Fix overriding in explicit nulls

[noti0na1]

Fix #13040 #13486 #13608.

During overriding check, Array[? <: FromJavaObject | Null].matchs(Array[? <: Object | Null]) is tested.
The TypeBounds ? <: FromJavaObject | Null is normalized with ? <: Any using & (in def isSubArg).
Since Any <:< FromJavaObject, the intersection result is arbitrary, and in this case, is ? <: Any.
Then, ? <: Any is no longer a subtype of ? <: Object | Null, and the match test fails.

This fix will handle FromJavaObject specially when computing the intersection of two TypeBounds. We will try to preserve the information of FromJavaObject in upper bounds.

// This & will try to preserve the FromJavaObjects type in upper bounds
// For example, (? <: FromJavaObjects | Null) & (? <: Any),
// we want to get (? <: FromJavaObjects | Null) intead of (? <: Any),
// because we may check FromJavaObjects | Null <:< Object | Null later.
def bounds_&(tp1: TypeBounds, tp2: TypeBounds) =
  if tp1.hi.containsFromJavaObject
    && (tp1.hi frozen_<:< tp2.hi)
    && (tp2.lo frozen_<:< tp1.lo) then
    // FromJavaObject in tp1.hi guarantees tp2.hi <:< tp1.hi
    // prefer tp1 if FromJavaObject is in its hi
    tp1
  else if tp2.hi.containsFromJavaObject
    && (tp2.hi frozen_<:< tp1.hi)
    && (tp1.lo frozen_<:< tp2.lo) then
    // Similarly, prefer tp2 if FromJavaObject is in its hi
    tp2
  else
    // Use regular & to solve other cases
    tp1 & tp2

[noti0na1]

At https://github.com/lampepfl/dotty/blob/01f040bbf7b703a7aa0e8dc03799e0e142073410/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L564 , would is be better to write impl.matchesLoosely(mbrDenot, alwaysCompareTypes = true)?

According to def matchesType in TypeComparer.scala, "A function implementing tp1 matches tp2", although the result should be same.

[smarter]

although the result should be same.

Yes, matching should be symmetric, if it isn't it's a bug, so the order doesn't matter.

[smarter]

Maybe we're looking at this the wrong way? FromJavaObject is defined as a type alias of Object (https://github.com/lampepfl/dotty/blob/eb8773ecbedd32b7f7f6822696f3c3df3cde6930/compiler/src/dotty/tools/dotc/core/Definitions.scala#L407), but maybe it should be a type alias of Object | Null with explicit nulls?

[noti0na1]

@smarter We don't need to change the FromJavaObject alias in explicit nulls, because we are going nullify the types from Java anyway.

I added some logic in glb and lub now to handle FromJavaObject union.

[smarter]

Thank you!