SAM types cannot be sealed or final

Previously we caught this in Typer, but doing it in the `SAMType`
extractor is more robust because it is used in overloading resolution,
and this will be needed in a subsequent commit of this PR to avoid
selecting the wrong overload in some cases.

This required dropping the final flag from the compiler-generated
ContextFunctionN, instead we explicitly prevent subclasses in RefChecks.

Author: smarter

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

@@ -133,8 +133,7 @@ class Definitions {
           ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: Nil, decls)
       }
     }
-    val flags0 = Trait | NoInits
-    val flags = if (name.isContextFunction) flags0 | Final else flags0
+    val flags = Trait | NoInits
     newPermanentClassSymbol(ScalaPackageClass, name, flags, completer)
   }
 

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

@@ -5043,7 +5043,7 @@ object Types {
         NoType
     }
     def isInstantiatable(tp: Type)(using Context): Boolean = zeroParamClass(tp) match {
-      case cinfo: ClassInfo =>
+      case cinfo: ClassInfo if !cinfo.cls.isOneOf(FinalOrSealed) =>
         val selfType = cinfo.selfType.asSeenFrom(tp, cinfo.cls)
         tp <:< selfType
       case _ =>

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -173,7 +173,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     CaseClassInInlinedCodeID,
     OverrideTypeMismatchErrorID,
     OverrideErrorID,
-    MatchableWarningID
+    MatchableWarningID,
+    CannotExtendFunctionID
 
   def errorNumber = ordinal - 2
 }

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1558,6 +1558,12 @@ import transform.SymUtils._
       def explain = ""
     }
 
+  class CannotExtendContextFunction(sym: Symbol)(using Context)
+    extends SyntaxMsg(CannotExtendFunctionID) {
+      def msg = em"""$sym cannot extend a context function class"""
+      def explain = ""
+    }
+
   class JavaEnumParentArgs(parent: Type)(using Context)
     extends TypeMsg(JavaEnumParentArgsID) {
       def msg = em"""not enough arguments for constructor Enum: ${hl("(name: String, ordinal: Int)")}: ${hl(parent.show)}"""

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

@@ -96,7 +96,7 @@ object RefChecks {
 
   /** Check that self type of this class conforms to self types of parents
    *  and required classes. Also check that only `enum` constructs extend
-   *  `java.lang.Enum`.
+   *  `java.lang.Enum` and no user-written class extends ContextFunctionN.
    */
   private def checkParents(cls: Symbol, parentTrees: List[Tree])(using Context): Unit = cls.info match {
     case cinfo: ClassInfo =>
@@ -132,6 +132,8 @@ object RefChecks {
             case _ =>
               false
           }
+      if psyms.exists(defn.isContextFunctionClass) then
+        report.error(CannotExtendContextFunction(cls), cls.sourcePos)
 
       /** Check that arguments passed to trait parameters conform to the parameter types
        *  in the current class. This is necessary since parameter types might be narrowed

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

@@ -1399,10 +1399,6 @@ class Typer extends Namer
                   else
                     report.error(ex"result type of lambda is an underspecified SAM type $pt", tree.srcPos)
                     pt
-                if (pt.classSymbol.isOneOf(FinalOrSealed)) {
-                  val offendingFlag = pt.classSymbol.flags & FinalOrSealed
-                  report.error(ex"lambda cannot implement $offendingFlag ${pt.classSymbol}", tree.srcPos)
-                }
                 TypeTree(targetTpe)
               case _ =>
                 if (mt.isParamDependent)