The reason this fails is because the Static Mesh Component header is missing. Add #include "Components/StaticMeshComponent.h" to the top of the Actor's header file.
Starting a hot reload by clicking the Compile button in the Editor fails after adding a Static Mesh Component to an Actor code class. This only occurs in the binary version of the Engine, and only if the project was originally a Blueprint-only project. The error does not occur if the project was originally a code project, or if the Engine was built from source code.
Yes, this error did not occur in 4.19.2.
The build fails with the following error: error C2440: '=': cannot convert from 'UStaticMeshComponent *' to 'USceneComponent *'
The build completes successfully.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-65826 in the post.