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Change Untyped from Null to Nothing for Trees #13883

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Nov 5, 2021
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Change Untyped from Null to Nothing for Trees #13883

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11 changes: 2 additions & 9 deletions compiler/src/dotty/tools/dotc/ast/Trees.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -19,13 +19,7 @@ import Decorators._

object Trees {

// Note: it would be more logical to make Untyped = Nothing.
// However, this interacts in a bad way with Scala's current type inference.
// In fact, we cannot write something like Select(pre, name), where pre is
// of type Tree[Nothing]; type inference will treat the Nothing as an uninstantiated
// value and will not infer Nothing as the type parameter for Select.
// We should come back to this issue once type inference is changed.
type Untyped = Null
type Untyped = Nothing

/** The total number of created tree nodes, maintained if Stats.enabled */
@sharable var ntrees: Int = 0
Expand All @@ -44,8 +38,7 @@ object Trees {
*
* - You can never observe a `tpe` which is `null` (throws an exception)
* - So when creating a typed tree with `withType` we can re-use
* the existing tree transparently, assigning its `tpe` field,
* provided it was `null` before.
* the existing tree transparently, assigning its `tpe` field.
* - It is impossible to embed untyped trees in typed ones.
* - Typed trees can be embedded in untyped ones provided they are rooted
* in a TypedSplice node.
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