Backport "Do not propagate TypeErrors of ops from TypeComparer#tryAlso" to LTS

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

@@ -1851,7 +1851,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       // check whether `op2` generates a weaker constraint than `op1`
       val leftConstraint = constraint
       constraint = preConstraint
-      if !(op && subsumes(leftConstraint, constraint, preConstraint)) then
+      val res = try op catch case _: TypeError => false
+      if !(res && subsumes(leftConstraint, constraint, preConstraint)) then
         if constr != noPrinter && !subsumes(constraint, leftConstraint, preConstraint) then
           constr.println(i"CUT - prefer $leftConstraint over $constraint")
         constraint = leftConstraint

tests/pos-deep-subtype/i21015.scala

@@ -0,0 +1,36 @@
+
+type Init[Coll[_], A, T <: Tuple] = T match
+  case EmptyTuple   => A
+  case head *: rest => InitCons[Coll, A, head, rest]
+
+type InitCons[Coll[_], A, H, Rest <: Tuple] = H match
+  case Int => Init[Coll, Coll[A], Rest]
+  case _   => Unit
+
+def fillVector[A, T <: Tuple](dims: T)(x: => A): Init[Vector, A, T] =
+  dims match
+    case _: EmptyTuple                => x
+    case (p : (head *: rest)) =>
+      val (head *: rest) = p
+      head match
+        case size: Int => fillVector(rest)(Vector.fill(size)(x))
+        case _         => ()
+
+
+object Minimization:
+
+  type M1[A] = Int match
+    case 1 => M2[A]
+
+  type M2[A] = Int match
+    case 2 => M1[Option[A]]
+
+  def m1[A](x: A): M1[A] = ???
+
+  val _: M1[Int] = m1(1) // was error
+  val _: M1[Int] = m1[Int](1) // ok
+  val _: M1[Int] =
+    val x = m1(1)
+    x // ok
+
+end Minimization