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
+