Same issue right now, all this time later.

Simply adding UNITY_INITIALIZE_OUTPUT(Varyings, OUT); beneath Varyings OUT; fixed it.

What's interesting to me is, the offending code that triggered this error in my project... is identical to the example UNITY gives:

https://docs.unity3d.com/Packages/[email protected]/manual/writing-shaders-urp-basic-unlit-structure.html

I guess UNITY themselves aren't bothered by the errors their own code generates? Dunno.