Keep constraints for subtype tests with ProtoTypes

A subtype test `A <:< P` where `P` is a prototype was implemented
as `P.isMatchedBy(A)`. `isMatchedBy` did not keep the constraint
which meant that information was lost instead of being propagated.

Author: odersky

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

@@ -1227,7 +1227,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
   /** Defer constraining type variables when compared against prototypes */
   def isMatchedByProto(proto: ProtoType, tp: Type): Boolean = tp.stripTypeVar match {
     case tp: TypeParamRef if constraint contains tp => true
-    case _ => proto.isMatchedBy(tp)
+    case _ => proto.isMatchedBy(tp, keepConstraint = true)
   }
 
   /** Narrow gadt.bounds for the type parameter referenced by `tr` to include

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

@@ -1665,7 +1665,7 @@ object Types {
 
   /** A trait for proto-types, used as expected types in typer */
   trait ProtoType extends Type {
-    def isMatchedBy(tp: Type)(implicit ctx: Context): Boolean
+    def isMatchedBy(tp: Type, keepConstraint: Boolean = false)(implicit ctx: Context): Boolean
     def fold[T](x: T, ta: TypeAccumulator[T])(implicit ctx: Context): T
     def map(tm: TypeMap)(implicit ctx: Context): ProtoType
 

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

@@ -277,10 +277,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     private[this] var _ok = true
 
     def ok: Boolean = _ok
-    def ok_=(x: Boolean): Unit = {
-      assert(x || ctx.reporter.errorsReported || !ctx.typerState.isCommittable) // !!! DEBUG
-      _ok = x
-    }
+    def ok_=(x: Boolean): Unit = _ok = x
 
     /** The function's type after widening and instantiating polytypes
      *  with TypeParamRefs in constraint set
@@ -1118,8 +1115,11 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
   /** Is given method reference applicable to type arguments `targs` and argument trees `args`?
    *  @param  resultType   The expected result type of the application
    */
-  def isApplicable(methRef: TermRef, targs: List[Type], args: List[Tree], resultType: Type)(implicit ctx: Context): Boolean =
-    ctx.test(implicit ctx => new ApplicableToTrees(methRef, targs, args, resultType).success)
+  def isApplicable(methRef: TermRef, targs: List[Type], args: List[Tree], resultType: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
+    def isApp(implicit ctx: Context): Boolean =
+      new ApplicableToTrees(methRef, targs, args, resultType).success
+    if (keepConstraint) isApp else ctx.test(implicit ctx => isApp)
+  }
 
   /** Is given method reference applicable to type arguments `targs` and argument trees `args` without inferring views?
     *  @param  resultType   The expected result type of the application
@@ -1137,8 +1137,8 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
    *  possibly after inserting an `apply`?
    *  @param  resultType   The expected result type of the application
    */
-  def isApplicable(tp: Type, targs: List[Type], args: List[Tree], resultType: Type)(implicit ctx: Context): Boolean =
-    onMethod(tp, isApplicable(_, targs, args, resultType))
+  def isApplicable(tp: Type, targs: List[Type], args: List[Tree], resultType: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean =
+    onMethod(tp, isApplicable(_, targs, args, resultType, keepConstraint))
 
   /** Is given type applicable to argument types `args`, possibly after inserting an `apply`?
    *  @param  resultType   The expected result type of the application
@@ -1491,7 +1491,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
           )
           if (alts2.isEmpty && !ctx.isAfterTyper)
             alts.filter(alt =>
-              isApplicable(alt, targs, args, resultType)
+              isApplicable(alt, targs, args, resultType, keepConstraint = false)
             )
           else
             alts2
@@ -1511,14 +1511,14 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         }
 
       case pt @ PolyProto(targs1, pt1) if targs.isEmpty =>
-        val alts1 = alts filter pt.isMatchedBy
+        val alts1 = alts.filter(pt.isMatchedBy(_))
         resolveOverloaded(alts1, pt1, targs1.tpes)
 
       case defn.FunctionOf(args, resultType, _, _) =>
         narrowByTypes(alts, args, resultType)
 
       case pt =>
-        val compat = alts.filter(normalizedCompatible(_, pt))
+        val compat = alts.filter(normalizedCompatible(_, pt, keepConstraint = false))
         if (compat.isEmpty)
           /*
            * the case should not be moved to the enclosing match

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

@@ -35,12 +35,21 @@ object ProtoTypes {
     def isCompatible(tp: Type, pt: Type)(implicit ctx: Context): Boolean =
       (tp.widenExpr relaxed_<:< pt.widenExpr) || viewExists(tp, pt)
 
-    /** Test compatibility after normalization in a fresh typerstate. */
-    def normalizedCompatible(tp: Type, pt: Type)(implicit ctx: Context): Boolean =
-      ctx.test { implicit ctx =>
+    /** Test compatibility after normalization.
+     *  Do this in a fresh typerstate unless `keepConstraint` is true.
+     */
+    def normalizedCompatible(tp: Type, pt: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
+      def testCompat(implicit ctx: Context): Boolean = {
         val normTp = normalize(tp, pt)
         isCompatible(normTp, pt) || pt.isRef(defn.UnitClass) && normTp.isParameterless
       }
+      if (keepConstraint)
+        tp.widenSingleton match {
+          case poly: PolyType => normalizedCompatible(tp, pt, keepConstraint = false)
+          case _ => testCompat
+        }
+      else ctx.test(implicit ctx => testCompat)
+    }
 
     private def disregardProto(pt: Type)(implicit ctx: Context): Boolean = pt.dealias match {
       case _: OrType => true
@@ -89,7 +98,7 @@ object ProtoTypes {
 
   /** A trait for prototypes that match all types */
   trait MatchAlways extends ProtoType {
-    def isMatchedBy(tp1: Type)(implicit ctx: Context): Boolean = true
+    def isMatchedBy(tp1: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = true
     def map(tm: TypeMap)(implicit ctx: Context): ProtoType = this
     def fold[T](x: T, ta: TypeAccumulator[T])(implicit ctx: Context): T = x
     override def toString: String = getClass.toString
@@ -131,13 +140,13 @@ object ProtoTypes {
       case _ => false
     }
 
-    override def isMatchedBy(tp1: Type)(implicit ctx: Context): Boolean = {
+    override def isMatchedBy(tp1: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
       name == nme.WILDCARD || hasUnknownMembers(tp1) ||
       {
         val mbr = if (privateOK) tp1.member(name) else tp1.nonPrivateMember(name)
         def qualifies(m: SingleDenotation) =
           memberProto.isRef(defn.UnitClass) ||
-          tp1.isValueType && compat.normalizedCompatible(NamedType(tp1, name, m), memberProto)
+          tp1.isValueType && compat.normalizedCompatible(NamedType(tp1, name, m), memberProto, keepConstraint)
             // Note: can't use `m.info` here because if `m` is a method, `m.info`
             //       loses knowledge about `m`'s default arguments.
         mbr match { // hasAltWith inlined for performance
@@ -234,8 +243,11 @@ object ProtoTypes {
   extends UncachedGroundType with ApplyingProto with FunOrPolyProto {
     override def resultType(implicit ctx: Context): Type = resType
 
-    def isMatchedBy(tp: Type)(implicit ctx: Context): Boolean =
-      typer.isApplicable(tp, Nil, unforcedTypedArgs, resultType)
+    def isMatchedBy(tp: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
+      val args = unforcedTypedArgs
+      def isPoly(tree: Tree) = tree.tpe.widenSingleton.isInstanceOf[PolyType]
+      typer.isApplicable(tp, Nil, args, resultType, keepConstraint && !args.exists(isPoly))
+    }
 
     def derivedFunProto(args: List[untpd.Tree] = this.args, resultType: Type, typer: Typer = this.typer): FunProto =
       if ((args eq this.args) && (resultType eq this.resultType) && (typer eq this.typer)) this
@@ -379,7 +391,7 @@ object ProtoTypes {
 
     override def resultType(implicit ctx: Context): Type = resType
 
-    def isMatchedBy(tp: Type)(implicit ctx: Context): Boolean =
+    def isMatchedBy(tp: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean =
       ctx.typer.isApplicable(tp, argType :: Nil, resultType) || {
         resType match {
           case SelectionProto(name: TermName, mbrType, _, _) =>
@@ -422,7 +434,7 @@ object ProtoTypes {
 
     override def resultType(implicit ctx: Context): Type = resType
 
-    override def isMatchedBy(tp: Type)(implicit ctx: Context): Boolean = {
+    override def isMatchedBy(tp: Type, keepConstraint: Boolean)(implicit ctx: Context): Boolean = {
       def isInstantiatable(tp: Type) = tp.widen match {
         case tp: PolyType => tp.paramNames.length == targs.length
         case _ => false