JSType getGreatestSubtypeHelper(JSType that) {
if (that.isRecordType()) {
RecordType thatRecord = that.toMaybeRecordType();
RecordTypeBuilder builder = new RecordTypeBuilder(registry);
builder.setSynthesized(true);
// The greatest subtype consists of those *unique* properties of both
// record types. If any property conflicts, then the NO_TYPE type
// is returned.
for (String property : getOwnPropertyNames()) {
if (thatRecord.hasProperty(property)
&& !thatRecord.getPropertyType(property).isInvariant(getPropertyType(property))) {
return registry.getNativeObjectType(JSTypeNative.NO_TYPE);
}
builder.addProperty(property, getPropertyType(property), getPropertyNode(property));
}
for (String property : thatRecord.getOwnPropertyNames()) {
if (!hasProperty(property)) {
builder.addProperty(
property, thatRecord.getPropertyType(property), thatRecord.getPropertyNode(property));
}
}
return builder.build();
}
JSType greatestSubtype = registry.getNativeType(JSTypeNative.NO_OBJECT_TYPE);
JSType thatRestrictedToObj =
registry.getNativeType(JSTypeNative.OBJECT_TYPE).getGreatestSubtype(that);
if (!thatRestrictedToObj.isEmptyType()) {
// In this branch, the other type is some object type. We find
// the greatest subtype with the following algorithm:
// 1) For each property "x" of this record type, take the union
// of all classes with a property "x" with a compatible property type.
// and which are a subtype of {@code that}.
// 2) Take the intersection of all of these unions.
for (String propName : getOwnPropertyNames()) {
JSType propType = getPropertyType(propName);
UnionTypeBuilder builder = new UnionTypeBuilder(registry);
for (ObjectType alt : registry.getEachReferenceTypeWithProperty(propName)) {
JSType altPropType = alt.getPropertyType(propName);
if (altPropType != null
&& !alt.isEquivalentTo(this)
&& alt.isSubtype(that)
&& propType.isInvariant(altPropType)) {
builder.addAlternate(alt);
}
}
greatestSubtype = greatestSubtype.getLeastSupertype(builder.build());
}
}
return greatestSubtype;
}
/** Determines if typeA is a subtype of typeB */
static boolean isSubtype(ObjectType typeA, RecordType typeB) {
// typeA is a subtype of record type typeB iff:
// 1) typeA has all the properties declared in typeB.
// 2) And for each property of typeB,
// 2a) if the property of typeA is declared, it must be equal
// to the type of the property of typeB,
// 2b) otherwise, it must be a subtype of the property of typeB.
//
// To figure out why this is true, consider the following pseudo-code:
// /** @type {{a: (Object,null)}} */ var x;
// /** @type {{a: !Object}} */ var y;
// var z = {a: {}};
// x.a = null;
//
// y cannot be assigned to x, because line 4 would violate y's declared
// properties. But z can be assigned to x. Even though z and y are the
// same type, the properties of z are inferred--and so an assignment
// to the property of z would not violate any restrictions on it.
for (String property : typeB.getOwnPropertyNames()) {
if (!typeA.hasProperty(property)) {
return false;
}
JSType propA = typeA.getPropertyType(property);
JSType propB = typeB.getPropertyType(property);
if (typeA.isPropertyTypeDeclared(property)) {
// If one declared property isn't invariant,
// then the whole record isn't covariant.
if (!propA.isInvariant(propB)) {
return false;
}
} else {
// If one inferred property isn't a subtype,
// then the whole record isn't covariant.
if (!propA.isSubtype(propB)) {
return false;
}
}
}
return true;
}