Fix PR27374 - Remove the implicit reduced-arity-extension in tuple.
This patch removes libc++'s tuple extension which allowed it to be
constructed from fewer initializers than elements; with the remaining
elements being default constructed. However the implicit version of
this extension breaks conforming code. For example:
int fun(std::string);
int fun(std::tuple<std::string, int>);
int x = fun("hello"); // ambigious
Because existing code may already depend on this extension it can be re-enabled
by defining _LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION.
Note that the explicit version of this extension is still supported,
although it's somewhat less useful than the implicit one.
llvm-svn: 289158
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: bd688258baf97e840c2bd0c4c307503042b1620c
diff --git a/include/tuple b/include/tuple
index 251443e..553d8e5 100644
--- a/include/tuple
+++ b/include/tuple
@@ -477,6 +477,12 @@
base base_;
+#if defined(_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION)
+ static constexpr bool _EnableImplicitReducedArityExtension = true;
+#else
+ static constexpr bool _EnableImplicitReducedArityExtension = false;
+#endif
+
template <class ..._Args>
struct _PackExpandsToThisTuple : false_type {};
@@ -703,11 +709,17 @@
) {}
template <class ..._Up,
+ bool _PackIsTuple = _PackExpandsToThisTuple<_Up...>::value,
typename enable_if
<
_CheckArgsConstructor<
- sizeof...(_Up) <= sizeof...(_Tp)
- && !_PackExpandsToThisTuple<_Up...>::value
+ sizeof...(_Up) == sizeof...(_Tp)
+ && !_PackIsTuple
+ >::template __enable_implicit<_Up...>() ||
+ _CheckArgsConstructor<
+ _EnableImplicitReducedArityExtension
+ && sizeof...(_Up) < sizeof...(_Tp)
+ && !_PackIsTuple
>::template __enable_implicit<_Up...>(),
bool
>::type = false
@@ -735,7 +747,12 @@
_CheckArgsConstructor<
sizeof...(_Up) <= sizeof...(_Tp)
&& !_PackExpandsToThisTuple<_Up...>::value
- >::template __enable_explicit<_Up...>(),
+ >::template __enable_explicit<_Up...>() ||
+ _CheckArgsConstructor<
+ !_EnableImplicitReducedArityExtension
+ && sizeof...(_Up) < sizeof...(_Tp)
+ && !_PackExpandsToThisTuple<_Up...>()
+ >::template __enable_implicit<_Up...>(),
bool
>::type = false
>