Thank you for the comment, it's very helpful.

Would be nice if this can further be reviewed and commited to the repo!

Fixed formatting & reverted a deleted FIXME comment.

In D119077#3300271, @nridge wrote:

I haven't looked at the patch in detail, but one high level question: have you considered the possibility of adding these highlightings during the findExplicitReferences phase, rather than in CollectExtraHighlightings? (I haven't thought through whether that would work, just wondering if you have. The reason this is worth asking is that if we can get findExplicitReferences to handle overloaded operator calls, other clangd features that use findExplicitReferences would benefit from knowing about such calls as well.)

Two changes:

1. Added tests for overloaded operators into SemanticHighlightingTests.
2. Fixed the way tokens are expanded. Previously operator<< would be split into two highlighting tokens: operator and <<. Now it's all one highlighting token. The same is true for all other compound operator keyword statements.

Fixed a bug I introduced by removing a null-check. I added the null-check back in again.