Breaking change: Removing std::tuple's "reduced-arity-initialization" extension on uses-allocator constructors.

Hi All,

I will be removing the “reduced-arity-initialization” extension from the uses-allocator constructors in std::tuple. This is a breaking change. I plan to make this change immediately without first deprecating the behavior.

I’m hoping that usage of this extension with uses-allocator construction is rare and this change will cause minimal breakage.

This will not affect the extension in non-uses-allocator cases. Nor will it affect conforming uses-allocator construction. This change is needed to fix a number of bugs and to ensure future changes in tuple don’t cause more breakage.

Please let me know if you have any objections.

Below is a more in-depth description of the problem and solution.

The extension was removed in r266409.