Cut backport of std::make_unique

[JoeLoser]

• Previously, folly supported compilers which did not implement
  std::make_unique, so there exists a backport implementation.
• Now that folly only supports C++14-compliant compilers, remove the
  backport.
• Fix call sites which were relying on unqualified make_unique being
  found in the folly namespace to explicitly use std::make_unique.

[JoeLoser]

Ping @yfeldblum

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[yfeldblum]

Now that folly only supports C++14-compliant compilers, remove the backport.

Note that some parts of folly may be used with C++14 compilers but with C++11 standard libraries.

[JoeLoser]

I wasn't aware of that bit, but are there any parts of folly in particular which need to be C++11-standard-library compatible? Based on the unqualified make_unique calls, it seems like nearly all of the call sites in folly namespace already use std::make_unique, so anyone depending on those headers is already using a standard library which has implemented std::make_unique. Do we have real code in the wild depending on this backport besides the two call sites I fixed in folly?

Does the backport live in folly as an easy way for downstream projects that are still using C++11-standard-library to get it, despite most of folly library already using std::make_unique?

[yfeldblum]

Are there any parts of folly in particular which need to be C++11-standard-library compatible?

Well, compatible with the gcc49 implementation of the standard library in C++14 mode. This version has a partial but not a full implementation of the C++14 standard library spec.

Note that the gcc49 implementation of the standard library in C++14 mode ships with std::make_unique.

Beyond std::make_unique, for other names, there isn't really a system to know in advance.

[yfeldblum]

Re this PR, there is code internally which still uses folly::make_unique, so that would need to be changed first.

[JoeLoser]

I grepped through Thrift and Eden and there weren't any explicit uses of folly::make_unique (though there is a lot of using std::make_unique and using namespace std at the top of .cpp files to bring in make_unique into lookup for ADL.

Do you have any interest in fixing the internal code in a sweeping codemod or the like?

[yfeldblum]

Yes if it's easy, no if it's hard. :)

I did just land one codemod internally, although more will be needed.

Based on a quick search, it looks like React Native is using folly::make_unique.

[JoeLoser]

Here is a PR which replaces the usages of folly::make_unique with std::make_unique in the react-native project.

[JoeLoser]

ping @yfeldblum since facebook/react-native#26730 landed