code: private static boolean functionalInterfaceReturnsExactlyVoid(
        Type interfaceType, VisitorState state) {
      return state.getTypes().findDescriptorType(interfaceType).getReturnType().getKind()
          == TypeKind.VOID;
    }
docs: Checks that the return value of a functional interface is void. Note, we do not use
  ASTHelpers.isVoidType here, return values of Void are actually type-checked. Only
  void-returning functions silently ignore return values of any type.