/** 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;
}
