http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55722
Bug #: 55722
Summary: failed static_assert won't trigger a second time
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: r...@gcc.gnu.org
Created attachment 28986
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28986
reduced testcase
#include <type_traits>
#include <utility>
template<typename _Signature>
struct _Bind_simple;
template<typename _Callable, typename... _Args>
struct _Bind_simple<_Callable(_Args...)>
{
template<typename _Callable2, typename... _Args2>
explicit
_Bind_simple(_Callable2&& __callable, _Args2&&... __args)
{ }
};
template<typename _Func, typename... _BoundArgs>
struct _Bind_simple_helper
{
typedef typename std::decay<_Func>::type __func_type;
typedef _Bind_simple<__func_type(typename std::decay<_BoundArgs>::type...)>
type;
typedef std::result_of<__func_type(typename
std::decay<_BoundArgs>::type...)> __result_of;
};
template<typename _Callable>
struct __assert_callable
{
static_assert(_Callable::value, "Invalid arguments for callable type");
};
template<typename _Func, typename... _Args>
__assert_callable<std::false_type>
__check_callable(...);
template<typename _Func, typename... _Args,
typename _Tp = _Bind_simple_helper<_Func, _Args...>,
typename = typename _Tp::__result_of::type>
typename _Tp::type
__check_callable(int);
template<typename _Callable, typename... _Args>
decltype(__check_callable<_Callable, _Args...>(0))
__bind_simple(_Callable&& __callable, _Args&&... __args)
{
typedef _Bind_simple_helper<_Callable, _Args...> __helper_type;
typedef typename __helper_type::type __bindexpr_type;
return __bindexpr_type( std::forward<_Callable>(__callable),
std::forward<_Args>(__args)...);
}
template<typename _Callable, typename... _Args>
void
call(_Callable&& __f, _Args&&... __args)
{
auto __bound_functor = __bind_simple(std::forward<_Callable>(__f),
std::forward<_Args>(__args)...);
}
void f(int&);
void test01()
{
call(f);
#ifndef ONLY_ONE
call(f, 1, 2);
#endif
}
When compiled with ONLY_ONE defined this produces a nice "static assertion
failed" message, as intended.
$ g++ -std=c++11 -DONLY_ONE
sa.cc: In instantiation of ‘struct
__assert_callable<std::integral_constant<bool, false> >’:
sa.cc:42:51: required by substitution of ‘template<class _Callable, class ...
_Args> decltype (__check_callable<_Callable, _Args ...>(0))
__bind_simple(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args
= {}]’
sa.cc:55:39: required from ‘void call(_Callable&&, _Args&& ...) [with
_Callable = void (&)(int&); _Args = {}]’
sa.cc:62:9: required from here
sa.cc:28:5: error: static assertion failed: Invalid arguments for callable type
static_assert(_Callable::value, "Invalid arguments for callable type");
^
But if ONLY_ONE is not defined, so there's a second invalid call(), the nice
clear diagnostic turns into an unpleasant mess:
$ g++ -std=c++11
sa.cc: In instantiation of ‘struct
__assert_callable<std::integral_constant<bool, false> >’:
sa.cc:42:51: required by substitution of ‘template<class _Callable, class ...
_Args> decltype (__check_callable<_Callable, _Args ...>(0))
__bind_simple(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args
= {}]’
sa.cc:55:39: required from ‘void call(_Callable&&, _Args&& ...) [with
_Callable = void (&)(int&); _Args = {}]’
sa.cc:62:9: required from here
sa.cc:28:5: error: static assertion failed: Invalid arguments for callable type
static_assert(_Callable::value, "Invalid arguments for callable type");
^
sa.cc: In instantiation of ‘decltype (__check_callable<_Callable, _Args
...>(0)) __bind_simple(_Callable&&, _Args&& ...) [with _Callable = void
(&)(int&); _Args = {int, int}; decltype (__check_callable<_Callable, _Args
...>(0)) = __assert_callable<std::integral_constant<bool, false> >]’:
sa.cc:55:39: required from ‘void call(_Callable&&, _Args&& ...) [with
_Callable = void (&)(int&); _Args = {int, int}]’
sa.cc:64:15: required from here
sa.cc:47:96: error: could not convert ‘_Bind_simple<void (*(int,
int))(int&)>((* & std::forward<void (&)(int&)>((* & __callable))), (* &
std::forward<int>((* & __args#0))), (* & std::forward<int>((* & __args#1))))’
from ‘__bindexpr_type {aka _Bind_simple<void (*(int, int))(int&)>}’ to
‘__assert_callable<std::integral_constant<bool, false> >’
return __bindexpr_type( std::forward<_Callable>(__callable),
std::forward<_Args>(__args)...);
^
sa.cc: In instantiation of ‘_Bind_simple<_Callable(_Args
...)>::_Bind_simple(_Callable2&&, _Args2&& ...) [with _Callable2 = void
(&)(int&); _Args2 = {int, int}; _Callable = void (*)(int&); _Args = {int,
int}]’:
sa.cc:47:96: required from ‘decltype (__check_callable<_Callable, _Args
...>(0)) __bind_simple(_Callable&&, _Args&& ...) [with _Callable = void
(&)(int&); _Args = {int, int}; decltype (__check_callable<_Callable, _Args
...>(0)) = __assert_callable<std::integral_constant<bool, false> >]’
sa.cc:55:39: required from ‘void call(_Callable&&, _Args&& ...) [with
_Callable = void (&)(int&); _Args = {int, int}]’
sa.cc:64:15: required from here
sa.cc:12:7: warning: unused parameter ‘__callable’ [-Wunused-parameter]
_Bind_simple(_Callable2&& __callable, _Args2&&... __args)
^
sa.cc:12:7: warning: unused parameter ‘__args#0’ [-Wunused-parameter]
sa.cc:12:7: warning: unused parameter ‘__args#1’ [-Wunused-parameter]
sa.cc: In function ‘decltype (__check_callable<_Callable, _Args ...>(0))
__bind_simple(_Callable&&, _Args&& ...) [with _Callable = void (&)(int&); _Args
= {int, int}; decltype (__check_callable<_Callable, _Args ...>(0)) =
__assert_callable<std::integral_constant<bool, false> >]’:
sa.cc:48:3: warning: control reaches end of non-void function [-Wreturn-type]
}
^
Why doesn't the static_assertion happen for the second call?
I'm trying to use this static assertion to simplify diagnostics for invalid
std::thread initializations or std::call_once uses, but it only helps for the
first such error.
