I have mixed (java + groovy) project.

SeanceTemplate.java

public interface SeanceTemplate<S> {
    interface Callback<T, S> {
        T doInSeance(S seance);
    }
    <T> T execute(Callback<T, S> callback);
}

SeanceTemplateImpl.groovy

@CompileStatic
class SeanceTemplateImpl<S> implements SeanceTemplate<S> {
    @Override
    public <T> T execute(Callback<T, S> callback) {
      ...
    }
}

produces compilation error:
ERROR in /home/mtrofimov/dev/main/src/com/mtrofimov/seance/impl/SeanceTemplateImpl.groovy (at line 0)
[ERROR] package com.mtrofimov.seance.impl
[ERROR] ^
[ERROR] Internal compiler error: java.lang.NullPointerException at org.codehaus.jdt.groovy.internal.compiler.ast.GroovyClassScope.fixupTypeParameters(GroovyClassScope.java:559)

I'm using

• eclipse mars.1
• groovy 2.4.3
• groovy-eclipse-compiler 2.9.2-01

I found similar/related issue https://issuetracker.springsource.com/browse/STS-3930 (if it could help)