From 11c794e0799ecfadaf35f6d603ec451c449624bb Mon Sep 17 00:00:00 2001
From: Daniel Arndt <daniel.arndt@iwr.uni-heidelberg.de>
Date: Thu, 6 Sep 2018 21:54:09 +0200
Subject: [PATCH] Check for the ability to call non-constepr functions in
 constexpr ones

---
 cmake/checks/check_01_cxx_features.cmake | 29 ++++++++++++++++++++++++
 include/deal.II/base/config.h.in         |  1 +
 include/deal.II/base/utilities.h         |  6 +++--
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/cmake/checks/check_01_cxx_features.cmake b/cmake/checks/check_01_cxx_features.cmake
index 9b44082564..a9a6ce7ea0 100644
--- a/cmake/checks/check_01_cxx_features.cmake
+++ b/cmake/checks/check_01_cxx_features.cmake
@@ -23,6 +23,7 @@
 #
 #   DEAL_II_HAVE_ATTRIBUTE_FALLTHROUGH
 #   DEAL_II_HAVE_CXX11_IS_TRIVIALLY_COPYABLE
+#   DEAL_II_HAVE_CXX14_CONSTEXPR_CAN_CALL_NONCONSTEXPR
 #   DEAL_II_HAVE_CXX17_SPECIAL_MATH_FUNCTIONS
 #   DEAL_II_HAVE_FP_EXCEPTIONS
 #   DEAL_II_HAVE_COMPLEX_OPERATOR_OVERLOADS
@@ -552,6 +553,7 @@ UNSET_IF_CHANGED(CHECK_CXX_FEATURES_FLAGS_SAVED
   "${CMAKE_REQUIRED_FLAGS}${DEAL_II_CXX_VERSION_FLAG}${DEAL_II_WITH_CXX14}${DEAL_II_WITH_CXX17}"
   DEAL_II_HAVE_ATTRIBUTE_FALLTHROUGH
   DEAL_II_HAVE_CXX11_IS_TRIVIALLY_COPYABLE
+  DEAL_II_HAVE_CXX14_CONSTEXPR_CAN_CALL_NONCONSTEXPR
   DEAL_II_HAVE_CXX17_SPECIAL_MATH_FUNCTIONS
   DEAL_II_HAVE_FP_EXCEPTIONS
   DEAL_II_HAVE_COMPLEX_OPERATOR_OVERLOADS
@@ -668,6 +670,33 @@ CHECK_CXX_SOURCE_COMPILES(
   "
   DEAL_II_HAVE_COMPLEX_OPERATOR_OVERLOADS)
 
+#
+# As long as there exists an argument value such that an invocation of the
+# function or constructor could be an evaluated subexpression of a core constant
+# expression, C++14 allows to call non-constexpr functions from constexpr
+# functions. Unfortunately, not all compilers obey the standard in this regard.
+#
+CHECK_CXX_SOURCE_COMPILES(
+  "
+  void bar()
+  {}
+
+  constexpr int
+  foo(const int n)
+  {
+    if(!(n >= 0))
+      bar();
+    return n;
+  }
+
+  int main()
+  {
+    constexpr unsigned int n=foo(1);
+    return n;
+  }
+  "
+  DEAL_II_HAVE_CXX14_CONSTEXPR_CAN_CALL_NONCONSTEXPR)
+
 #
 # Not all compilers with C++17 support include the new special math
 # functions. Check this separately so that we can use C++17 compilers that don't
diff --git a/include/deal.II/base/config.h.in b/include/deal.II/base/config.h.in
index af22c18563..c32f0dd5b7 100644
--- a/include/deal.II/base/config.h.in
+++ b/include/deal.II/base/config.h.in
@@ -122,6 +122,7 @@
  */
 
 #cmakedefine DEAL_II_HAVE_CXX11_IS_TRIVIALLY_COPYABLE
+#cmakedefine DEAL_II_HAVE_CXX14_CONSTEXPR_CAN_CALL_NONCONSTEXPR
 #cmakedefine DEAL_II_HAVE_CXX17_SPECIAL_MATH_FUNCTIONS
 #cmakedefine DEAL_II_HAVE_FP_EXCEPTIONS
 #cmakedefine DEAL_II_HAVE_COMPLEX_OPERATOR_OVERLOADS
diff --git a/include/deal.II/base/utilities.h b/include/deal.II/base/utilities.h
index 491796bd18..44d83cd612 100644
--- a/include/deal.II/base/utilities.h
+++ b/include/deal.II/base/utilities.h
@@ -354,7 +354,8 @@ namespace Utilities
   pow(const unsigned int base, const int iexp)
   {
 #ifdef DEAL_II_WITH_CXX14
-#  if defined(DEAL_II_HAVE_BUILTIN_EXPECT) && defined(__INTEL_COMPILER)
+#  ifdef DEAL_II_HAVE_CXX14_CONSTEXPR_CAN_CALL_NONCONSTEXPR
+#    if defined(DEAL_II_HAVE_BUILTIN_EXPECT) && defined(__INTEL_COMPILER)
     if (!(iexp >= 0))
       ::dealii::deal_II_exceptions::internals::issue_error_noreturn(
         ::dealii::deal_II_exceptions::internals::abort_or_throw_on_exception,
@@ -364,8 +365,9 @@ namespace Utilities
         "iexp>=0",
         "ExcMessage(\"The exponent must not be negative!\")",
         ExcMessage("The exponent must not be negative!"));
-#  else
+#    else
     Assert(iexp >= 0, ExcMessage("The exponent must not be negative!"));
+#    endif
 #  endif
 #endif
     // The "exponentiation by squaring" algorithm used below has to be
-- 
2.39.5