Fix #1991: Use classtag where available in unapply

1 files changed, 20 insertions, 13 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 2b57cf778..820044b8e 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/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)
@@ -1123,7 +1130,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     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] =>
+      case UnApply(fn, Nil, arg :: Nil) if tree.body.isInstanceOf[untpd.Typed] && !body1.tpe.isError =>
         // A typed pattern `x @ (_: T)` with an implicit `ctag: ClassTag[T]`
         // was rewritten to `x @ ctag(_)`.
         // Rewrite further to `ctag(x @ _)`