Simplify & update InstantiateModel

Martin had tweaked my i14218 fix to make it more conservative: in the
(NN, UU) case (i.e. no inferred bounds, only a UU upper bound declared)
for covariant type parameters, revert back to minimising to Nothing
rather than maximising to the declared bound.

Author: dwijnand

compiler/test/dotty/tools/dotc/typer/InstantiateModel.scala

@@ -4,22 +4,16 @@ package typer
 
 // Modelling the decision in IsFullyDefined
 object InstantiateModel:
-  enum LB  { case NN; case LL; case L1                             }; import LB.*
-  enum UB  { case AA; case UU; case U1                             }; import UB.*
-  enum Var { case V; case NotV                                     }; import Var.*
-  enum MSe { case M; case NotM                                     }; import MSe.*
-  enum Bot { case Fail; case Ok; case Flip                         }; import Bot.*
-  enum Act { case Min; case Max; case ToMax; case Skip; case False }; import Act.*
+  enum LB { case NN; case LL; case L1 }; import LB.*
+  enum UB { case AA; case UU; case U1 }; import UB.*
+  enum Decision { case Min; case Max; case ToMax; case Skip; case Fail }; import Decision.*
 
   // NN/AA = Nothing/Any
   // LL/UU = the original bounds, on the type parameter
   // L1/U1 = the constrained bounds, on the type variable
-  // V     = variance >= 0 ("non-contravariant")
-  // MSe   = minimisedSelected
-  // Bot   = IfBottom
   // ToMax = delayed maximisation, via addition to toMaximize
   // Skip  = minimisedSelected "hold off instantiating"
-  // False = return false
+  // Fail  = IfBottom.fail's bail option
 
   // there are 9 combinations:
   // # | LB | UB | d | // d = direction
@@ -34,24 +28,27 @@ object InstantiateModel:
   // 8 | NN | UU | 0 |             T       <: UU
   // 9 | NN | AA | 0 |             T
 
-  def decide(lb: LB, ub: UB, v: Var, bot: Bot, m: MSe): Act = (lb, ub) match
+  def instDecision(lb: LB, ub: UB, v: Int, ifBottom: IfBottom, min: Boolean) = (lb, ub) match
     case (L1, AA) => Min
     case (L1, UU) => Min
     case (LL, U1) => Max
     case (NN, U1) => Max
 
-    case (L1, U1) => if m==M || v==V then Min else ToMax
-    case (LL, UU) => if m==M || v==V then Min else ToMax
-    case (LL, AA) => if m==M || v==V then Min else ToMax
-
-    case (NN, UU) => bot match
-      case _    if m==M => Max
-    //case Ok   if v==V => Min   // removed, i14218 fix
-      case Fail if v==V => False
-      case _            => ToMax
-
-    case (NN, AA) => bot match
-      case _    if m==M => Skip
-      case Ok   if v==V => Min
-      case Fail if v==V => False
-      case _            => ToMax
+    case (L1, U1) => if min then Min else pickVar(v, Min, Min, ToMax)
+    case (LL, UU) => if min then Min else pickVar(v, Min, Min, ToMax)
+    case (LL, AA) => if min then Min else pickVar(v, Min, Min, ToMax)
+
+    case (NN, UU) => ifBottom match
+      case _ if min      => Max
+      case IfBottom.ok   => pickVar(v, Min, ToMax, ToMax)
+      case IfBottom.fail => pickVar(v, Fail, Fail, ToMax)
+      case IfBottom.flip => ToMax
+
+    case (NN, AA) => ifBottom match
+      case _ if min      => Skip
+      case IfBottom.ok   => pickVar(v, Min, Min, ToMax)
+      case IfBottom.fail => pickVar(v, Fail, Fail, ToMax)
+      case IfBottom.flip => ToMax
+
+  def pickVar[A](v: Int, cov: A, inv: A, con: A) =
+    if v > 0 then cov else if v == 0 then inv else con