/**
* Return the greatest lower bound of two types. That is, return the largest type that is a
* subtype of both inputs. If none exists return {@code thisType}.
*/
public JReferenceType strengthenType(JReferenceType thisType, JReferenceType thatType) {
if (thisType == thatType) {
return thisType;
}
if (thisType.isNullType() || thatType.isNullType()) {
return JReferenceType.NULL_TYPE;
}
if (thisType.canBeNull() != thatType.canBeNull()) {
// If either is non-nullable, the result should be non-nullable.
return strengthenType(thisType.strengthenToNonNull(), thatType.strengthenToNonNull());
}
if (typeOracle.castSucceedsTrivially(thisType, thatType)) {
return thisType;
}
if (typeOracle.castSucceedsTrivially(thatType, thisType)) {
return thatType;
}
// This types are incompatible; ideally this code should not be reached, but there are two
// situations where this happens:
// 1 - unrelated interfaces;
// 2 - unsafe code.
// The original type is preserved in this case.
return thisType;
}
/**
* Return the least upper bound of two types. That is, the "smallest" type that is a supertype of
* both types. In this lattice there the smallest element might no exist, there might be multiple
* minimal elements neither of which is smaller than the others. E.g.
*
* <p>{@code I J | \ /| | \ / | | x | | / \ | | / \ | A B }
*
* <p>where I and J are interfaces, A and B are classes and both A and B implement I and J. In
* this case both I and J are generalizing the types A and B.
*/
private JReferenceType generalizeTypes(JReferenceType thisType, JReferenceType thatType) {
if (!thisType.canBeNull() && !thatType.canBeNull()) {
// Nullability is an orthogonal property, so remove non_nullability and perform the
// generalization on the nullable types, and if both were NOT nullable then strengthen the
// result to NOT nullable.
//
// not_nullable(A) v not_nullable(B) = not_nullable(A v B)
JReferenceType nulllableGeneralizer =
generalizeTypes(thisType.weakenToNullable(), thatType.weakenToNullable());
return nulllableGeneralizer.strengthenToNonNull();
}
thisType = thisType.weakenToNullable();
thatType = thatType.weakenToNullable();
// From here on nullability does not need to be considered.
// Generalization for exact types is as follows.
// exact(A) v null = exact(A)
// A v null = A
if (thatType.isNullType()) {
return thisType;
}
// null v exact(A) = exact(A)
// null v A = A
if (thisType.isNullType()) {
return thatType;
}
// exact(A) v exact(A) = exact(A)
// A v A = A
if (thisType == thatType) {
return thisType;
}
// exact(A) v exact(B) = A v B
// A v exact(B) = A v B
// exact(A) v B = A v B
// A v B = A v B
return generalizeUnderlyingTypes(thisType.getUnderlyingType(), thatType.getUnderlyingType());
}