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Backport "Add addendum to private val parameter variance error message" to 3.3 LTS #585

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Backport "Add addendum to private val parameter variance error message" to 3.3 LTS #585

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15 changes: 14 additions & 1 deletion compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,20 @@ class VarianceChecker(using Context) {
i"\n${hl("enum case")} ${towner.name} requires explicit declaration of $tvar to resolve this issue.\n$example"
else
""
em"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum"
val privateParamAddendum =
if sym.flags.is(ParamAccessor) && sym.flags.is(Private) then
val varOrVal = if sym.is(Mutable) then "var" else "val"
val varFieldInstead = if sym.is(Mutable) then " and add\na field inside the class instead" else ""
s"""
|
|Implementation limitation: ${hl(f"private $varOrVal")} parameters cannot be inferred to be local
|and therefore are always variance-checked.
|
|Potential fix: remove the ${hl(f"private $varOrVal")} modifiers on the parameter ${sym.name}$varFieldInstead.
""".stripMargin
else
""
em"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum$privateParamAddendum"
if (migrateTo3 &&
(sym.owner.isConstructor || sym.ownersIterator.exists(_.isAllOf(ProtectedLocal))))
report.migrationWarning(
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21 changes: 21 additions & 0 deletions tests/neg/i22620.check
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@@ -0,0 +1,21 @@
-- Error: tests/neg/i22620.scala:4:34 ----------------------------------------------------------------------------------
4 |class PrivateTest[-M](private val v: ArrayBuffer[M]) // error
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| contravariant type M occurs in invariant position in type scala.collection.mutable.ArrayBuffer[M] of value v
|
| Implementation limitation: private val parameters cannot be inferred to be local
| and therefore are always variance-checked.
|
| Potential fix: remove the private val modifiers on the parameter v.
|
-- Error: tests/neg/i22620.scala:6:37 ----------------------------------------------------------------------------------
6 |class PrivateTestMut[-M](private var v: ArrayBuffer[M]) // error
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| contravariant type M occurs in invariant position in type scala.collection.mutable.ArrayBuffer[M] of variable v
|
| Implementation limitation: private var parameters cannot be inferred to be local
| and therefore are always variance-checked.
|
| Potential fix: remove the private var modifiers on the parameter v and add
| a field inside the class instead.
|
4 changes: 4 additions & 0 deletions tests/neg/i22620.scala
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Expand Up @@ -2,3 +2,7 @@
import scala.collection.mutable.ArrayBuffer

class PrivateTest[-M](private val v: ArrayBuffer[M]) // error

class PrivateTestMut[-M](private var v: ArrayBuffer[M]) // error

class PrivateTestParamOnly[-M](v: ArrayBuffer[M]) // no error