Not sure if this addresses your specific problem, but I also had semantic errors. They just came out of the blue after having a working project.
Anyway, I fixed it with a single option in workspace settings by setting:
“Build configuration for indexer: Use active build configuration”
The other option was “Use the build configuration specified in the project’s indexer settings” and this is the one that wasn’t working. I think it may have been corrupted after an eclipse crash.