Make function::base private

This diff removes the `function::base` nested alias from `function`'s
public interface.

Author: ispeters

include/exec/function.hpp

@@ -376,10 +376,15 @@ namespace experimental::execution
     : _func::_func_impl<STDEXEC::sender_tag(Args...), _func::_sigs_from_t<Return, false>, queries<>>
     , _func::_func_ops_crtp<function<Return(Args...)>>
   {
+   private:
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return, false>,
                                    queries<>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 
@@ -388,9 +393,14 @@ namespace experimental::execution
     : _func::_func_impl<STDEXEC::sender_tag(Args...), _func::_sigs_from_t<Return, true>, queries<>>
     , _func::_func_ops_crtp<function<Return(Args...) noexcept>>
   {
+   private:
     using base =
       _func::_func_impl<STDEXEC::sender_tag(Args...), _func::_sigs_from_t<Return, true>, queries<>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 
@@ -400,9 +410,14 @@ namespace experimental::execution
     : _func::_func_impl<STDEXEC::sender_tag(Args...), _func::_canonical_t<Sigs>, queries<>>
     , _func::_func_ops_crtp<function<STDEXEC::sender_tag(Args...), Sigs>>
   {
+   private:
     using base =
       _func::_func_impl<STDEXEC::sender_tag(Args...), _func::_canonical_t<Sigs>, queries<>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 
@@ -413,10 +428,15 @@ namespace experimental::execution
                         _func::_canonical_t<queries<Queries...>>>
     , _func::_func_ops_crtp<function<Return(Args...), queries<Queries...>>>
   {
+   private:
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return, false>,
                                    _func::_canonical_t<queries<Queries...>>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 
@@ -427,10 +447,15 @@ namespace experimental::execution
                         _func::_canonical_t<queries<Queries...>>>
     , _func::_func_ops_crtp<function<Return(Args...) noexcept, queries<Queries...>>>
   {
+   private:
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_sigs_from_t<Return, true>,
                                    _func::_canonical_t<queries<Queries...>>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 
@@ -445,10 +470,15 @@ namespace experimental::execution
                                      STDEXEC::completion_signatures<Sigs...>,
                                      queries<Queries...>>>
   {
+   private:
     using base = _func::_func_impl<STDEXEC::sender_tag(Args...),
                                    _func::_canonical_t<STDEXEC::completion_signatures<Sigs...>>,
                                    _func::_canonical_t<queries<Queries...>>>;
 
+    template <class, class, template <class> class, template <class> class>
+    friend struct std::basic_common_reference;
+
+   public:
     using base::base;
   };
 }  // namespace experimental::execution

test/exec/test_function.cpp

@@ -297,20 +297,13 @@ namespace
       exec::function<ex::sender_tag(int),
                      ex::completion_signatures<ex::set_stopped_t(), ex::set_value_t(int)>>;
 
-    SECTION("the function types are not the same as each other...")
+    SECTION("the function types are not the same as each other")
     {
       STATIC_REQUIRE(!std::same_as<func1_t, func2_t>);
       STATIC_REQUIRE(!std::same_as<func1_t, func3_t>);
       STATIC_REQUIRE(!std::same_as<func2_t, func3_t>);
     }
 
-    SECTION("...but they all inherit from the same _func_impl base")
-    {
-      STATIC_REQUIRE(std::same_as<func1_t::base, func2_t::base>);
-      STATIC_REQUIRE(std::same_as<func1_t::base, func3_t::base>);
-      STATIC_REQUIRE(std::same_as<func2_t::base, func3_t::base>);
-    }
-
     SECTION("move-construction works in every direction between all three types")
     {
       STATIC_REQUIRE(std::constructible_from<func1_t, func1_t>);
@@ -385,16 +378,11 @@ namespace
     using func2_t =
       exec::function<int(int), exec::queries<double(query2_t, int), int(query1_t) noexcept>>;
 
-    SECTION("the function types are not the same as each other...")
+    SECTION("the function types are not the same as each other")
     {
       STATIC_REQUIRE(!std::same_as<func1_t, func2_t>);
     }
 
-    SECTION("...but they both inherit from the same _func_impl base")
-    {
-      STATIC_REQUIRE(std::same_as<func1_t::base, func2_t::base>);
-    }
-
     SECTION("move construction works in all directions with both types")
     {
       STATIC_REQUIRE(std::constructible_from<func1_t, func1_t>);