add fix for GCC.

[erip]

Motivation

Why are you making this change?

To fix build with GCC so PyTorch can build with newest commits.

Summary

Updates return type to be explicitly pinned within the class. SFINAE etc. etc.

How does the code work?

Why did you choose this approach?

https://stackoverflow.com/a/14419935/2883245

Test Plan

How did you test this change?

Any change that adds functionality should add a unit test as well.

[erip]

cc @dreiss @passy Any chance of review/merge for this one? I've got PyTorch Java bindings building here with my fork of fbjni which incorporates these changes and tests pass with both compilers so hopefully not too controversial.

[dreiss]

I'm probably going to need a few days, unfortunately. Do you happen to know which version of GCC requires this?

[erip]

I think it's gcc 7.x - I can double check. I guess I misremembered - I think it was gcc 5.4. See godbolt.

[erip]

@dreiss - any updates on this? If there are things I can help with, I'm happy to.

[erip]

@dreiss Will you have any time to look at this in the next day or so? I was hoping to see Java bindings for Windows land in PyTorch 1.7.0, but this is now not possible since the 1.7.0 RC has already been cut. I'd still like to see it land ASAP and this is the last thing standing in the way. Since this is such a small PR, I'm hoping it can be imported into phabricator and land cleanly.

[facebook-github-bot]

@dreiss has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@erip has updated the pull request. You must reimport the pull request before landing.

[erip]

@dreiss thanks for making the required changes. Is there a reason my contributions are being committed directly to the repo instead of being merged? This is the second PR that's been ignored with nearly identical fixes being added directly to the repo.