public DataType requireOutputType() {
        if (outputType == null)
            throw new IllegalStateException("The output type of " + this + " is unresolved");
        return outputType;
    }
Returns the already assigned (during verification) output type, or throws an exception if no type is assigned.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/Expression.java