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Fix #5888: Disallow SAM type creation of final and sealed classes #5896

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Merged
merged 1 commit into from
Feb 15, 2019
Merged

Fix #5888: Disallow SAM type creation of final and sealed classes #5896

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3 changes: 3 additions & 0 deletions compiler/src/dotty/tools/dotc/core/Flags.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -638,6 +638,9 @@ object Flags {
/** value that's final or inline */
final val FinalOrInline: FlagSet = Final | Inline

/** class that's final or sealed */
final val FinalOrSealed: FlagSet = Final | Sealed

/** A covariant type parameter instance */
final val LocalCovariant: FlagConjunction = allOf(Local, Covariant)

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6 changes: 5 additions & 1 deletion compiler/src/dotty/tools/dotc/typer/Typer.scala
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Expand Up @@ -985,7 +985,11 @@ class Typer extends Namer
case SAMType(sam)
if !defn.isFunctionType(pt) && mt <:< sam =>
if (!isFullyDefined(pt, ForceDegree.all))
ctx.error(ex"result type of closure is an underspecified SAM type $pt", tree.sourcePos)
ctx.error(ex"result type of lambda is an underspecified SAM type $pt", tree.sourcePos)
else if (pt.classSymbol.is(FinalOrSealed)) {
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Should we move this check into the SAMType extractor?

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No I think it's better here. An extractor is for extracting, not reporting errors.

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I didn't mean to report an error in the extractor, just have the extractor return None on sealed and final classes. It seemed arbitrary to have a special error message for this. In scalac I get a type mismatch instead.:

scala> sealed trait Foo { def f(x: Int): Int }; val foo: Foo = (x: Int) => x
<console>:11: error: type mismatch;
 found   : Int => Int
 required: Foo
       sealed trait Foo { def f(x: Int): Int }; val foo: Foo = (x: Int) => x

val offendingFlag = pt.classSymbol.flags & FinalOrSealed
ctx.error(ex"lambda cannot implement $offendingFlag ${pt.classSymbol}", tree.sourcePos)
}
TypeTree(pt)
case _ =>
if (mt.isParamDependent) {
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8 changes: 8 additions & 0 deletions tests/neg/i5888.scala
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@@ -0,0 +1,8 @@
val x: String <:< Int = _.toInt // error

abstract final class Foo { def f(x: Int): Int }

val foo: Foo = x => x // error

val foo2: Foo = Predef.identity // error