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1f91ba7fcb79244fce1fe448c1d31fb5b0580584 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 343511158

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8dcd61fed44f4e56044aa4a899cc3124117fa21f by Abseil Team <absl-team@google.com>:

refactors result_of so it doesn't unnecessarily break in C++20 mode

std::result_of was deprecated in C++17 and removed in C++20. This CL
adds an implementation detail so it transparently continues to work
when external users turn up to C++20 or later.

The implementation is derived from C++17's specification of result_of
[depr.meta.types][1].

[1]: https://timsong-cpp.github.io/cppwp/n4659/depr.meta.types

Fixes #649

PiperOrigin-RevId: 343395432
GitOrigin-RevId: 1f91ba7fcb79244fce1fe448c1d31fb5b0580584
Change-Id: Id5d9c55dc7547737bade48a15f2f5740f414d811