Merge pull request #5896 from dotty-staging/fix-#5888

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

Author: odersky

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -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)
 

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

@@ -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)) {
+ val offendingFlag = pt.classSymbol.flags & FinalOrSealed
+ ctx.error(ex"lambda cannot implement $offendingFlag ${pt.classSymbol}", tree.sourcePos)
+ }
  TypeTree(pt)
  case _ =>
  if (mt.isParamDependent) {

tests/neg/i5888.scala

@@ -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
+