From 9823a98a4dbc86ee79852e7af9b75d772e5d2205 Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Tue, 1 Mar 2022 14:22:39 -0700
Subject: [PATCH] Provide a facility where we can test that a type is one of a
 list.

---
 include/deal.II/base/template_constraints.h | 45 ++++++++++++++++++++-
 1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/include/deal.II/base/template_constraints.h b/include/deal.II/base/template_constraints.h
index 1d39f1b1d7..6b254638fe 100644
--- a/include/deal.II/base/template_constraints.h
+++ b/include/deal.II/base/template_constraints.h
@@ -170,7 +170,11 @@ namespace internal
 
     /**
      * A helper class whose `value` member is true or false depending on
-     * whether all of the given boolean template arguments are true.
+     * whether all of the given boolean template arguments are `true`.
+     * The class works by comparing the list of boolean values
+     * `true, Values...` with the list `Values..., true` (i.e., with
+     * its rotated self). The two are only the same if `Values...` is
+     * a list of only `true` values.
      */
     template <bool... Values>
     struct all_true
@@ -179,6 +183,28 @@ namespace internal
         std::is_same<BoolStorage<Values..., true>,
                      BoolStorage<true, Values...>>::value;
     };
+
+
+    /**
+     * A class whose `value` member is set to `true` if any of the
+     * boolean template arguments are true.
+     */
+    template <bool... Values>
+    struct any_true;
+
+
+    template <bool V1, bool... Values>
+    struct any_true<V1, Values...>
+    {
+      static constexpr bool value = V1 || any_true<Values...>::value;
+    };
+
+
+    template <>
+    struct any_true<>
+    {
+      static constexpr bool value = false;
+    };
   } // namespace TemplateConstraints
 } // namespace internal
 
@@ -201,7 +227,7 @@ struct is_base_of_all
  * This struct is a generalization of std::is_same to template
  * parameter packs and tests if all of the types in the `Types...`
  * parameter pack are equal to the `Type` given as first template
- * argument. The result is stored in the member variable value.
+ * argument. The result is stored in the member variable `value`.
  */
 template <class Type, class... Types>
 struct all_same_as
@@ -212,6 +238,21 @@ struct all_same_as
 
 
 
+/**
+ * This struct is a generalization of std::is_same to template
+ * parameter packs and tests if any of the types in the `Types...`
+ * parameter pack are equal to the `Type` given as first template
+ * argument. The result is stored in the member variable `value`.
+ */
+template <class Type, class... Types>
+struct is_same_as_any_of
+{
+  static constexpr bool value = internal::TemplateConstraints::any_true<
+    std::is_same<Type, Types>::value...>::value;
+};
+
+
+
 /*
  * A generalization of `std::enable_if` that only works if
  * <i>all</i> of the given boolean template parameters are
-- 
2.39.5