Require scrutinees to be Scrutable

Author: odersky

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

@@ -140,5 +140,6 @@ class ReTyper extends Typer with ReChecking {
 
   override protected def addAccessorDefs(cls: Symbol, body: List[Tree])(using Context): List[Tree] = body
   override protected def checkEqualityEvidence(tree: tpd.Tree, pt: Type)(using Context): Unit = ()
+  override protected def checkScrutable(sel: Tree)(using Context): Unit = ()
   override protected def matchingApply(methType: MethodOrPoly, pt: FunProto)(using Context): Boolean = true
-}
+}

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

@@ -1294,6 +1294,8 @@ class Typer extends Namer
     assignType(cpy.Closure(tree)(env1, meth1, target), meth1, target)
   }
 
+  protected def checkScrutable(sel: Tree)(using Context): Unit = adapt(sel, defn.ScrutableType)
+
   def typedMatch(tree: untpd.Match, pt: Type)(using Context): Tree =
     tree.selector match {
       case EmptyTree =>
@@ -1315,9 +1317,10 @@ class Typer extends Namer
           typed(desugar.makeCaseLambda(tree.cases, checkMode, protoFormals.length).withSpan(tree.span), pt)
         }
       case _ =>
-        if (tree.isInline) checkInInlineContext("inline match", tree.srcPos)
         val sel1 = typedExpr(tree.selector)
         val selType = fullyDefinedType(sel1.tpe, "pattern selector", tree.span).widen
+        if tree.isInline then checkInInlineContext("inline match", tree.srcPos)
+        else checkScrutable(sel1)
 
         /** Extractor for match types hidden behind an AppliedType/MatchAlias */
         object MatchTypeInDisguise {

tests/run-macros/i8514.check

@@ -1,4 +1,4 @@
-List(class Object, class Classable, class Any)
-List(class A, class Object, class Classable, class Any)
-List(class B, class A, class Object, class Classable, class Any)
-List(class C, class B, class A, class Object, class Classable, class Any)
+List(class Object, class Scrutable, class Any)
+List(class A, class Object, class Scrutable, class Any)
+List(class B, class A, class Object, class Scrutable, class Any)
+List(class C, class B, class A, class Object, class Scrutable, class Any)

tests/run-macros/i8514b.check

@@ -1,5 +1,5 @@
 B
 A[[T >: scala.Nothing <: scala.Any] => P[T], scala.Predef.String]
 java.lang.Object
-scala.Classable
+scala.Scrutable
 scala.Any

tests/run/scrutable.check

@@ -1,2 +1,4 @@
 class java.lang.Integer
+int
 class java.lang.String
+string

tests/run/scrutable.scala

@@ -1,6 +1,12 @@
 def foo[T](x: Any): Scrutable =
   println(x.getClass())
-  x
+  x match
+    case x: Int =>
+      println("int")
+      x
+    case x: String =>
+      println("string")
+      x
 
 @main def Test =
   val x: Scrutable = foo(1)