[PATCH] D139779: [Support] Add transformOptional

[Kazu Hirata via Phabricator via llvm-commits]

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llvm::Optional<T> has transform, which is equivalent to
std::optional<T>::transform.  The problem is that
std::optional<T>::transform won't be available until C++23, implying
that we probably cannot use it in our codebase untli 2028 or so.  We
certainly don't want to keep llvm::Optional just for transform.

This patch adds transformOptional to STLForwardCompat.h so that we can
use transform during the migration to std::optional and beyond.

I've shamelessly borrowed the implementation and test from
llvm/include/llvm/ADT/Optional.h and
llvm/unittests/ADT/OptionalTest.cpp, respectively.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139779

Files:
  llvm/include/llvm/ADT/STLForwardCompat.h
  llvm/unittests/ADT/STLForwardCompatTest.cpp

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