More details on Lazy Vals

[allanrenucci]

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[dottybot]

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[smarter]

Some things that may be worth mentioning:

• There's an apparent deadlock somewhere in the implementation: lazy val locking #2275
• Recursively calling a lazy val behaves differently: in Scala 2 it's formally unspecified but the behavior is that the value gets computed again, in Dotty the recursive call always returns null (or 0 for primitives). This can lead to hard to track bugs, that sometimes can be fixed only by replacing a lazy val by a manual equivalent (see Recursively calling a lazy val is broken, prevents bootstrapping tests from passing #1856, Fixes #1856: mimic Scalac sematic of recursive lazy vals. #1892). It also means that a lazy val can never have a non-nullable type, which could end up being a very annoying restriction.