trivial lazy self-reference deadlocks on reference ... can we detect this case?

[tpolecat]

Compiler version

3.0.0

Minimized code

scala> lazy val a: Int = a                                                                          
lazy val a: Int

scala> a         
<hangs>

Suggestion

In Scala 2 this situation results in a stack overflow, but in Scala 3 it hangs awaiting an object monitor. It also happens with given values that reference themselves directly, which seem to desugar to a lazy val. I ran into this when someone new to doobie was trying to bring an instance into scope by saying

given Meta[Json] = Meta[Json]

and his program deadlocked trying to summon the instance. This ends up being a common beginner mistake.

In any case my suggestion is that we detect at least this specific case at compile time because it can be very hard to track down.

[som-snytt]

less trivial #10947

[nicolasstucki]

#13011 propose a way to mitigate simple instances of this issue.

[odersky]

We now get:

-- Warning: --------------------------------------------------------------------
1 |lazy val a: Int = a
  |                  ^
  |                  Infinite loop in function body
  |                  a