Given I3 < I2 < I1 and C < B < A The groovy runtime does not correctly pick the most specialized method when the argument type is an interface that extends two other interfaces and the parameter type is a class that implements the same structure. It's much easier to explain it in code, so I've attached a test case. MainGroovy produces: I1 I2 Exception in thread "main" groovy.lang.GroovyRuntimeException: Ambiguous method overloading for method Foo#foo. Cannot resolve which method to invoke for {class C} due to overlapping prototypes between: {interface I2} {interface I3} at groovy.lang.MetaClassImpl.chooseMostSpecificParams(MetaClassImpl.java:2383) at groovy.lang.MetaClassImpl.chooseMethod(MetaClassImpl.java:2336) at groovy.lang.MetaClassImpl.getMethodWithoutCaching(MetaClassImpl.java:1002) at groovy.lang.MetaClassImpl.getMethodWithCaching(MetaClassImpl.java:949) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:762) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:742) at org.codehaus.groovy.runtime.Invoker.invokePojoMethod(Invoker.java:87) at org.codehaus.groovy.runtime.Invoker.invokeMethod(Invoker.java:75) at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.java:66) at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodN(ScriptBytecodeAdapter.java:165) at MainGroovy.main(MainGroovy.groovy:9) Expected: I1 I2 I3