From b3f7e6de98dc66c6108f4598ea649437911ffb10 Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Mon, 18 Jul 2022 15:54:28 -0600
Subject: [PATCH] Declare 'enable_if_all_t' in analogy to std::enable_if_t.

While there, also use it in fe_system.h.
---
 include/deal.II/base/template_constraints.h | 51 ++++++++++++++++++++-
 include/deal.II/fe/fe_system.h              |  4 +-
 2 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/include/deal.II/base/template_constraints.h b/include/deal.II/base/template_constraints.h
index 0d49d35b8f..fd333a27d1 100644
--- a/include/deal.II/base/template_constraints.h
+++ b/include/deal.II/base/template_constraints.h
@@ -259,7 +259,27 @@ struct is_same_as_any_of
 /*
  * A generalization of `std::enable_if` that only works if
  * <i>all</i> of the given boolean template parameters are
- * true.
+ * true. See [here](https://en.cppreference.com/w/cpp/types/enable_if)
+ * for what `std::enable_if` does.
+ *
+ * @note
+ * In contrast to `std::enable_if`, this template has no additional
+ * template type (which for `std::enable_if` is defaulted to `void`).
+ * As a consequence, this structure cannot be used for anything other
+ * than enabling or disabling a template declaration; in particular
+ * it cannot be used to set the return type of a function as one might
+ * do with something like
+ * @code
+ *   template <typename T>
+ *   typename std::enable_if<std::is_floating_point<T>::value, T>::type
+ *   abs (const T t);
+ * @endcode
+ * which declares a function template `abs()` that can only be
+ * instantiated if `T` is a floating point type; this function then
+ * returns an object of type `T` as indicated by the last argument to
+ * `std::enable_if`. The reason `enable_if_all` does not allow providing
+ * this additional type is that variadic templates (here, the list of
+ * `bool` arguments) must be the last template argument.
  */
 template <bool... Values>
 struct enable_if_all
@@ -268,6 +288,35 @@ struct enable_if_all
 
 
 
+/*
+ * A generalization of `std::enable_if_t` that only works if
+ * <i>all</i> of the given boolean template parameters are
+ * true. See [here](https://en.cppreference.com/w/cpp/types/enable_if)
+ * for what `std::enable_if_t` does.
+ *
+ * @note
+ * In contrast to `std::enable_if_t`, this template has no additional
+ * template type (which for `std::enable_if` is defaulted to `void`).
+ * As a consequence, this structure cannot be used for anything other
+ * than enabling or disabling a template declaration; in particular
+ * it cannot be used to set the return type of a function as one might
+ * do with something like
+ * @code
+ *   template <typename T>
+ *   std::enable_if_t<std::is_floating_point<T>::value, T>
+ *   abs (const T t);
+ * @endcode
+ * which declares a function template `abs()` that can only be
+ * instantiated if `T` is a floating point type; this function then
+ * returns an object of type `T` as indicated by the last argument to
+ * `std::enable_if`. The reason `enable_if_all` does not allow providing
+ * this additional type is that variadic templates (here, the list of
+ * `bool` arguments) must be the last template argument.
+ */
+template <bool... Values>
+using enable_if_all_t = typename enable_if_all<Values...>::type;
+
+
 /**
  * A type trait that checks to see if a class behaves as an iterable container
  * that has a beginning and an end. This implies that the class either defines
diff --git a/include/deal.II/fe/fe_system.h b/include/deal.II/fe/fe_system.h
index b834c4c95e..2a86623626 100644
--- a/include/deal.II/fe/fe_system.h
+++ b/include/deal.II/fe/fe_system.h
@@ -536,12 +536,12 @@ public:
    */
   template <
     class... FEPairs,
-    typename = typename enable_if_all<
+    typename = enable_if_all_t<
       (std::is_same<typename std::decay<FEPairs>::type,
                     std::pair<std::unique_ptr<FiniteElement<dim, spacedim>>,
                               unsigned int>>::value ||
        std::is_base_of<FiniteElement<dim, spacedim>,
-                       typename std::decay<FEPairs>::type>::value)...>::type>
+                       typename std::decay<FEPairs>::type>::value)...>>
   FESystem(FEPairs &&...fe_pairs);
 
   /**
-- 
2.39.5