private void setOutputType(DataType outputType, Expression expression, TypeContext context) {
        DataType inputType = expression.setOutputType(outputType, context);
        if (inputType != null && ! DataType.STRING.isAssignableTo(inputType))
            throw new VerificationException(this, "This inputs a string, but '" + expression + "' requires type " + inputType);
    }
The default expression, or null if none
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/SwitchExpression.java