void setVariant(Variant value) {
        if (variant == value) return; // already OK
        if (variant != Variant.UNKNOWN) {
            throw new IllegalArgumentException("setVariant(" + value + ") only valid for UNKNOWN, not: " + variant);
        }
        // maybe even more checking would be useful
        this.variant = value;
    }
This class holds the extracted information after parsing a type
declaration (typically for a field).  Since types can be complex,
struct names (known or unknown), or even document names, this class
is somewhat complicated.
@author arnej27959
config-model/src/main/java/com/yahoo/schema/parser/ParsedType.java