Fix #6314: match type unsoundness

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1945,19 +1945,6 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
             true
         }
       case (AppliedType(tycon1, args1), AppliedType(tycon2, args2)) if tycon1 == tycon2 =>
-        // Unboxed xs.zip(ys).zip(zs).forall { case ((a, b), c) => f(a, b, c) }
-        def zip_zip_forall[A, B, C](xs: List[A], ys: List[B], zs: List[C])(f: (A, B, C) => Boolean): Boolean = {
-          xs match {
-            case x :: xs => ys match {
-              case y :: ys => zs match {
-                case z :: zs => f(x, y, z) && zip_zip_forall(xs, ys, zs)(f)
-                case _ => true
-              }
-              case _ => true
-            }
-            case _ => true
-          }
-        }
         def covariantIntersecting(tp1: Type, tp2: Type, tparam: TypeParamInfo): Boolean = {
           intersecting(tp1, tp2) || {
             // We still need to proof that `Nothing` is not a valid
@@ -1981,7 +1968,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
           }
         }
 
-        zip_zip_forall(args1, args2, tycon1.typeParams) {
+        (args1, args2, tycon1.typeParams).zipped.forall {
           (arg1, arg2, tparam) =>
             val v = tparam.paramVariance
             if (v > 0)
@@ -1994,7 +1981,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
               covariantIntersecting(arg1, arg2, tparam) && (isSameType(arg1, arg2) || {
                 // We can only trust a "no" from `isSameType` when both
                 // `arg1` and `arg2` are fully instantiated.
-                val fullyInstantiated = new TypeAccumulator[Boolean] {
+                def fullyInstantiated(tp: Type): Boolean = new TypeAccumulator[Boolean] {
                   override def apply(x: Boolean, t: Type) =
                     x && {
                       t match {
@@ -2003,9 +1990,8 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
                         case _ => foldOver(x, t)
                       }
                     }
-                }
-                !(fullyInstantiated.apply(true, arg1) &&
-                  fullyInstantiated.apply(true, arg2))
+                }.apply(true, tp)
+                !(fullyInstantiated(arg1) && fullyInstantiated(arg2))
               })
         }
       case (tp1: HKLambda, tp2: HKLambda) =>
@@ -2019,10 +2005,15 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
         intersecting(tp1.tp1, tp2) || intersecting(tp1.tp2, tp2)
       case (_, tp2: OrType)  =>
         intersecting(tp1, tp2.tp1) || intersecting(tp1, tp2.tp2)
+      case (tp1: AndType, tp2: AndType) =>
+        intersecting(tp1.tp1, tp2.tp1) && intersecting(tp1.tp2, tp2.tp2) ||
+        intersecting(tp1.tp1, tp2.tp2) && intersecting(tp1.tp2, tp2.tp1)
       case (tp1: AndType, _) =>
-        intersecting(tp1.tp1, tp2) && intersecting(tp1.tp2, tp2) && intersecting(tp1.tp1, tp1.tp2)
+        intersecting(tp1.tp1, tp2) && intersecting(tp1.tp2, tp2) ||
+        intersecting(tp1.tp2, tp2) && intersecting(tp1.tp1, tp2)
       case (_, tp2: AndType) =>
-        intersecting(tp1, tp2.tp1) && intersecting(tp1, tp2.tp2) && intersecting(tp2.tp1, tp2.tp2)
+        intersecting(tp1, tp2.tp1) && intersecting(tp1, tp2.tp2) ||
+        intersecting(tp1, tp2.tp2) && intersecting(tp1, tp2.tp1)
       case (tp1: TypeProxy, tp2: TypeProxy) =>
         intersecting(tp1.underlying, tp2) && intersecting(tp1, tp2.underlying)
       case (tp1: TypeProxy, _) =>

library/src-3.x/scala/compiletime/package.scala

@@ -11,4 +11,4 @@ package object compiletime {
   inline def constValue[T]: T = ???
 
   type S[X <: Int] <: Int
-}
+}

tests/neg/6314.scala

@@ -0,0 +1,14 @@
+object G {
+  final class X
+  final class Y
+
+  trait Test {
+    type Type
+    val i: Bar[Y & Type] = 1 // error
+  }
+
+  type Bar[A] = A match {
+    case X & Y => String
+    case Y => Int
+  }
+}

tests/run/tuples1.scala

@@ -94,4 +94,4 @@ object Test extends App {
   val us0: 0 = size(())
   val x3s1: 3 = size0(x3)
   val us1: 0 = size0(())
-}
+}