On 15/08/19 17:04 +0100, Jonathan Wakely wrote:
In C++17 a function can return a prvalue of a type that cannot be moved
or copied. The current implementation of std::is_invocable_r uses
std::is_convertible to test the conversion to R required by INVOKE<R>.
That fails for non-copyable prvalues, because std::is_convertible is
defined in terms of std::declval which uses std::add_rvalue_reference.
In C++17 conversion from R to R involves no copies and so is not the
same as conversion from R&& to R.
This commit changes std::is_invocable_r to check the conversion without
using std::is_convertible.
std::function also contains a similar check using std::is_convertible,
which can be fixed by simply reusing std::is_invocable_r (but because
std::is_invocable_r is not defined for C++11 it uses the underlying
std::__is_invocable_impl trait directly).
PR libstdc++/91456
* include/bits/std_function.h (__check_func_return_type): Remove.
(function::_Callable): Use std::__is_invocable_impl instead of
__check_func_return_type.
* include/std/type_traits (__is_invocable_impl): Add another defaulted
template parameter. Define a separate partial specialization for
INVOKE and INVOKE<void>. For INVOKE<R> replace is_convertible check
with a check that models delayed temporary materialization.
* testsuite/20_util/function/91456.cc: New test.
* testsuite/20_util/is_invocable/91456.cc: New test.
With some minor changes to __is_convertible_helper we could make that
usable by both std::is_convertible and __is_invokable_impl.
I don't plan to commit this now but might do at a later date.
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 44db2cade5d..4df3fee4c77 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1491,20 +1491,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
bool = __or_<is_void<_From>, is_function<_To>,
is_array<_To>>::value>
struct __is_convertible_helper
- {
- typedef typename is_void<_To>::type type;
- };
+ : public is_void<_To>::type
+ { };
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wctor-dtor-privacy"
template<typename _From, typename _To>
class __is_convertible_helper<_From, _To, false>
{
+ // Unlike declval, this doesn't add_rvalue_reference.
+ template<typename _From1>
+ static _From1 __declval();
+
template<typename _To1>
static void __test_aux(_To1) noexcept;
template<typename _From1, typename _To1,
- typename = decltype(__test_aux<_To1>(std::declval<_From1>()))>
+ typename = decltype(__test_aux<_To1>(__declval<_From1>()))>
static true_type
__test(int);
@@ -1513,14 +1516,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__test(...);
public:
- typedef decltype(__test<_From, _To>(0)) type;
+ using type = decltype(__test<_From, _To>(0));
};
#pragma GCC diagnostic pop
+ template<typename _Tp> struct add_rvalue_reference;
+
/// is_convertible
template<typename _From, typename _To>
struct is_convertible
- : public __is_convertible_helper<_From, _To>::type
+ : public __is_convertible_helper<typename add_rvalue_reference<_From>::type,
+ _To>::type
{ };
template<typename _From, typename _To,
@@ -1535,12 +1541,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _From, typename _To>
class __is_nt_convertible_helper<_From, _To, false>
{
+ // Unlike declval, this doesn't add_rvalue_reference.
+ template<typename _From1>
+ static _From1 __declval();
+
template<typename _To1>
static void __test_aux(_To1) noexcept;
template<typename _From1, typename _To1>
static
- __bool_constant<noexcept(__test_aux<_To1>(std::declval<_From1>()))>
+ __bool_constant<noexcept(__test_aux<_To1>(__declval<_From1>()))>
__test(int);
template<typename, typename>
@@ -1555,14 +1565,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// is_nothrow_convertible for C++11
template<typename _From, typename _To>
struct __is_nothrow_convertible
- : public __is_nt_convertible_helper<_From, _To>::type
+ : __is_nt_convertible_helper<typename add_rvalue_reference<_From>::type,
+ _To>::type
{ };
#if __cplusplus > 201703L
/// is_nothrow_convertible
template<typename _From, typename _To>
struct is_nothrow_convertible
- : public __is_nt_convertible_helper<_From, _To>::type
+ : __is_nt_convertible_helper<typename add_rvalue_reference<_From>::type,
+ _To>::type
{ };
/// is_nothrow_convertible_v
@@ -2896,35 +2908,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
: true_type
{ };
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wctor-dtor-privacy"
// Used for INVOKE<R> expressions to check the implicit conversion to R.
template<typename _Result, typename _Ret>
struct __is_invocable_impl<_Result, _Ret,
/* is_void<_Ret> = */ false,
__void_t<typename _Result::type>>
- {
- private:
- // The type of the INVOKE expression.
- // Unlike declval, this doesn't add_rvalue_reference.
- static typename _Result::type _S_get();
-
- template<typename _Tp>
- static void _S_conv(_Tp);
-
- // This overload is viable if INVOKE(f, args...) can convert to _Tp.
- template<typename _Tp, typename = decltype(_S_conv<_Tp>(_S_get()))>
- static true_type
- _S_test(int);
-
- template<typename _Tp>
- static false_type
- _S_test(...);
-
- public:
- using type = decltype(_S_test<_Ret>(1));
- };
-#pragma GCC diagnostic pop
+ : __is_convertible_helper<typename _Result::type, _Ret>
+ { };
template<typename _Fn, typename... _ArgTypes>
struct __is_invocable
