EricWF created this revision.
EricWF added reviewers: mclow.lists, rsmith.
EricWF added a subscriber: cfe-commits.
The reduced-arity-extension on tuple's implicit constructors breaks conforming
code. eg
```
#include <tuple>
#include <string>
using namespace std;
int count(tuple<string, string>) { return 2; }
int count(tuple<string, string, string>) { return 3; }
int main() {
int c = count({"abc", "def"}); // expected-error {{call to 'count' is
ambiguous}}
}
```
To fix this the I removed the reduced-arity-extension only on the implicit
constructors. This breaks the following code:
```
std::tuple<int, int, int> foo() { return {1, 2} }
```
But it still allows for
```
using Tup = std::tuple<int, int, int>;
Tup foo() { return Tup{1, 2}; }
```
@Marshall should we provide a way to turn this ctor back on in case in breaks a
bunch of real-world code? Maybe deprecate it for a release?
See also:
* http://llvm.org/PR27374
* http://wg21.link/lwg2419
https://reviews.llvm.org/D23495
Files:
include/tuple
Index: include/tuple
===================================================================
--- include/tuple
+++ include/tuple
@@ -706,7 +706,7 @@
typename enable_if
<
_CheckArgsConstructor<
- sizeof...(_Up) <= sizeof...(_Tp)
+ sizeof...(_Up) == sizeof...(_Tp)
&& !_PackExpandsToThisTuple<_Up...>::value
>::template __enable_implicit<_Up...>(),
bool
@@ -716,26 +716,30 @@
tuple(_Up&&... __u)
_NOEXCEPT_((
is_nothrow_constructible<base,
- typename __make_tuple_indices<sizeof...(_Up)>::type,
- typename __make_tuple_types<tuple, sizeof...(_Up)>::type,
- typename __make_tuple_indices<sizeof...(_Tp),
sizeof...(_Up)>::type,
- typename __make_tuple_types<tuple, sizeof...(_Tp),
sizeof...(_Up)>::type,
+ typename __make_tuple_indices<sizeof...(_Tp)>::type,
+ __tuple_types<_Tp...>,
+ __tuple_indices<>,
+ __tuple_types<>,
_Up...
>::value
))
- : base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
- typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
- typename __make_tuple_indices<sizeof...(_Tp),
sizeof...(_Up)>::type(),
- typename __make_tuple_types<tuple, sizeof...(_Tp),
sizeof...(_Up)>::type(),
+ : base_(typename __make_tuple_indices<sizeof...(_Tp)>::type{},
+ __tuple_types<_Tp...>{},
+ __tuple_indices<>{},
+ __tuple_types<>{},
_VSTD::forward<_Up>(__u)...) {}
template <class ..._Up,
typename enable_if
<
_CheckArgsConstructor<
sizeof...(_Up) <= sizeof...(_Tp)
&& !_PackExpandsToThisTuple<_Up...>::value
- >::template __enable_explicit<_Up...>(),
+ >::template __enable_explicit<_Up...>() ||
+ _CheckArgsConstructor<
+ sizeof...(_Up) < sizeof...(_Tp)
+ && !_PackExpandsToThisTuple<_Up...>::value
+ >::template __enable_implicit<_Up...>(),
bool
>::type = false
>
Index: include/tuple
===================================================================
--- include/tuple
+++ include/tuple
@@ -706,7 +706,7 @@
typename enable_if
<
_CheckArgsConstructor<
- sizeof...(_Up) <= sizeof...(_Tp)
+ sizeof...(_Up) == sizeof...(_Tp)
&& !_PackExpandsToThisTuple<_Up...>::value
>::template __enable_implicit<_Up...>(),
bool
@@ -716,26 +716,30 @@
tuple(_Up&&... __u)
_NOEXCEPT_((
is_nothrow_constructible<base,
- typename __make_tuple_indices<sizeof...(_Up)>::type,
- typename __make_tuple_types<tuple, sizeof...(_Up)>::type,
- typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type,
- typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type,
+ typename __make_tuple_indices<sizeof...(_Tp)>::type,
+ __tuple_types<_Tp...>,
+ __tuple_indices<>,
+ __tuple_types<>,
_Up...
>::value
))
- : base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
- typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
- typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
- typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
+ : base_(typename __make_tuple_indices<sizeof...(_Tp)>::type{},
+ __tuple_types<_Tp...>{},
+ __tuple_indices<>{},
+ __tuple_types<>{},
_VSTD::forward<_Up>(__u)...) {}
template <class ..._Up,
typename enable_if
<
_CheckArgsConstructor<
sizeof...(_Up) <= sizeof...(_Tp)
&& !_PackExpandsToThisTuple<_Up...>::value
- >::template __enable_explicit<_Up...>(),
+ >::template __enable_explicit<_Up...>() ||
+ _CheckArgsConstructor<
+ sizeof...(_Up) < sizeof...(_Tp)
+ && !_PackExpandsToThisTuple<_Up...>::value
+ >::template __enable_implicit<_Up...>(),
bool
>::type = false
>
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