I can confirm this bug, using 5.1.0.Final and 5.1.1.Final. Stack Trace for 5.1.1.Final is:

Exception 0 :
java.lang.ArrayIndexOutOfBoundsException: 0
at org.hibernate.validator.internal.util.ExecutableHelper.instanceMethodParametersResolveToSameTypes(ExecutableHelper.java:135)
at org.hibernate.validator.internal.util.ExecutableHelper.overrides(ExecutableHelper.java:98)
at org.hibernate.validator.internal.cdi.ValidationExtension.replaceWithOverriddenOrInterfaceMethod(ValidationExtension.java:426)
at org.hibernate.validator.internal.cdi.ValidationExtension.determineConstrainedMethod(ValidationExtension.java:276)
at org.hibernate.validator.internal.cdi.ValidationExtension.determineConstrainedCallables(ValidationExtension.java:263)
at org.hibernate.validator.internal.cdi.ValidationExtension.processAnnotatedType(ValidationExtension.java:243)