libstdc++: Avoid a move in std::function construction (LWG 2447)

This makes the std::function constructor use perfect forwarding, to
avoid an unnecessary move-construction of the target. This means we need
to rewrite the _Function_base::_Base_manager::_M_init_functor function
to use a forwarding reference, and so can reuse it for the clone
operation.

Also simplify the SFINAE constraints on the constructor, by combining
the !is_same_v<remove_cvref_t<F>, function> constraint into the
_Callable trait.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/std_function.h (_function_base::_Base_manager):
	Replace _M_init_functor with a function template using a
	forwarding reference, and a pair of _M_create function
	templates. Reuse _M_create for the clone operation.
	(function::_Decay_t): New alias template.
	(function::_Callable): Simplify by using _Decay.
	(function::function(F)): Change parameter to forwarding
	reference, as per LWG 2447. Add noexcept-specifier. Simplify
	constraints.
	(function::operator=(F&&)): Add noexcept-specifier.
	* testsuite/20_util/function/cons/lwg2774.cc: New test.
	* testsuite/20_util/function/cons/noexcept.cc: New test.

diff --git a/libstdc++-v3/include/bits/std_function.h b/libstdc++-v3/include/bits/std_function.h
index e081cd8..8dfbd11 100644 (file)
--- a/libstdc++-v3/include/bits/std_function.h
+++ b/libstdc++-v3/include/bits/std_function.h
@@ -127,7 +127,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         && __alignof__(_Functor) <= _M_max_align
         && (_M_max_align % __alignof__(_Functor) == 0));
 
-       typedef integral_constant<bool, __stored_locally> _Local_storage;
+       using _Local_storage = integral_constant<bool, __stored_locally>;
 
        // Retrieve a pointer to the function object
        static _Functor*
@@ -142,32 +142,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
            return __source._M_access<_Functor*>();
        }
 
-       // Clone a location-invariant function object that fits within
+      private:
+       // Construct a location-invariant function object that fits within
        // an _Any_data structure.
-       static void
-       _M_clone(_Any_data& __dest, const _Any_data& __source, true_type)
-       {
-         ::new (__dest._M_access()) _Functor(__source._M_access<_Functor>());
-       }
+       template<typename _Fn>
+         static void
+         _M_create(_Any_data& __dest, _Fn&& __f, true_type)
+         {
+           ::new (__dest._M_access()) _Functor(std::forward<_Fn>(__f));
+         }
 
-       // Clone a function object that is not location-invariant or
-       // that cannot fit into an _Any_data structure.
-       static void
-       _M_clone(_Any_data& __dest, const _Any_data& __source, false_type)
-       {
-         __dest._M_access<_Functor*>() =
-           new _Functor(*__source._M_access<const _Functor*>());
-       }
+       // Construct a function object on the heap and store a pointer.
+       template<typename _Fn>
+         static void
+         _M_create(_Any_data& __dest, _Fn&& __f, false_type)
+         {
+           __dest._M_access<_Functor*>()
+             = new _Functor(std::forward<_Fn>(__f));
+         }
 
-       // Destroying a location-invariant object may still require
-       // destruction.
+       // Destroy an object stored in the internal buffer.
        static void
        _M_destroy(_Any_data& __victim, true_type)
        {
          __victim._M_access<_Functor>().~_Functor();
        }
 
-       // Destroying an object located on the heap.
+       // Destroy an object located on the heap.
        static void
        _M_destroy(_Any_data& __victim, false_type)
        {
@@ -188,12 +189,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              __dest._M_access<const type_info*>() = nullptr;
 #endif
              break;
+
            case __get_functor_ptr:
              __dest._M_access<_Functor*>() = _M_get_pointer(__source);
              break;
 
            case __clone_functor:
-             _M_clone(__dest, __source, _Local_storage());
+             _M_init_functor(__dest,
+                 *const_cast<const _Functor*>(_M_get_pointer(__source)));
              break;
 
            case __destroy_functor:
@@ -203,9 +206,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          return false;
        }
 
-       static void
-       _M_init_functor(_Any_data& __functor, _Functor&& __f)
-       { _M_init_functor(__functor, std::move(__f), _Local_storage()); }
+       template<typename _Fn>
+         static void
+         _M_init_functor(_Any_data& __functor, _Fn&& __f)
+         noexcept(__and_<_Local_storage,
+                         is_nothrow_constructible<_Functor, _Fn>>::value)
+         {
+           _M_create(__functor, std::forward<_Fn>(__f), _Local_storage());
+         }
 
        template<typename _Signature>
          static bool
@@ -226,15 +234,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          static bool
          _M_not_empty_function(const _Tp&)
          { return true; }
-
-      private:
-       static void
-       _M_init_functor(_Any_data& __functor, _Functor&& __f, true_type)
-       { ::new (__functor._M_access()) _Functor(std::move(__f)); }
-
-       static void
-       _M_init_functor(_Any_data& __functor, _Functor&& __f, false_type)
-       { __functor._M_access<_Functor*>() = new _Functor(std::move(__f)); }
       };
 
     _Function_base() = default;
@@ -291,6 +290,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        return std::__invoke_r<_Res>(*_Base::_M_get_pointer(__functor),
                                     std::forward<_ArgTypes>(__args)...);
       }
+
+      template<typename _Fn>
+       static constexpr bool
+       _S_nothrow_init() noexcept
+       {
+         return __and_<typename _Base::_Local_storage,
+                       is_nothrow_constructible<_Functor, _Fn>>::value;
+       }
     };
 
   // Specialization for invalid types
@@ -329,19 +336,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
       private _Function_base
     {
+      // Equivalent to std::decay_t except that it produces an invalid type
+      // if the decayed type is the current specialization of std::function.
       template<typename _Func,
-              typename _Res2 = __invoke_result<_Func&, _ArgTypes...>>
+              bool _Self = is_same<__remove_cvref_t<_Func>, function>::value>
+       using _Decay_t
+         = typename __enable_if_t<!_Self, decay<_Func>>::type;
+
+      template<typename _Func,
+              typename _DFunc = _Decay_t<_Func>,
+              typename _Res2 = __invoke_result<_DFunc&, _ArgTypes...>>
        struct _Callable
        : __is_invocable_impl<_Res2, _Res>::type
        { };
 
-      // Used so the return type convertibility checks aren't done when
-      // performing overload resolution for copy construction/assignment.
-      template<typename _Tp>
-       struct _Callable<function, _Tp> : false_type { };
+      template<typename _Cond, typename _Tp = void>
+       using _Requires = __enable_if_t<_Cond::value, _Tp>;
 
-      template<typename _Cond, typename _Tp>
-       using _Requires = typename enable_if<_Cond::value, _Tp>::type;
+      template<typename _Functor>
+       using _Handler
+         = _Function_handler<_Res(_ArgTypes...), __decay_t<_Functor>>;
 
     public:
       typedef _Res result_type;
@@ -416,23 +430,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  If @a __f is a non-NULL function pointer or an object of type @c
        *  reference_wrapper<F>, this function will not throw.
        */
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 2774. std::function construction vs assignment
       template<typename _Functor,
-              typename = _Requires<__not_<is_same<_Functor, function>>, void>,
-              typename = _Requires<_Callable<_Functor>, void>>
-       function(_Functor __f)
+              typename = _Requires<_Callable<_Functor>>>
+       function(_Functor&& __f)
+       noexcept(_Handler<_Functor>::template _S_nothrow_init<_Functor>())
        : _Function_base()
        {
-         static_assert(is_copy_constructible<_Functor>::value,
+         static_assert(is_copy_constructible<__decay_t<_Functor>>::value,
              "std::function target must be copy-constructible");
-         static_assert(is_constructible<_Functor, _Functor>::value,
+         static_assert(is_constructible<__decay_t<_Functor>, _Functor>::value,
              "std::function target must be constructible from the "
              "constructor argument");
 
-         using _My_handler = _Function_handler<_Res(_ArgTypes...), _Functor>;
+         using _My_handler = _Handler<_Functor>;
 
          if (_My_handler::_M_not_empty_function(__f))
            {
-             _My_handler::_M_init_functor(_M_functor, std::move(__f));
+             _My_handler::_M_init_functor(_M_functor,
+                                          std::forward<_Functor>(__f));
              _M_invoker = &_My_handler::_M_invoke;
              _M_manager = &_My_handler::_M_manager;
            }
@@ -511,8 +528,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  reference_wrapper<F>, this function will not throw.
        */
       template<typename _Functor>
-       _Requires<_Callable<typename decay<_Functor>::type>, function&>
+       _Requires<_Callable<_Functor>, function&>
        operator=(_Functor&& __f)
+       noexcept(_Handler<_Functor>::template _S_nothrow_init<_Functor>())
        {
          function(std::forward<_Functor>(__f)).swap(*this);
          return *this;

diff --git a/libstdc++-v3/testsuite/20_util/function/cons/lwg2774.cc b/libstdc++-v3/testsuite/20_util/function/cons/lwg2774.cc
new file mode 100644 (file)
index 0000000..a606104
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function/cons/lwg2774.cc
@@ -0,0 +1,31 @@
+// { dg-do run { target c++11 } }
+#include <functional>
+#include <testsuite_hooks.h>
+
+struct Funk
+{
+  Funk() = default;
+  Funk(const Funk&) { ++copies; }
+  Funk(Funk&&) { ++moves; }
+
+  void operator()() const { }
+
+  static int copies;
+  static int moves;
+};
+
+int Funk::copies = 0;
+int Funk::moves = 0;
+
+int main()
+{
+  Funk e;
+  // LWG 2774 means there should be no move here:
+  std::function<void()> fc(e);
+  VERIFY(Funk::copies == 1);
+  VERIFY(Funk::moves == 0);
+  // And only one move here:
+  std::function<void()> fm(std::move(e));
+  VERIFY(Funk::copies == 1);
+  VERIFY(Funk::moves == 1);
+}

diff --git a/libstdc++-v3/testsuite/20_util/function/cons/noexcept.cc b/libstdc++-v3/testsuite/20_util/function/cons/noexcept.cc
new file mode 100644 (file)
index 0000000..6357198
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function/cons/noexcept.cc
@@ -0,0 +1,37 @@
+// { dg-do compile { target c++11 } }
+#include <functional>
+
+struct X
+{
+  void operator()(X*);
+
+  char bigness[100];
+};
+
+using F = std::function<void(X*)>;
+
+static_assert( std::is_nothrow_constructible<F>::value, "" );
+static_assert( std::is_nothrow_constructible<F, F>::value, "" );
+static_assert( ! std::is_nothrow_constructible<F, F&>::value, "" );
+static_assert( ! std::is_nothrow_constructible<F, const F&>::value, "" );
+static_assert( std::is_nothrow_constructible<F, std::nullptr_t>::value, "" );
+
+static_assert( ! std::is_nothrow_constructible<F, X>::value, "" );
+using R = std::reference_wrapper<X>;
+static_assert( std::is_nothrow_constructible<F, R>::value, "" );
+
+
+// The standard requires that construction from a function pointer type
+// does not throw, but doesn't require that the construction is noexcept.
+// Strengthening that noexcept for these types is a GCC extension.
+static_assert( std::is_nothrow_constructible<F, void(*)(X*)>::value, "" );
+// This is a GCC extension, not required by the standard:
+static_assert( std::is_nothrow_constructible<F, void(&)(X*)>::value, "" );
+// This is a GCC extension, not required by the standard:
+static_assert( std::is_nothrow_constructible<F, void(X::*)()>::value, "" );
+
+auto c = [](X*){};
+static_assert( std::is_nothrow_constructible<F, decltype(+c)>::value, "" );
+// The standard allows this to throw, but as a GCC extenension we store
+// closures with no captures in the std::function, so this is noexcept too:
+static_assert( std::is_nothrow_constructible<F, decltype(c)>::value, "" );