Give up on InferExpectedTypeSuite.map/flatMap

But keep the extraction of the instDecision logic, and keep the tests
cases I used in studying this change.

Also simplify and 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

Diff for: compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -240,25 +240,12 @@ object Inferencing {
           && {
             var fail = false
             var skip = false
-            val direction = instDirection(tvar.origin)
-            if minimizeSelected then
-              if direction <= 0 && tvar.hasLowerBound then
-                skip = instantiate(tvar, fromBelow = true)
-              else if direction >= 0 && tvar.hasUpperBound then
-                skip = instantiate(tvar, fromBelow = false)
-              // else hold off instantiating unbounded unconstrained variable
-            else if direction != 0 then
-              skip = instantiate(tvar, fromBelow = direction < 0)
-            else if variance >= 0 && tvar.hasLowerBound then
-              skip = instantiate(tvar, fromBelow = true)
-            else if (variance > 0 || variance == 0 && !tvar.hasUpperBound)
-                && force.ifBottom == IfBottom.ok
-            then // if variance == 0, prefer upper bound if one is given
-              skip = instantiate(tvar, fromBelow = true)
-            else if variance >= 0 && force.ifBottom == IfBottom.fail then
-              fail = true
-            else
-              toMaximize = tvar :: toMaximize
+            instDecision(tvar, variance, minimizeSelected, force.ifBottom) match
+              case Decision.Min   => skip = instantiate(tvar, fromBelow = true)
+              case Decision.Max   => skip = instantiate(tvar, fromBelow = false)
+              case Decision.Skip  => // hold off instantiating unbounded unconstrained variable
+              case Decision.Fail  => fail = true
+              case Decision.ToMax => toMaximize ::= tvar
             !fail && (skip || foldOver(x, tvar))
           }
         case tp => foldOver(x, tp)
@@ -452,9 +439,32 @@ object Inferencing {
       if (!cmp.isSubTypeWhenFrozen(constrained.lo, original.lo)) 1 else 0
     val approxAbove =
       if (!cmp.isSubTypeWhenFrozen(original.hi, constrained.hi)) 1 else 0
+    //println(i"instDirection($param) = $approxAbove - $approxBelow  original=[$original] constrained=[$constrained]")
     approxAbove - approxBelow
   }
 
+  /** The instantiation decision for given poly param computed from the constraint. */
+  enum Decision { case Min; case Max; case ToMax; case Skip; case Fail }
+  private def instDecision(tvar: TypeVar, v: Int, minimizeSelected: Boolean, ifBottom: IfBottom)(using Context): Decision =
+    import Decision.*
+    val direction = instDirection(tvar.origin)
+    val dec = if minimizeSelected then
+      if direction <= 0 && tvar.hasLowerBound then Min
+      else if direction >= 0 && tvar.hasUpperBound then Max
+      else Skip
+    else if direction != 0 then if direction < 0 then Min else Max
+    else if tvar.hasLowerBound then if v >= 0 then Min else ToMax
+    else ifBottom match
+      // What's left are unconstrained tvars with at most a non-Any param upperbound:
+      // * IfBottom.flip will always maximise to the param upperbound, for all variances
+      // * IfBottom.fail will fail the IFD check, for covariant or invariant tvars, maximise contravariant tvars
+      // * IfBottom.ok will minimise to Nothing covariant and unbounded invariant tvars, and max to Any the others
+      case IfBottom.ok   => if v > 0 || v == 0 && !tvar.hasUpperBound then Min else ToMax // prefer upper bound if one is given
+      case IfBottom.fail => if v >= 0 then Fail else ToMax
+      case ifBottom_flip => ToMax
+    //println(i"instDecision($tvar, v=v, minimizedSelected=$minimizeSelected, $ifBottom) dir=$direction = $dec")
+    dec
+
   /** Following type aliases and stripping refinements and annotations, if one arrives at a
    *  class type reference where the class has a companion module, a reference to
    *  that companion module. Otherwise NoType
@@ -651,7 +661,7 @@ trait Inferencing { this: Typer =>
 
     val ownedVars = state.ownedVars
     if (ownedVars ne locked) && !ownedVars.isEmpty then
-      val qualifying = ownedVars -- locked
+      val qualifying = (ownedVars -- locked).toList
       if (!qualifying.isEmpty) {
         typr.println(i"interpolate $tree: ${tree.tpe.widen} in $state, pt = $pt, owned vars = ${state.ownedVars.toList}%, %, qualifying = ${qualifying.toList}%, %, previous = ${locked.toList}%, % / ${state.constraint}")
         val resultAlreadyConstrained =
@@ -687,6 +697,10 @@ trait Inferencing { this: Typer =>
 
         def constraint = state.constraint
 
+        trace(i"interpolateTypeVars($tree: ${tree.tpe}, $pt, $qualifying)", typr, (_: Any) => i"$qualifying\n$constraint\n${ctx.gadt}") {
+        //println(i"$constraint")
+        //println(i"${ctx.gadt}")
+
         /** Values of this type report type variables to instantiate with variance indication:
          *    +1  variable appears covariantly, can be instantiated from lower bound
          *    -1  variable appears contravariantly, can be instantiated from upper bound
@@ -804,6 +818,7 @@ trait Inferencing { this: Typer =>
         end doInstantiate
 
         doInstantiate(filterByDeps(toInstantiate))
+        }
       }
     end if
     tree

Diff for: compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -18,10 +18,11 @@ import config.Printers.typr
 import Inferencing.*
 import ErrorReporting.*
 import util.SourceFile
+import util.Spans.{NoSpan, Span}
 import TypeComparer.necessarySubType
+import reporting.*
 
 import scala.annotation.internal.sharable
-import dotty.tools.dotc.util.Spans.{NoSpan, Span}
 
 object ProtoTypes {
 
@@ -83,6 +84,7 @@ object ProtoTypes {
      *  fits the given expected result type.
      */
     def constrainResult(mt: Type, pt: Type)(using Context): Boolean =
+    trace(i"constrainResult($mt, $pt)", typr):
       val savedConstraint = ctx.typerState.constraint
       val res = pt.widenExpr match {
         case pt: FunProto =>

Diff for: 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

Diff for: presentation-compiler/test/dotty/tools/pc/tests/InferExpectedTypeSuite.scala

@@ -221,34 +221,31 @@ class InferExpectedTypeSuite extends BasePCSuite:
          |""".stripMargin
     )
 
-  @Ignore("Generic functions are not handled correctly.")
   @Test def flatmap =
     check(
       """|val _ : List[Int] = List().flatMap(_ => @@)
          |""".stripMargin,
-      """|IterableOnce[Int]
-         |""".stripMargin
+      """|IterableOnce[Nothing]
+         |""".stripMargin // ideally IterableOnce[Int], but can't change interpolateTypeVars
     )
 
-  @Ignore("Generic functions are not handled correctly.")
   @Test def map =
     check(
       """|val _ : List[Int] = List().map(_ => @@)
          |""".stripMargin,
-      """|Int
-         |""".stripMargin
+      """|Nothing
+         |""".stripMargin // ideally Int, but can't change interpolateTypeVars
     )
 
-  @Ignore("Generic functions are not handled correctly.")
   @Test def `for-comprehension` =
     check(
       """|val _ : List[Int] =
          |  for {
          |    _ <- List("a", "b")
          |  } yield @@
          |""".stripMargin,
-      """|Int
-         |""".stripMargin
+      """|Nothing
+         |""".stripMargin // ideally Int, but can't change interpolateTypeVars
     )
 
 // bounds

Diff for: tests/pos/i21390.TrieMap.scala

@@ -0,0 +1,12 @@
+// Minimised from scala.collection.concurrent.LNode
+// Useful as a minimisation of how,
+// If we were to change the type interpolation
+// to minimise to the inferred "X" type,
+// then this is a minimisation of how the (ab)use of
+// GADT constraints to handle class type params
+// can fail PostTyper, -Ytest-pickler, and probably others.
+
+import scala.language.experimental.captureChecking
+
+class Foo[X](xs: List[X]):
+  def this(a: X, b: X) = this(if (a == b) then a :: Nil else a :: b :: Nil)

Diff for: tests/pos/i21390.zio.scala

@@ -0,0 +1,59 @@
+// A minimisation of a community build failure in PR 21390
+// To see why changing the instantiation direction in interpolateTypeVars
+// using the same logic as IsFullyDefined.
+class Has[A]
+object Has:
+  class Union[B, C]
+  object Union:
+    given HasHasUnion[B0 <: Has[?], C0 <: Has[?]]: Union[B0, C0] = ???
+
+class Lay[+D]:
+  def and1[B1 >: D, C1](that: Lay[C1])(using Has.Union[B1, C1]): Lay[B1 & C1] = ???
+  def and2[B2 >: D, C2](that: Lay[C2])(using Has.Union[B2, C2]): Lay[B2 & C2] = ???
+
+class J; type X = Has[J]
+class K; type Y = Has[K]
+class L; type Z = Has[L]
+
+def t1(x: Lay[X], y: Lay[Y], z: Lay[Z]): Lay[X & Y & Z] = x.and1(y).and2(z)
+
+/*
+
+Here's what goes wrong in the tvar instantiation, in method t1:
+
+1)  <== constrainResult(method and1, (using x$2: Union[B1, C1]): Lay[B1 & C1], ?{ and2: ? }) = true
+2)  ==> Has.Union[B0, C0] <: Has.Union[B1, C1 := Y]?
+3)  <== Has.Union[B0, C0] <: Has.Union[B1, C1 := Y] = OK
+
+1)  B1 >: X                   B2 >: B1 & C1
+2)  B1 >: X           C1 := Y B2 >: B1 & Y  B0 <: Has[?] C0 <: Has[?]
+3)  B1 >: X <: Has[?] C1 := Y B2 >: B1 & Y  B0 := B1     C0 := Y
+
+1) Check that the result of and1 fits the expected .and2 call, inferring any necessary constraints
+2) Initiate the check that calling HasHasUnion matches the needed Has.Union[B1, C1] parameter
+3) In inferring that the need B0 := B1 and C0 := Y, we end up inferring B0's `<: Has[?]` on B1.
+
+4a) <== B1.instantiate(fromBelow = true ) = X
+4b) <== B1.instantiate(fromBelow = false) = Has[?]
+5a) <== B2.instantiate(fromBelow = true)  = X & Y
+5b) <== B2.instantiate(fromBelow = true)  = Y
+6)  <== constrainResult(method and2, (using x$2: Has.Union[B2, C2]): Lay[B2 & C2], Lay[X & Y & Z]) = true
+
+4a) B2 >: X & Y
+4b) B2 >: Y & Has[?]
+5a) B2 := X & Y
+5b) B2 := Y
+6a) B2 >: X & Y  C2 <: Z
+6b) B2 >: Y      C2 <: X & Z
+
+4) With the extra upper bound constraint, we end up maximising to Has[?] (4b) instead of minimising to X (4a)
+5) Which leads to instantiating B2 to just Y (5b) instead of X & Y (5a)
+6) Which leads the constraints from the result of and2 to infer X & Z (6b) instead of just Z (6a)
+
+-- [E007] Type Mismatch Error: tests/pos/i21390.zio.scala:14:73 ------------------------------------
+14 |def t1(x: Lay[X], y: Lay[Y], z: Lay[Z]): Lay[X & Y & Z] = x.and1(y).and2(z)
+   |                                                                         ^
+   |                                                                         Found:    (z : Lay[Z])
+   |                                                                         Required: Lay[X & Z]
+
+*/