Fixes to support case id: T <- ...

This was not classified as a pattern binding before, so no filtering was applied.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1279,16 +1279,19 @@ object desugar {
      */
     def makeFor(mapName: TermName, flatMapName: TermName, enums: List[Tree], body: Tree): Tree = trace(i"make for ${ForYield(enums, body)}", show = true) {
 
-      /** Make a function value pat => body.
-       *  If pat is a var pattern id: T then this gives (id: T) => body
-       *  Otherwise this gives { case pat => body }, where `pat` is allowed to be
-       *  refutable only if `checkMode` is MatchCheck.None.
+      /** Let `pat` be `gen`'s pattern. Make a function value `pat => body`.
+       *  If `pat` is a var pattern `id: T` then this gives `(id: T) => body`.
+       *  Otherwise this gives `{ case pat => body }`, where `pat` is checked to be
+       *  irrefutable if `gen`'s checkMode is GenCheckMode.Check.
        */
-      def makeLambda(pat: Tree, body: Tree, checkMode: MatchCheck): Tree = pat match {
-        case IdPattern(named, tpt) =>
-          Function(derivedValDef(pat, named, tpt, EmptyTree, Modifiers(Param)) :: Nil, body)
+      def makeLambda(gen: GenFrom, body: Tree): Tree = gen.pat match {
+        case IdPattern(named, tpt) if gen.checkMode != GenCheckMode.FilterAlways =>
+          Function(derivedValDef(gen.pat, named, tpt, EmptyTree, Modifiers(Param)) :: Nil, body)
         case _ =>
-          makeCaseLambda(CaseDef(pat, EmptyTree, body) :: Nil, checkMode)
+          val matchCheckMode =
+            if (gen.checkMode == GenCheckMode.Check) MatchCheck.IrrefutableGenFrom
+            else MatchCheck.None
+          makeCaseLambda(CaseDef(gen.pat, EmptyTree, body) :: Nil, matchCheckMode)
       }
 
       /** If `pat` is not an Identifier, a Typed(Ident, _), or a Bind, wrap
@@ -1360,16 +1363,20 @@ object desugar {
         }
       }
 
-      def needsFilter(gen: GenFrom): Boolean =
-        gen.checkMode != GenCheckMode.Filter ||
-        IdPattern.unapply(gen.pat).isDefined ||
-        isIrrefutable(gen.pat, gen.expr)
+      def needsNoFilter(gen: GenFrom): Boolean =
+        if (gen.checkMode == GenCheckMode.FilterAlways) // pattern was prefixed by `case`
+          isIrrefutable(gen.pat, gen.expr)
+        else (
+          gen.checkMode != GenCheckMode.FilterNow ||
+          IdPattern.unapply(gen.pat).isDefined ||
+          isIrrefutable(gen.pat, gen.expr)
+        )
 
       /** rhs.name with a pattern filter on rhs unless `pat` is irrefutable when
        *  matched against `rhs`.
        */
       def rhsSelect(gen: GenFrom, name: TermName) = {
-        val rhs = if (needsFilter(gen)) gen.expr else makePatFilter(gen.expr, gen.pat)
+        val rhs = if (needsNoFilter(gen)) gen.expr else makePatFilter(gen.expr, gen.pat)
         Select(rhs, name)
       }
 
@@ -1379,10 +1386,10 @@ object desugar {
 
       enums match {
         case (gen: GenFrom) :: Nil =>
-          Apply(rhsSelect(gen, mapName), makeLambda(gen.pat, body, checkMode(gen)))
+          Apply(rhsSelect(gen, mapName), makeLambda(gen, body))
         case (gen: GenFrom) :: (rest @ (GenFrom(_, _, _) :: _)) =>
           val cont = makeFor(mapName, flatMapName, rest, body)
-          Apply(rhsSelect(gen, flatMapName), makeLambda(gen.pat, cont, checkMode(gen)))
+          Apply(rhsSelect(gen, flatMapName), makeLambda(gen, cont))
         case (gen: GenFrom) :: (rest @ GenAlias(_, _) :: _) =>
           val (valeqs, rest1) = rest.span(_.isInstanceOf[GenAlias])
           val pats = valeqs map { case GenAlias(pat, _) => pat }
@@ -1395,7 +1402,7 @@ object desugar {
           val vfrom1 = new GenFrom(makeTuple(allpats), rhs1, GenCheckMode.Ignore)
           makeFor(mapName, flatMapName, vfrom1 :: rest1, body)
         case (gen: GenFrom) :: test :: rest =>
-          val filtered = Apply(rhsSelect(gen, nme.withFilter), makeLambda(gen.pat, test, MatchCheck.None))
+          val filtered = Apply(rhsSelect(gen, nme.withFilter), makeLambda(gen, test))
           val genFrom = GenFrom(gen.pat, filtered, GenCheckMode.Ignore)
           makeFor(mapName, flatMapName, genFrom :: rest, body)
         case _ =>

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -121,7 +121,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   object GenCheckMode {
     val Ignore = new GenCheckMode(0)  // neither filter nor check since filtering was done before
     val Check = new GenCheckMode(1)   // check that pattern is irrefutable
-    val Filter = new GenCheckMode(2)  // filter out non-matching elements
+    val FilterNow = new GenCheckMode(2)  // filter out non-matching elements since we are not in -strict
+    val FilterAlways = new GenCheckMode(3) // filter out non-matching elements since pattern is prefixed by `case`
   }
 
   // ----- Modifiers -----------------------------------------------------

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1737,8 +1737,9 @@ object Parsers {
     def generatorRest(pat: Tree, casePat: Boolean): GenFrom =
       atSpan(startOffset(pat), accept(LARROW)) {
         val checkMode =
-          if (casePat || !ctx.settings.strict.value) GenCheckMode.Filter  // don't filter under -strict
-          else GenCheckMode.Check
+          if (casePat) GenCheckMode.FilterAlways
+          else if (ctx.settings.strict.value) GenCheckMode.Check
+          else GenCheckMode.FilterNow  // filter for now, to keep backwards compat
         GenFrom(pat, expr(), checkMode)
       }
 

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -568,7 +568,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case ForDo(enums, expr) =>
         forText(enums, expr, keywordStr(" do "))
       case GenFrom(pat, expr, checkMode) =>
-        (Str("case ") provided checkMode == untpd.GenCheckMode.Filter) ~
+        (Str("case ") provided checkMode == untpd.GenCheckMode.FilterAlways) ~
         toText(pat) ~ " <- " ~ toText(expr)
       case GenAlias(pat, expr) =>
         toText(pat) ~ " = " ~ toText(expr)

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

@@ -607,7 +607,7 @@ trait Checking {
     def fail(pat: Tree, pt: Type): Boolean = {
       var reportedPt = pt.dropAnnot(defn.UncheckedAnnot)
       if (!pat.tpe.isSingleton) reportedPt = reportedPt.widen
-      val problem = if (pat.tpe <:< pt) "is more specialized than" else "does not match"
+      val problem = if (pat.tpe <:< reportedPt) "is more specialized than" else "does not match"
       val fix = if (isPatDef) "`: @unchecked` after" else "`case ` before"
       ctx.errorOrMigrationWarning(
         ex"""pattern's type ${pat.tpe} $problem the right hand side expression's type $reportedPt