wip: C++20: Make sigc::ptr_fun() constexprc++20

This needs C++20 because it needs constexpr std::invoke().

However, I don't currently have a compiler/library that has constexpr
std::invoke().

5 files changed, 37 insertions, 7 deletions

diff --git a/sigc++/functors/ptr_fun.h b/sigc++/functors/ptr_fun.h
index c680a2e..bef305c 100644
--- a/sigc++/functors/ptr_fun.h
+++ b/sigc++/functors/ptr_fun.h
@@ -82,18 +82,18 @@ protected:
 public:
 
   /// Constructs an invalid functor.
-  pointer_functor() = default;
+  constexpr pointer_functor() = default;
 
   /** Constructs a pointer_functor2 object that wraps an existing function.
    * @param func Pointer to function that will be invoked from operator()().
    */
-  explicit pointer_functor(function_type func) : func_ptr_(func) {}
+  constexpr explicit pointer_functor(function_type func) : func_ptr_(func) {}
 
   /** Execute the wrapped function.
    * @param a Arguments to be passed on to the function.
    * @return The return value of the function invocation.
    */
-  T_return operator()(type_trait_take_t<T_args>... a) const {
+  constexpr T_return operator()(type_trait_take_t<T_args>... a) const {
     return std::invoke(func_ptr_, a...);
   }
 };
@@ -105,7 +105,7 @@ public:
  * @ingroup ptr_fun
  */
 template <typename T_return, typename... T_args>
-inline decltype(auto) ptr_fun(T_return (*func)(T_args...))
+inline constexpr decltype(auto) ptr_fun(T_return (*func)(T_args...))
 {
   return pointer_functor<T_return(T_args...)>(func);
 }
diff --git a/sigc++/functors/slot.h b/sigc++/functors/slot.h
index 5cda9cc..6befc02 100644
--- a/sigc++/functors/slot.h
+++ b/sigc++/functors/slot.h
@@ -48,7 +48,7 @@ namespace internal
  * (argument-dependent lookup).
  */
 template <typename T_out, typename T_in>
-inline T_out function_pointer_cast(T_in in)
+inline constexpr T_out function_pointer_cast(T_in in)
 {
   // The double reinterpret_cast suppresses a warning from gcc8 with the
   // -Wcast-function-type option.
@@ -78,13 +78,13 @@ public:
    * The notification callback is registered using visit_each().
    * @param functor The functor contained by the new slot_rep object.
    */
-  inline explicit typed_slot_rep(const T_functor& functor)
+  inline constexpr explicit typed_slot_rep(const T_functor& functor)
   : slot_rep(nullptr), functor_(std::make_unique<adaptor_type>(functor))
   {
     sigc::visit_each_trackable(slot_do_bind(this), *functor_);
   }
 
-  inline typed_slot_rep(const typed_slot_rep& src)
+  inline constexpr typed_slot_rep(const typed_slot_rep& src)
   : slot_rep(src.call_), functor_(std::make_unique<adaptor_type>(*src.functor_))
   {
     sigc::visit_each_trackable(slot_do_bind(this), *functor_);
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 36e728b..80d611d 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -26,6 +26,7 @@ set (TEST_SOURCE_FILES
   test_bind_return.cc
   test_compose.cc
   test_connection.cc
+  test_constexpr.cc
   test_copy_invalid_slot.cc
   test_cpp11_lambda.cc
   test_custom.cc
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 735d76b..39aecfa 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -33,6 +33,7 @@ check_PROGRAMS = \
   test_bind_return \
   test_compose \
   test_connection \
+  test_constexpr \
   test_copy_invalid_slot \
   test_cpp11_lambda \
   test_custom \
@@ -77,6 +78,7 @@ test_bind_refptr_SOURCES     = test_bind_refptr.cc $(sigc_test_util)
 test_bind_return_SOURCES     = test_bind_return.cc $(sigc_test_util)
 test_compose_SOURCES         = test_compose.cc $(sigc_test_util)
 test_connection_SOURCES      = test_connection.cc $(sigc_test_util)
+test_constexpr_SOURCES       = test_constexpr.cc $(sigc_test_util)
 test_copy_invalid_slot_SOURCES = test_copy_invalid_slot.cc $(sigc_test_util)
 test_cpp11_lambda_SOURCES    = test_cpp11_lambda.cc $(sigc_test_util)
 test_custom_SOURCES          = test_custom.cc $(sigc_test_util)
diff --git a/tests/test_constexpr.cc b/tests/test_constexpr.cc
new file mode 100644
index 0000000..98e773b
--- /dev/null
+++ b/tests/test_constexpr.cc
@@ -0,0 +1,27 @@
+/* Copyright 2002 - 2016, The libsigc++ Development Team
+ *  Assigned to public domain.  Use as you wish without restriction.
+ */
+
+#include "testutilities.h"
+#include <sigc++/sigc++.h>
+
+namespace {
+
+constexpr
+int foo(int a, int b) {
+    return a + b;
+}
+
+} // end anonymous namespace
+
+int
+main()
+{
+  constexpr auto slot = sigc::ptr_fun(&foo);
+  constexpr auto result = slot(1, 2);
+  if (result != 3) {
+      return EXIT_FAILURE;
+  }
+
+  EXIT_SUCCESS;
+}