Use constrainPatternType also for unapply patterns

Author: odersky

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

@@ -927,7 +927,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
      *   - If a type proxy P is not a reference to a class, P's supertype is in G
      */
     def isSubTypeOfParent(subtp: Type, tp: Type)(implicit ctx: Context): Boolean =
-      if (subtp <:< tp) true
+      if (constrainPatternType(subtp, tp)) true
       else tp match {
         case tp: TypeRef if tp.symbol.isClass => isSubTypeOfParent(subtp, tp.firstParent)
         case tp: TypeProxy => isSubTypeOfParent(subtp, tp.superType)

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

@@ -185,7 +185,7 @@ object Inferencing {
    *  Invariant refinement can be assumed if `PatternType`'s class(es) are final or
    *  case classes (because of `RefChecks#checkCaseClassInheritanceInvariant`).
    */
-  def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context) = {
+  def constrainPatternType(tp: Type, pt: Type)(implicit ctx: Context): Boolean = {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
       case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
       case tp: TypeProxy => refinementIsInvariant(tp.underlying)
@@ -206,7 +206,7 @@ object Inferencing {
     }
 
     val widePt = if (ctx.scala2Mode || refinementIsInvariant(tp)) pt else widenVariantParams(pt)
-    (tp <:< widePt)(ctx.addMode(Mode.GADTflexible))
+    tp <:< widePt
   }
 
   /** The list of uninstantiated type variables bound by some prefix of type `T` which

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

@@ -568,7 +568,7 @@ class Typer extends Namer
       def typedTpt = checkSimpleKinded(typedType(tree.tpt))
       def handlePattern: Tree = {
         val tpt1 = typedTpt
-        if (!ctx.isAfterTyper) constrainPatternType(tpt1.tpe, pt)
+        if (!ctx.isAfterTyper) constrainPatternType(tpt1.tpe, pt)(ctx.addMode(Mode.GADTflexible))
         // special case for an abstract type that comes with a class tag
         tryWithClassTag(ascription(tpt1, isWildcard = true), pt)
       }

tests/neg/i3989b.scala

@@ -3,7 +3,7 @@ object Test extends App {
   case class B[+X](val x: X) extends A[X]
   class C[+X](x: Any) extends B[Any](x) with A[X] // error
   def f(a: A[Int]): Int = a match {
-    case a: B[_] => a.x
+    case B(i) => i
     case _ => 0
   }
   f(new C[Int]("foo"))

tests/neg/i3989c.scala

@@ -0,0 +1,15 @@
+import scala.Option
+object Test extends App {
+  trait A[+X]
+  class B[+X](val x: X) extends A[X]
+  object B {
+    def unapply[X](b: B[X]): Option[X] = Some(b.x)
+  }
+
+  class C[+X](x: Any) extends B[Any](x) with A[X]
+  def f(a: A[Int]): Int = a match {
+    case B(i) => i   // error
+    case _ => 0
+  }
+  f(new C[Int]("foo"))
+}