The static compiler is trying to resolve classes which it shouldn't. Imagine the following scenario:
- a project written in Java exposes class `A` which has a private method using `D` as a parameter
- `D` is an implementation dependency of `A`, meaning it's not exposed in its public API, so consumers of `A` should never require `D` to build against `A`
- a project `B` written in Groovy consumes `A` and tries to call a public method of `A`
- if the class is using `@CompileStatic`, compilation FAILS with a NoClassDefFoundError
- if the class doesn't use `@CompileStatic`, compilation passes as expected
The reason is that when we compile `B`, we only put on the compile classpath the API dependencies of `A`. But Groovy breaks because when it inspects class `A`, it tries to load internal implementation details (private fields, methods) and as such fails.
This is a big problem because it effectively breaks the contract of API/implementation separation, and forces either the producer to declare `D` as an implementation dependency, which it's not, or redeclare `D` as an implementation dependency of `B`, which it isn't either, just to make the compiler happy!
The Gradle build itself is suffering from this. This is not nice because it forces compilation of things we don't need, and forces us to redeclare dependencies that we shouldn't.
To reproduce, checkout this project: https://github.com/melix/groovy-compiler-bug
and run `./gradlew build`