From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Sat, 25 Feb 2023 21:46:04 +0000 (-0700)
Subject: Introduce a macro to use for 'requires' clauses.
X-Git-Tag: v9.5.0-rc1~513^2~5
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7893c94803ea7be87f0a2a9b14a32d24ba606148;p=dealii.git

Introduce a macro to use for 'requires' clauses.
---

diff --git a/include/deal.II/base/config.h.in b/include/deal.II/base/config.h.in
index b2b830344d..6cf17cf239 100644
--- a/include/deal.II/base/config.h.in
+++ b/include/deal.II/base/config.h.in
@@ -150,6 +150,18 @@
 #cmakedefine DEAL_II_HAVE_CXX17
 #cmakedefine DEAL_II_HAVE_CXX20
 
+/**
+ * If we have C++20 available, we can have concepts and requires
+ * clauses. We want to avoid using too many `#ifdef` statements, so
+ * define a convenience macro that allows us to write a 'requires'
+ * clause that is simply removed when not using C++20.
+ */
+#ifdef DEAL_II_HAVE_CXX20
+#  define DEAL_II_CXX20_REQUIRES(condition) requires(condition)
+#else
+#  define DEAL_II_CXX20_REQUIRES(condition)
+#endif
+
 #cmakedefine DEAL_II_HAVE_FP_EXCEPTIONS
 #cmakedefine DEAL_II_HAVE_COMPLEX_OPERATOR_OVERLOADS
 #cmakedefine DEAL_II_HAVE_CXX17_BESSEL_FUNCTIONS
@@ -174,9 +186,10 @@
 #cmakedefine DEAL_II_FALLTHROUGH @DEAL_II_FALLTHROUGH@
 #cmakedefine DEAL_II_CONSTEXPR @DEAL_II_CONSTEXPR@
 
-// defined for backwards compatibility with older deal.II versions
+// The following two are defined for backwards compatibility with older deal.II versions:
 #define DEAL_II_WITH_CXX11
 #define DEAL_II_WITH_CXX14
+
 #ifdef DEAL_II_HAVE_CXX17
 #  define DEAL_II_WITH_CXX17
 #endif