Merge pull request #1994 from dotty-staging/fix-#1991

Fix #1991: Use classtag where available in unapply

Author: odersky

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

@@ -916,7 +916,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         val result = assignType(cpy.UnApply(tree)(unapplyFn, unapplyImplicits, unapplyPatterns), ownType)
         unapp.println(s"unapply patterns = $unapplyPatterns")
         if ((ownType eq selType) || ownType.isError) result
-        else Typed(result, TypeTree(ownType))
+        else tryWithClassTag(Typed(result, TypeTree(ownType)), selType)
       case tp =>
         val unapplyErr = if (tp.isError) unapplyFn else notAnExtractor(unapplyFn)
         val typedArgsErr = args mapconserve (typed(_, defn.AnyType))

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

@@ -523,18 +523,8 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       def handlePattern: Tree = {
         val tpt1 = typedTpt
         // special case for an abstract type that comes with a class tag
-        tpt1.tpe.dealias match {
-          case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper =>
-            inferImplicit(defn.ClassTagType.appliedTo(tref),
-               EmptyTree, tpt1.pos)(ctx.retractMode(Mode.Pattern)) match {
-              case SearchSuccess(arg, _, _, _) =>
-                return typed(untpd.Apply(untpd.TypedSplice(arg), tree.expr), pt)
-              case _ =>
-            }
-          case _ =>
-            if (!ctx.isAfterTyper) tpt1.tpe.<:<(pt)(ctx.addMode(Mode.GADTflexible))
-        }
-        ascription(tpt1, isWildcard = true)
+        if (!ctx.isAfterTyper) tpt1.tpe.<:<(pt)(ctx.addMode(Mode.GADTflexible))
+        tryWithClassTag(ascription(tpt1, isWildcard = true), pt)
       }
       cases(
         ifPat = handlePattern,
@@ -543,6 +533,23 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     }
   }
 
+  /** For a typed tree `e: T`, if `T` is an abstract type for which an implicit class tag `ctag`
+   *  exists, rewrite to `ctag(e)`.
+   *  @pre We are in pattern-matching mode (Mode.Pattern)
+   */
+  def tryWithClassTag(tree: Typed, pt: Type)(implicit ctx: Context) = tree.tpt.tpe.dealias match {
+    case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper =>
+      require(ctx.mode.is(Mode.Pattern))
+      inferImplicit(defn.ClassTagType.appliedTo(tref),
+                    EmptyTree, tree.tpt.pos)(ctx.retractMode(Mode.Pattern)) match {
+        case SearchSuccess(clsTag, _, _, _) =>
+          typed(untpd.Apply(untpd.TypedSplice(clsTag), untpd.TypedSplice(tree.expr)), pt)
+        case _ =>
+          tree
+      }
+    case _ => tree
+  }
+
   def typedNamedArg(tree: untpd.NamedArg, pt: Type)(implicit ctx: Context) = track("typedNamedArg") {
     val arg1 = typed(tree.arg, pt)
     assignType(cpy.NamedArg(tree)(tree.name, arg1), arg1)
@@ -1121,15 +1128,13 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def typedBind(tree: untpd.Bind, pt: Type)(implicit ctx: Context): Tree = track("typedBind") {
     val pt1 = fullyDefinedType(pt, "pattern variable", tree.pos)
     val body1 = typed(tree.body, pt1)
-    typr.println(i"typed bind $tree pt = $pt1 bodytpe = ${body1.tpe}")
     body1 match {
-      case UnApply(fn, Nil, arg :: Nil) if tree.body.isInstanceOf[untpd.Typed] =>
-        // A typed pattern `x @ (_: T)` with an implicit `ctag: ClassTag[T]`
-        // was rewritten to `x @ ctag(_)`.
-        // Rewrite further to `ctag(x @ _)`
-        assert(fn.symbol.owner == defn.ClassTagClass)
+      case UnApply(fn, Nil, arg :: Nil) if fn.symbol.owner == defn.ClassTagClass && !body1.tpe.isError =>
+        // A typed pattern `x @ (e: T)` with an implicit `ctag: ClassTag[T]`
+        // was rewritten to `x @ ctag(e)` by `tryWithClassTag`.
+        // Rewrite further to `ctag(x @ e)`
         tpd.cpy.UnApply(body1)(fn, Nil,
-            typed(untpd.Bind(tree.name, arg).withPos(tree.pos), arg.tpe) :: Nil)
+            typed(untpd.Bind(tree.name, untpd.TypedSplice(arg)).withPos(tree.pos), arg.tpe) :: Nil)
       case _ =>
         val sym = newPatternBoundSym(tree.name, body1.tpe, tree.pos)
         assignType(cpy.Bind(tree)(tree.name, body1), sym)

tests/run/i1991.scala

@@ -0,0 +1,29 @@
+import scala.reflect.ClassTag
+
+class A[Foo](implicit tag: ClassTag[Foo]) {
+  object ExtractFoo {
+    def unapply(foo: Foo): Boolean = true
+  }
+
+  def isFoo(x: Any) = x match {
+    case ExtractFoo() => true
+    //case foo: Foo => true
+    case _ => false
+  }
+
+  def testBind(x: Any) = x match {
+    case foo0: Foo =>
+      (foo0: Foo)
+    case foo1 @ (_: Foo) =>
+      (foo1: Foo)
+    case foo2 @ ExtractFoo() =>
+      (foo2: Foo)
+  }
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert((new A[String]).isFoo("foo")) // OK
+    assert(!(new A[String]).isFoo(42)) // OK in scalac, fails in Dotty
+  }
+}