/DifferentialCentripetalAccelerationConstraint.h"
 #include "trajopt/constraint/differential/DifferentialTangentialVelocityConstraint.h"
-#include "trajopt/util/AppendVariant.h"
+#include "trajopt/util/VariantCat.h"
 
 namespace trajopt {
 
 using DifferentialConstraint =
-    decltype(_append_variant(Constraint{}, AngularVelocityConstraint{},
-                             DifferentialTangentialVelocityConstraint{},
-                             DifferentialCentripetalAccelerationConstraint{}));
+    variant_cat_t<Constraint, AngularVelocityConstraint,
+                  DifferentialTangentialVelocityConstraint,
+                  DifferentialCentripetalAccelerationConstraint>;
 
 }  // namespace trajopt

include/trajopt/constraint/holonomic/HolonomicConstraint.h

@@ -6,12 +6,12 @@
 #include "trajopt/constraint/Constraint.h"
 #include "trajopt/constraint/PointAtConstraint.h"
 #include "trajopt/constraint/holonomic/HolonomicVelocityConstraint.h"
-#include "trajopt/util/AppendVariant.h"
+#include "trajopt/util/VariantCat.h"
 
 namespace trajopt {
 
-using HolonomicConstraint = decltype(_append_variant(
-    Constraint{}, AngularVelocityConstraint{}, HolonomicVelocityConstraint{},
-    PointAtConstraint{}));
+using HolonomicConstraint =
+    variant_cat_t<Constraint, AngularVelocityConstraint,
+                  HolonomicVelocityConstraint, PointAtConstraint>;
 
 }  // namespace trajopt

include/trajopt/util/VariantCat.h

@@ -0,0 +1,32 @@
+// Copyright (c) TrajoptLib contributors
+
+#pragma once
+
+#include <variant>
+
+namespace trajopt {
+
+namespace detail {
+
+//! @cond Doxygen_Suppress
+
+template <typename Variant, typename... Args>
+struct variant_cat;
+
+template <typename... Args0, typename... Args1>
+struct variant_cat<std::variant<Args0...>, Args1...> {
+  using type = std::variant<Args0..., Args1...>;
+};
+
+//! @endcond
+
+}  // namespace detail
+
+/**
+ * Defines a new variant type that appends the types Args... to the variant type
+ * Variant.
+ */
+template <typename Variant, typename... Args>
+using variant_cat_t = typename detail::variant_cat<Variant, Args...>::type;
+
+}  // namespace trajopt