Help implement Metals' infer expected type feature (#21390)

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

@@ -3268,9 +3268,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
   /** The trace of comparison operations when performing `op` */
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean)(using Context): String =
-    val cmp = explainingTypeComparer(short)
-    inSubComparer(cmp)(op)
-    cmp.lastTrace(header)
+    explaining(cmp => { op(cmp); cmp.lastTrace(header) }, short)
+
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean)(using Context): T =
+    inSubComparer(explainingTypeComparer(short))(op)
 
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     inSubComparer(matchReducer)(op)
@@ -3440,6 +3441,9 @@ object TypeComparer {
   def explained[T](op: ExplainingTypeComparer => T, header: String = "Subtype trace:", short: Boolean = false)(using Context): String =
     comparing(_.explained(op, header, short))
 
+  def explaining[T](op: ExplainingTypeComparer => T, short: Boolean = false)(using Context): T =
+    comparing(_.explaining(op, short))
+
   def reduceMatchWith[T](op: MatchReducer => T)(using Context): T =
     comparing(_.reduceMatchWith(op))
 
@@ -3871,7 +3875,7 @@ class ExplainingTypeComparer(initctx: Context, short: Boolean) extends TypeCompa
   override def recur(tp1: Type, tp2: Type): Boolean =
     def moreInfo =
       if Config.verboseExplainSubtype || ctx.settings.verbose.value
-      then s" ${tp1.getClass} ${tp2.getClass}"
+      then s" ${tp1.className} ${tp2.className}"
       else ""
     val approx = approxState
     def approxStr = if short then "" else approx.show

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

@@ -691,11 +691,22 @@ object TypeOps:
       val hiBound = instantiate(bounds.hi, skolemizedArgTypes)
       val loBound = instantiate(bounds.lo, skolemizedArgTypes)
 
-      def check(using Context) = {
-        if (!(lo <:< hiBound)) violations += ((arg, "upper", hiBound))
-        if (!(loBound <:< hi)) violations += ((arg, "lower", loBound))
+      def check(tp1: Type, tp2: Type, which: String, bound: Type)(using Context) = {
+        val isSub = TypeComparer.explaining { cmp =>
+          val isSub = cmp.isSubType(tp1, tp2)
+          if !isSub then
+            if !ctx.typerState.constraint.domainLambdas.isEmpty then
+              typr.println(i"${ctx.typerState.constraint}")
+            if !ctx.gadt.symbols.isEmpty then
+              typr.println(i"${ctx.gadt}")
+            typr.println(cmp.lastTrace(i"checkOverlapsBounds($lo, $hi, $arg, $bounds)($which)"))
+            //trace.dumpStack()
+          isSub
+        }//(using ctx.fresh.setSetting(ctx.settings.verbose, true)) // uncomment to enable moreInfo in ExplainingTypeComparer recur
+        if !isSub then violations += ((arg, which, bound))
       }
-      check(using checkCtx)
+      check(lo, hiBound, "upper", hiBound)(using checkCtx)
+      check(loBound, hi, "lower", loBound)(using checkCtx)
     }
 
     def loop(args: List[Tree], boundss: List[TypeBounds]): Unit = args match

compiler/src/dotty/tools/dotc/reporting/trace.scala

@@ -27,6 +27,18 @@ object trace extends TraceSyntax:
   object log extends TraceSyntax:
     inline def isEnabled: true = true
     protected val isForced = false
+
+  def dumpStack(limit: Int = -1): Unit = {
+    val out = Console.out
+    val exc = new Exception("Dump Stack")
+    var stack = exc.getStackTrace
+      .filter(e => !e.getClassName.startsWith("dotty.tools.dotc.reporting.TraceSyntax"))
+      .filter(e => !e.getClassName.startsWith("dotty.tools.dotc.reporting.trace"))
+    if limit >= 0 then
+      stack = stack.take(limit)
+    exc.setStackTrace(stack)
+    exc.printStackTrace(out)
+  }
 end trace
 
 /** This module is carefully optimized to give zero overhead if Config.tracingEnabled

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

@@ -571,7 +571,7 @@ trait Applications extends Compatibility {
           fail(TypeMismatch(methType.resultType, resultType, None))
 
         // match all arguments with corresponding formal parameters
-        matchArgs(orderedArgs, methType.paramInfos, 0)
+        if success then matchArgs(orderedArgs, methType.paramInfos, 0)
       case _ =>
         if (methType.isError) ok = false
         else fail(em"$methString does not take parameters")
@@ -666,7 +666,7 @@ trait Applications extends Compatibility {
      *  @param n   The position of the first parameter in formals in `methType`.
      */
     def matchArgs(args: List[Arg], formals: List[Type], n: Int): Unit =
-      if (success) formals match {
+      formals match {
         case formal :: formals1 =>
 
           def checkNoVarArg(arg: Arg) =
@@ -878,7 +878,9 @@ trait Applications extends Compatibility {
     init()
 
     def addArg(arg: Tree, formal: Type): Unit =
-      typedArgBuf += adapt(arg, formal.widenExpr)
+      val typedArg = adapt(arg, formal.widenExpr)
+      typedArgBuf += typedArg
+      ok = ok & !typedArg.tpe.isError
 
     def makeVarArg(n: Int, elemFormal: Type): Unit = {
       val args = typedArgBuf.takeRight(n).toList
@@ -943,7 +945,7 @@ trait Applications extends Compatibility {
       var typedArgs = typedArgBuf.toList
       def app0 = cpy.Apply(app)(normalizedFun, typedArgs) // needs to be a `def` because typedArgs can change later
       val app1 =
-        if (!success || typedArgs.exists(_.tpe.isError)) app0.withType(UnspecifiedErrorType)
+        if !success then app0.withType(UnspecifiedErrorType)
         else {
           if isJavaAnnotConstr(methRef.symbol) then
             // #19951 Make sure all arguments are NamedArgs for Java annotations

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

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 =>

compiler/src/dotty/tools/dotc/util/Signatures.scala

@@ -651,7 +651,7 @@ object Signatures {
    *
    * @param err            The error message to inspect.
    * @param params         The parameters that were given at the call site.
-   * @param alreadyCurried Index of paramss we are currently in.
+   * @param paramssIndex   Index of paramss we are currently in.
    *
    * @return A pair composed of the index of the best alternative (0 if no alternatives
    *         were found), and the list of alternatives.

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