Scala 3.3.0-RC2: inline value not considered to be immutable path

[soronpo]

The following compiles in Scala 3.2.2

Compiler version

v3.3.0-RC2

Minimized code

trait Bar[T]
given [T]: Bar[T] with {}
inline def foo[V](inline value: V)(using Bar[value.type]) : Unit = {} //error

Output

(value : V) is not a valid singleton type, since it is not an immutable path
inline def foo[V](inline value: V)(using Bar[value.type]) : Unit = {}

Expectation

No error.

[nicolasstucki]

That error is expected. This bug was fixed in #15511.

Inline parameters are not immutable paths because they may refer to any number of distinct paths (0,1, 2,..). Consider this example.

inline def f(inline x: X)(using F[x.type]): Unit = ()
inline def g(inline x: X)(using F[x.type]): Unit = { x; () }
inline def h(inline x: X)(using F[x.type]): Unit = { x; x; () }

f(new X) // inlined as `()`. The value of the path is not created, and therefore it would be unsound to select it.

g(new X) // inlined as `{ new X; () }`. The value of the path is created after the path is selected. Not sure if that would be sound.

h(new X) // inlined as `{ new X; new X; () }`. Two values are created, and therefore two paths represent the same inline argument. It would be unsound to choose one of them.