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bishaboshabishabosha
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Merge pull request #9709 from dotty-staging/topic/enum-unification
fix #9260 - Enums copy variance of parent
2 parents f143077 + d4769b9 commit 6daa7b5

30 files changed

+156
-43
lines changed

‎compiler/src/dotty/tools/dotc/ast/Desugar.scala

Lines changed: 4 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -136,8 +136,9 @@ object desugar {
136136
)
137137

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/** A derived type definition watching `sym` */
139-
def derivedTypeParam(sym: TypeSymbol)(using Context): TypeDef =
140-
TypeDef(sym.name, DerivedFromParamTree().watching(sym)).withFlags(TypeParam)
139+
def derivedTypeParamWithVariance(sym: TypeSymbol)(using Context): TypeDef =
140+
val variance = VarianceFlags & sym.flags
141+
TypeDef(sym.name, DerivedFromParamTree().watching(sym)).withFlags(TypeParam | Synthetic | variance)
141142

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/** A value definition copied from `vdef` with a tpt typetree derived from it */
143144
def derivedTermParam(vdef: ValDef)(using Context): ValDef =
@@ -407,7 +408,7 @@ object desugar {
407408

408409
val originalTparams = constr1.tparams
409410
val originalVparamss = constr1.vparamss
410-
lazy val derivedEnumParams = enumClass.typeParams.map(derivedTypeParam)
411+
lazy val derivedEnumParams = enumClass.typeParams.map(derivedTypeParamWithVariance)
411412
val impliedTparams =
412413
if (isEnumCase) {
413414
val tparamReferenced = typeParamIsReferenced(

‎compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala

Lines changed: 11 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -12,6 +12,7 @@ import util.SrcPos
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import config.Printers.variances
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import config.Feature.migrateTo3
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import reporting.trace
15+
import printing.Formatting.hl
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/** Provides `check` method to check that all top-level definitions
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* in tree are variance correct. Does not recurse inside methods.
@@ -166,7 +167,16 @@ class VarianceChecker(using Context) {
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private object Traverser extends TreeTraverser {
167168
def checkVariance(sym: Symbol, pos: SrcPos) = Validator.validateDefinition(sym) match {
168169
case Some(VarianceError(tvar, required)) =>
169-
def msg = i"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym"
170+
def msg =
171+
val enumAddendum =
172+
val towner = tvar.owner
173+
if towner.isAllOf(EnumCase) && towner.isClass && tvar.is(Synthetic) then
174+
val example =
175+
"See an example at http://dotty.epfl.ch/docs/reference/enums/adts.html#parameter-variance-of-enums"
176+
i"\n${hl("enum case")} ${towner.name} requires explicit declaration of $tvar to resolve this issue.\n$example"
177+
else
178+
""
179+
i"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum"
170180
if (migrateTo3 &&
171181
(sym.owner.isConstructor || sym.ownersIterator.exists(_.isAllOf(ProtectedLocal))))
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report.migrationWarning(

‎docs/docs/reference/enums/adts.md

Lines changed: 50 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -94,6 +94,56 @@ enum Color(val rgb: Int) {
9494
}
9595
```
9696

97+
### Parameter Variance of Enums
98+
99+
By default, parameterized cases of enums with type parameters will copy the type parameters of their parent, along
100+
with any variance notations. As usual, it is important to use type parameters carefully when they are variant, as shown
101+
below:
102+
103+
The following `View` enum has a contravariant type parameter `T` and a single case `Refl`, representing a function
104+
mapping a type `T` to itself:
105+
```scala
106+
enum View[-T]:
107+
case Refl(f: T => T)
108+
```
109+
The definition of `Refl` is incorrect, as it uses contravariant type `T` in the covariant result position of a
110+
function type, leading to the following error:
111+
```scala
112+
-- Error: View.scala:2:12 --------
113+
2 | case Refl(f: T => T)
114+
| ^^^^^^^^^
115+
|contravariant type T occurs in covariant position in type T => T of value f
116+
|enum case Refl requires explicit declaration of type T to resolve this issue.
117+
```
118+
Because `Refl` does not declare explicit parameters, it looks to the compiler like the following:
119+
```scala
120+
enum View[-T]:
121+
case Refl[/*synthetic*/-T1](f: T1 => T1) extends View[T1]
122+
```
123+
124+
The compiler has inferred for `Refl` the contravariant type parameter `T1`, following `T` in `View`.
125+
We can now clearly see that `Refl` needs to declare its own non-variant type parameter to correctly type `f`,
126+
and can remedy the error by the following change to `Refl`:
127+
128+
```diff
129+
enum View[-T]:
130+
- case Refl(f: T => T)
131+
+ case Refl[R](f: R => R) extends View[R]
132+
```
133+
Above, type `R` is chosen as the parameter for `Refl` to highlight that it has a different meaning to
134+
type `T` in `View`, but any name will do.
135+
136+
After some further changes, a more complete implementation of `View` can be given as follows and be used
137+
as the function type `T => U`:
138+
139+
```scala
140+
enum View[-T, +U] extends (T => U):
141+
case Refl[R](f: R => R) extends View[R, R]
142+
143+
final def apply(t: T): U = this match
144+
case refl: Refl[r] => refl.f(t)
145+
```
146+
97147
### Syntax of Enums
98148

99149
Changes to the syntax fall in two categories: enum definitions and cases inside enums.

‎tests/neg-custom-args/fatal-warnings/enum-variance.check

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,10 @@
1+
-- Error: tests/neg-custom-args/fatal-warnings/enum-variance.scala:2:12 ------------------------------------------------
2+
2 | case Refl(f: T => T) // error: enum case Refl requires explicit declaration of type T
3+
| ^^^^^^^^^
4+
| contravariant type T occurs in covariant position in type T => T of value f
5+
| enum case Refl requires explicit declaration of type T to resolve this issue.
6+
| See an example at http://dotty.epfl.ch/docs/reference/enums/adts.html#parameter-variance-of-enums
7+
-- Error: tests/neg-custom-args/fatal-warnings/enum-variance.scala:5:16 ------------------------------------------------
8+
5 | case Refl[-T](f: T => T) extends ExplicitView[T] // error: contravariant type T occurs in covariant position
9+
| ^^^^^^^^^
10+
| contravariant type T occurs in covariant position in type T => T of value f

‎tests/neg-custom-args/fatal-warnings/enum-variance.scala

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,11 @@
1+
enum View[-T]:
2+
case Refl(f: T => T) // error: enum case Refl requires explicit declaration of type T
3+
4+
enum ExplicitView[-T]: // desugared version of View
5+
case Refl[-T](f: T => T) extends ExplicitView[T] // error: contravariant type T occurs in covariant position
6+
7+
enum InvariantView[-T, +U] extends (T => U):
8+
case Refl[T](f: T => T) extends InvariantView[T, T]
9+
10+
final def apply(t: T): U = this match
11+
case refl: Refl[t] => refl.f(t)

‎tests/neg-custom-args/typeclass-derivation2.scala

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -122,7 +122,7 @@ object TypeLevel {
122122

123123
// An algebraic datatype
124124
enum Lst[+T] {
125-
case Cons(hd: T, tl: Lst[T])
125+
case Cons[T](hd: T, tl: Lst[T]) extends Lst[T]
126126
case Nil
127127
}
128128

‎tests/neg/enum-tparams.scala

Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1,10 +1,10 @@
11
object Test {
22

33
enum Opt[+T] {
4-
case S(x: T) extends Opt[T]
4+
case S[T](x: T) extends Opt[T]
55
case I(x: Int) extends Opt[Int]
6-
case V() extends Opt[`T`]
7-
case P(x: List[T]) extends Opt[String]
6+
case V[T]() extends Opt[`T`]
7+
case P[T](x: List[T]) extends Opt[String]
88
case N extends Opt[Nothing]
99
}
1010

@@ -37,4 +37,4 @@ object Test {
3737
E.C4() // OK
3838
E.C5[List, Id]() // OK
3939
E.C5() // OK
40-
}
40+
}

‎tests/neg/enums.scala

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -36,7 +36,7 @@ enum Captured[T] {
3636
}
3737

3838
enum Option[+T] derives Eql {
39-
case Some(x: T)
39+
case Some[T](x: T) extends Option[T]
4040
case None
4141
}
4242

‎tests/neg/i4470c.scala

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1,9 +1,9 @@
11
object DuplicatedEnum {
22
enum Maybe[+T] { // error
3-
case Some(x: T)
3+
case Some[T](x: T) extends Maybe[T]
44
}
55

66
enum Maybe[+T] { // error
7-
case Some(x: T)
7+
case Some[T](x: T) extends Maybe[T]
88
}
99
}

‎tests/neg/i5495.scala

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,4 @@
11
lazy enum LazyList[+A] { // error: sealed abstract types cannot be lazy enum
2-
case :: (head: A, tail: LazyList[A])
2+
case ::[A] (head: A, tail: LazyList[A]) extends LazyList[A]
33
case Nil
4-
}
4+
}

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