[PATCH] D138539: Use std::nullopt_t instead of NoneType (NFC)

[Fangrui Song via Phabricator via cfe-commits]

MaskRay added a comment.

In D138539#4164313 <https://reviews.llvm.org/D138539#4164313>, @steakhal wrote:

> Oh, I forgot to mention why this change breaks the equality operator of `llvm::StringSet`. It's because the `StringMap::operator==` will try to compare the `value` as well, which is of type `std::nullopt_t` and that has no equality comparison operator.
> Previously, the `enum class NoneType` has a default one.

Since `std::unordered_set` and counterparts like `absl::flat_hash_set` provide `operator==`, I think providing `StringMap::operator==` is fine. I don't think whether the operation is commonly used, though.


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  https://reviews.llvm.org/D138539/new/

https://reviews.llvm.org/D138539