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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@289158 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index 1d2cc9e..1fd1d49 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -155,3 +155,30 @@
Defining this macro and then building libc++ with hidden visibility gives a
build of libc++ which does not export any symbols, which can be useful when
building statically for inclusion into another library.
+
+**_LIBCPP_ENABLE_TUPLE_IMPLICIT_REDUCED_ARITY_EXTENSION**:
+ This macro is used to re-enable an extension in `std::tuple` which allowed
+ it to be implicitly constructed from fewer initializers than contained
+ elements. Elements without an initializer are default constructed. For example:
+
+ .. code-block:: cpp
+
+ std::tuple<std::string, int, std::error_code> foo() {
+ return {"hello world", 42}; // default constructs error_code
+ }
+
+
+ Since libc++ 4.0 this extension has been disabled by default. This macro
+ may be defined to re-enable it in order to support existing code that depends
+ on the extension. New use of this extension should be discouraged.
+ See `PR 27374 <http://llvm.org/PR27374>`_ for more information.
+
+ Note: The "reduced-arity-initialization" extension is still offered but only
+ for explicit conversions. Example:
+
+ .. code-block:: cpp
+
+ auto foo() {
+ using Tup = std::tuple<std::string, int, std::error_code>;
+ return Tup{"hello world", 42}; // explicit constructor called. OK.
+ }