Scala3 has dropped general type projection because it was unsound:
It was possible to compile code which was failing at runtime.
(question edited to reflect comments)
Consider the following scala3 code : scastie
class A:
class X:
def outer : A.this.type = A.this
class B extends A
class C extends A
val b0 = new B
val b1 = b0
val b2 = new B
val c0 = new C
val c1 = c0
val c2 = new C
val b0x : A#X = new b0.X
val pathTypeMatch = b0x match
case _ : c2.X => "c2.X"
case _ : c1.X => "c1.x"
case _ : c0.X => "c0.X"
case _ : b2.X => "b2.X"
case _ : b1.X => "b1.X"
case _ : b0.X => "b0.X"
case _ => "ELSE"
pathTypeMatch // "b1.x"
val projectionTypeMatch = b0x match
case _ : C#X => "C#X"
case _ : B#X => "B#X"
case _ : A#X => "A#X"
case _ => "ELSE"
projectionTypeMatch // "C#X" !!!
val failingTypeMatch = b0x match
case cx : C#X =>
val c : C = cx.outer // Fails at runtime
Code compiles, but fails at runtime with "class B cannot be cast to class C".
The compiler considers C#X
and B#X
erasures to be A#X
, so that case cx:C#X
branch matches. From there, it is legitimate to consider cx.outer
to be a C
(which is wrong), hence the exception.
No warning is issued. No TypeTest is involved.
When looking into the bytecode of projectionTypeMatch
, the test of all 3 branches is strictly identical.
In type-patterns:
(A type-pattern T is ...) a reference to a class
C
,p.C
, orT#C
. This type pattern matches any non-null instance of the given class. Note that the prefix of the class, if it exists, is relevant for determining class instances.
Indeed, when the match is done against path-dependent types, the behavior is the expected one (pathTypeMatch
is b1.x
). In particular, the compiler had to take X outer into account in the match.
One would expect the match against projection-types to be consistent, isn't it ?
What is the intention ?
It seems the restriction of type projections to concrete types in scala3 would allow a consistent behavior. Is it so ?
See https://github.com/lampepfl/dotty/issues/16728.
Fixed by https://github.com/lampepfl/dotty/pull/17136
The observed behavior did not reflect intention.
scala3 compiler has been fixed: 3.3.1-RC1-bin-20230327-a569057-NIGHTLY
behaves as expected.
Note the faulty behavior may be seen as a flavor of general type projection unsoundness, and cannot be fixed in scala2.