Refactor function type logic (#18193)

* Rename `isFucntionType` to `isFunctionNType` 
  (tests for FunctionN/ContextFunctionN)
* Rename `isFunctionOrPolyType` to `isFunctionType` 
  (tests for FunctionN/ContextFunctionN/PolyFunction/ErasedFunction)
* Rename `isRefinedFunctionType` to `isPolyOrErasedFunctionType`
  (tests for PolyFunction/ErasedFunction)
* Define `isPolyFunctionType`
* Refactor uses of
`isPolyFunctionType`/`isErasedFunctionType`/`isPolyOrErasedFunctionType`
* Simplify/generalize erasure of refined function types

Followup of
dotty-staging@6a349b5#r121364261

Author: smarter

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -968,7 +968,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       && tree.isTerm
       && {
         val qualType = tree.qualifier.tpe
-        hasRefinement(qualType) && !defn.isRefinedFunctionType(qualType)
+        hasRefinement(qualType) && !defn.isPolyOrErasedFunctionType(qualType)
       }
     def loop(tree: Tree): Boolean = tree match
       case TypeApply(fun, _) =>

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -187,7 +187,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               capt.println(i"solving $t")
               refs.solve()
             traverse(parent)
-          case t @ RefinedType(_, nme.apply, rinfo) if defn.isFunctionOrPolyType(t) =>
+          case t @ RefinedType(_, nme.apply, rinfo) if defn.isFunctionType(t) =>
             traverse(rinfo)
           case tp: TypeVar =>
           case tp: TypeRef =>
@@ -638,7 +638,7 @@ class CheckCaptures extends Recheck, SymTransformer:
         case expected @ CapturingType(eparent, refs) =>
           CapturingType(recur(eparent), refs, boxed = expected.isBoxed)
         case expected @ defn.FunctionOf(args, resultType, isContextual)
-          if defn.isNonRefinedFunction(expected) && defn.isFunctionType(actual) && !defn.isNonRefinedFunction(actual) =>
+          if defn.isNonRefinedFunction(expected) && defn.isFunctionNType(actual) && !defn.isNonRefinedFunction(actual) =>
           val expected1 = toDepFun(args, resultType, isContextual)
           expected1
         case _ =>
@@ -707,7 +707,7 @@ class CheckCaptures extends Recheck, SymTransformer:
           val (eargs, eres) = expected.dealias.stripCapturing match
             case defn.FunctionOf(eargs, eres, _) => (eargs, eres)
             case expected: MethodType => (expected.paramInfos, expected.resType)
-            case expected @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(expected) => (rinfo.paramInfos, rinfo.resType)
+            case expected @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionNType(expected) => (rinfo.paramInfos, rinfo.resType)
             case _ => (aargs.map(_ => WildcardType), WildcardType)
           val aargs1 = aargs.zipWithConserve(eargs) { (aarg, earg) => adapt(aarg, earg, !covariant) }
           val ares1 = adapt(ares, eres, covariant)
@@ -769,7 +769,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case actual @ AppliedType(tycon, args) if defn.isNonRefinedFunction(actual) =>
               adaptFun(actual, args.init, args.last, expected, covariant, insertBox,
                   (aargs1, ares1) => actual.derivedAppliedType(tycon, aargs1 :+ ares1))
-            case actual @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionOrPolyType(actual) =>
+            case actual @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(actual) =>
               // TODO Find a way to combine handling of generic and dependent function types (here and elsewhere)
               adaptFun(actual, rinfo.paramInfos, rinfo.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
@@ -779,7 +779,7 @@ class CheckCaptures extends Recheck, SymTransformer:
               adaptFun(actual, actual.paramInfos, actual.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
                   actual.derivedLambdaType(paramInfos = aargs1, resType = ares1))
-            case actual @ RefinedType(p, nme, rinfo: PolyType) if defn.isFunctionOrPolyType(actual) =>
+            case actual @ RefinedType(p, nme, rinfo: PolyType) if defn.isFunctionType(actual) =>
               adaptTypeFun(actual, rinfo.resType, expected, covariant, insertBox,
                 ares1 =>
                   val rinfo1 = rinfo.derivedLambdaType(rinfo.paramNames, rinfo.paramInfos, ares1)
@@ -996,7 +996,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case CapturingType(parent, refs) =>
               healCaptureSet(refs)
               traverse(parent)
-            case tp @ RefinedType(parent, rname, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
+            case tp @ RefinedType(parent, rname, rinfo: MethodType) if defn.isFunctionType(tp) =>
               traverse(rinfo)
             case tp: TermLambda =>
               val saved = allowed

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -54,7 +54,7 @@ extends tpd.TreeTraverser:
         val boxedRes = recur(res)
         if boxedRes eq res then tp
         else tp1.derivedAppliedType(tycon, args.init :+ boxedRes)
-      case tp1 @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionOrPolyType(tp1) =>
+      case tp1 @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(tp1) =>
         val boxedRinfo = recur(rinfo)
         if boxedRinfo eq rinfo then tp
         else boxedRinfo.toFunctionType(isJava = false, alwaysDependent = true)
@@ -231,7 +231,7 @@ extends tpd.TreeTraverser:
                 tp.derivedAppliedType(tycon1, args1 :+ res1)
           else
             tp.derivedAppliedType(tycon1, args.mapConserve(arg => this(arg)))
-        case tp @ RefinedType(core, rname, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
+        case tp @ RefinedType(core, rname, rinfo: MethodType) if defn.isFunctionType(tp) =>
           val rinfo1 = apply(rinfo)
           if rinfo1 ne rinfo then rinfo1.toFunctionType(isJava = false, alwaysDependent = true)
           else tp
@@ -329,7 +329,7 @@ extends tpd.TreeTraverser:
               args.last, CaptureSet.empty, currentCs ++ outerCs)
             tp.derivedAppliedType(tycon1, args1 :+ resType1)
         tp1.capturing(outerCs)
-      case tp @ RefinedType(parent, nme.apply, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
+      case tp @ RefinedType(parent, nme.apply, rinfo: MethodType) if defn.isFunctionType(tp) =>
         propagateDepFunctionResult(mapOver(tp), currentCs ++ outerCs)
           .capturing(outerCs)
       case _ =>

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

@@ -1475,6 +1475,7 @@ class Definitions {
   def PolyFunctionType = PolyFunctionClass.typeRef
 
   lazy val ErasedFunctionClass = requiredClass("scala.runtime.ErasedFunction")
+  def ErasedFunctionType = ErasedFunctionClass.typeRef
 
   /** If `cls` is a class in the scala package, its name, otherwise EmptyTypeName */
   def scalaClassName(cls: Symbol)(using Context): TypeName = cls.denot match
@@ -1709,21 +1710,29 @@ class Definitions {
    *  - scala.FunctionN
    *  - scala.ContextFunctionN
    */
-  def isFunctionType(tp: Type)(using Context): Boolean =
+  def isFunctionNType(tp: Type)(using Context): Boolean =
     isNonRefinedFunction(tp.dropDependentRefinement)
 
-  /** Is `tp` a specialized, refined function type? Either an `ErasedFunction` or a `PolyFunction`. */
-  def isRefinedFunctionType(tp: Type)(using Context): Boolean =
-    tp.derivesFrom(defn.PolyFunctionClass) || isErasedFunctionType(tp)
+  /** Does `tp` derive from `PolyFunction` or `ErasedFunction`? */
+  def isPolyOrErasedFunctionType(tp: Type)(using Context): Boolean =
+    isPolyFunctionType(tp) || isErasedFunctionType(tp)
+
+  /** Does `tp` derive from `PolyFunction`? */
+  def isPolyFunctionType(tp: Type)(using Context): Boolean =
+    tp.derivesFrom(defn.PolyFunctionClass)
+
+  /** Does `tp` derive from `ErasedFunction`? */
+  def isErasedFunctionType(tp: Type)(using Context): Boolean =
+    tp.derivesFrom(defn.ErasedFunctionClass)
 
   /** Returns whether `tp` is an instance or a refined instance of:
    *  - scala.FunctionN
    *  - scala.ContextFunctionN
    *  - ErasedFunction
    *  - PolyFunction
    */
-  def isFunctionOrPolyType(tp: Type)(using Context): Boolean =
-    isFunctionType(tp) || isRefinedFunctionType(tp)
+  def isFunctionType(tp: Type)(using Context): Boolean =
+    isFunctionNType(tp) || isPolyOrErasedFunctionType(tp)
 
   private def withSpecMethods(cls: ClassSymbol, bases: List[Name], paramTypes: Set[TypeRef]) =
     if !ctx.settings.Yscala2Stdlib.value then
@@ -1830,7 +1839,7 @@ class Definitions {
       case tp1 @ RefinedType(parent, nme.apply, mt: MethodType) if isErasedFunctionType(parent) && mt.isContextualMethod =>
         tp1
       case tp1 =>
-        if tp1.typeSymbol.name.isContextFunction && isFunctionType(tp1) then tp1
+        if tp1.typeSymbol.name.isContextFunction && isFunctionNType(tp1) then tp1
         else NoType
 
   /** Is `tp` an context function type? */
@@ -1858,13 +1867,10 @@ class Definitions {
   /* Returns a list of erased booleans marking whether parameters are erased, for a function type. */
   def erasedFunctionParameters(tp: Type)(using Context): List[Boolean] = tp.dealias match {
     case RefinedType(parent, nme.apply, mt: MethodType) => mt.erasedParams
-    case tp if isFunctionType(tp) => List.fill(functionArity(tp)) { false }
+    case tp if isFunctionNType(tp) => List.fill(functionArity(tp)) { false }
     case _ => Nil
   }
 
-  def isErasedFunctionType(tp: Type)(using Context): Boolean =
-    tp.derivesFrom(defn.ErasedFunctionClass)
-
   /** A whitelist of Scala-2 classes that are known to be pure */
   def isAssuredNoInits(sym: Symbol): Boolean =
     (sym `eq` SomeClass) || isTupleClass(sym)

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

@@ -509,8 +509,7 @@ class TypeApplications(val self: Type) extends AnyVal {
    *  Handles `ErasedFunction`s and poly functions gracefully.
    */
   final def functionArgInfos(using Context): List[Type] = self.dealias match
-    case RefinedType(parent, nme.apply, mt: MethodType) if defn.isErasedFunctionType(parent) => (mt.paramInfos :+ mt.resultType)
-    case RefinedType(parent, nme.apply, mt: MethodType) if parent.typeSymbol eq defn.PolyFunctionClass => (mt.paramInfos :+ mt.resultType)
+    case RefinedType(parent, nme.apply, mt: MethodType) if defn.isPolyOrErasedFunctionType(parent) => (mt.paramInfos :+ mt.resultType)
     case _ => self.dropDependentRefinement.dealias.argInfos
 
   /** Argument types where existential types in arguments are disallowed */

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

@@ -666,15 +666,17 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 isSubType(info1, info2)
 
             if defn.isFunctionType(tp2) then
-              tp1w.widenDealias match
-                case tp1: RefinedType =>
-                  return isSubInfo(tp1.refinedInfo, tp2.refinedInfo)
-                case _ =>
-            else if tp2.parent.typeSymbol == defn.PolyFunctionClass then
-              tp1.member(nme.apply).info match
-                case info1: PolyType =>
-                  return isSubInfo(info1, tp2.refinedInfo)
-                case _ =>
+              if defn.isPolyFunctionType(tp2) then
+                // TODO should we handle ErasedFunction is this same way?
+                tp1.member(nme.apply).info match
+                  case info1: PolyType =>
+                    return isSubInfo(info1, tp2.refinedInfo)
+                  case _ =>
+              else
+                tp1w.widenDealias match
+                  case tp1: RefinedType =>
+                    return isSubInfo(tp1.refinedInfo, tp2.refinedInfo)
+                  case _ =>
 
           val skipped2 = skipMatching(tp1w, tp2)
           if (skipped2 eq tp2) || !Config.fastPathForRefinedSubtype then

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

@@ -560,21 +560,16 @@ object TypeErasure {
     case _ => false
   }
 
-  /** The erasure of `PolyFunction { def apply: $applyInfo }` */
-  def erasePolyFunctionApply(applyInfo: Type)(using Context): Type =
-    assert(applyInfo.isInstanceOf[PolyType])
-    val res = applyInfo.resultType
-    val paramss = res.paramNamess
-    assert(paramss.length == 1)
-    erasure(defn.FunctionType(paramss.head.length,
-      isContextual = res.isImplicitMethod))
-
-  def eraseErasedFunctionApply(erasedFn: MethodType)(using Context): Type =
-    val fnType = defn.FunctionType(
-      n = erasedFn.erasedParams.count(_ == false),
-      isContextual = erasedFn.isContextualMethod,
-    )
-    erasure(fnType)
+  /** The erasure of `(PolyFunction | ErasedFunction) { def apply: $applyInfo }` */
+  def eraseRefinedFunctionApply(applyInfo: Type)(using Context): Type =
+    def functionType(info: Type): Type = info match {
+      case info: PolyType =>
+        functionType(info.resultType)
+      case info: MethodType =>
+        assert(!info.resultType.isInstanceOf[MethodicType])
+        defn.FunctionType(n = info.erasedParams.count(_ == false))
+    }
+    erasure(functionType(applyInfo))
 }
 
 import TypeErasure._
@@ -592,7 +587,7 @@ import TypeErasure._
  */
 class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConstructor: Boolean, isSymbol: Boolean, inSigName: Boolean) {
 
-  /**  The erasure |T| of a type T. 
+  /**  The erasure |T| of a type T.
    *
    *   If computing the erasure of T requires erasing a WildcardType or an
    *   uninstantiated type variable, then an exception signaling an internal
@@ -659,10 +654,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
         else SuperType(eThis, eSuper)
       case ExprType(rt) =>
         defn.FunctionType(0)
-      case RefinedType(parent, nme.apply, refinedInfo) if parent.typeSymbol eq defn.PolyFunctionClass =>
-        erasePolyFunctionApply(refinedInfo)
-      case RefinedType(parent, nme.apply, refinedInfo: MethodType) if defn.isErasedFunctionType(parent) =>
-        eraseErasedFunctionApply(refinedInfo)
+      case RefinedType(parent, nme.apply, refinedInfo) if defn.isPolyOrErasedFunctionType(parent) =>
+        eraseRefinedFunctionApply(refinedInfo)
       case tp: TypeVar if !tp.isInstantiated =>
         assert(inSigName, i"Cannot erase uninstantiated type variable $tp")
         WildcardType
@@ -943,13 +936,11 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
         sigName(defn.FunctionOf(Nil, rt))
       case tp: TypeVar if !tp.isInstantiated =>
         tpnme.Uninstantiated
-      case tp @ RefinedType(parent, nme.apply, _) if parent.typeSymbol eq defn.PolyFunctionClass =>
+      case tp @ RefinedType(parent, nme.apply, _) if defn.isPolyOrErasedFunctionType(parent) =>
         // we need this case rather than falling through to the default
         // because RefinedTypes <: TypeProxy and it would be caught by
         // the case immediately below
         sigName(this(tp))
-      case tp @ RefinedType(parent, nme.apply, refinedInfo) if defn.isErasedFunctionType(parent) =>
-        sigName(this(tp))
       case tp: TypeProxy =>
         sigName(tp.underlying)
       case tp: WildcardType =>

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

@@ -708,7 +708,7 @@ object Types {
       }
       findMember(name, pre, required, excluded)
     }
-    
+
     /** The implicit members with given name. If there are none and the denotation
      *  contains private members, also look for shadowed non-private implicits.
      */
@@ -1875,20 +1875,25 @@ object Types {
      *  @param alwaysDependent if true, always create a dependent function type.
      */
     def toFunctionType(isJava: Boolean, dropLast: Int = 0, alwaysDependent: Boolean = false)(using Context): Type = this match {
-      case mt: MethodType if !mt.isParamDependent =>
-        val formals1 = if (dropLast == 0) mt.paramInfos else mt.paramInfos dropRight dropLast
-        val isContextual = mt.isContextualMethod && !ctx.erasedTypes
-        val result1 = mt.nonDependentResultApprox match {
-          case res: MethodType => res.toFunctionType(isJava)
-          case res => res
-        }
-        val funType = defn.FunctionOf(
-          formals1 mapConserve (_.translateFromRepeated(toArray = isJava)),
-          result1, isContextual)
-        if alwaysDependent || mt.isResultDependent then
-          RefinedType(funType, nme.apply, mt)
-        else funType
-      case poly @ PolyType(_, mt: MethodType) if !mt.isParamDependent =>
+      case mt: MethodType =>
+        assert(!mt.isParamDependent)
+        def nonDependentFunType =
+          val formals1 = if (dropLast == 0) mt.paramInfos else mt.paramInfos dropRight dropLast
+          val isContextual = mt.isContextualMethod && !ctx.erasedTypes
+          val result1 = mt.nonDependentResultApprox match {
+            case res: MethodType => res.toFunctionType(isJava)
+            case res => res
+          }
+          defn.FunctionOf(
+            formals1 mapConserve (_.translateFromRepeated(toArray = isJava)),
+            result1, isContextual)
+        if mt.hasErasedParams then
+          RefinedType(defn.ErasedFunctionType, nme.apply, mt)
+        else if alwaysDependent || mt.isResultDependent then
+          RefinedType(nonDependentFunType, nme.apply, mt)
+        else nonDependentFunType
+      case poly @ PolyType(_, mt: MethodType) =>
+        assert(!mt.isParamDependent)
         RefinedType(defn.PolyFunctionType, nme.apply, poly)
     }
 
@@ -4071,9 +4076,9 @@ object Types {
       def addInto(tp: Type): Type = tp match
         case tp @ AppliedType(tycon, args) if tycon.typeSymbol == defn.RepeatedParamClass =>
           tp.derivedAppliedType(tycon, addInto(args.head) :: Nil)
-        case tp @ AppliedType(tycon, args) if defn.isFunctionType(tp) =>
+        case tp @ AppliedType(tycon, args) if defn.isFunctionNType(tp) =>
           wrapConvertible(tp.derivedAppliedType(tycon, args.init :+ addInto(args.last)))
-        case tp @ RefinedType(parent, rname, rinfo) if defn.isFunctionOrPolyType(tp) =>
+        case tp @ RefinedType(parent, rname, rinfo) if defn.isFunctionType(tp) =>
           wrapConvertible(tp.derivedRefinedType(parent, rname, addInto(rinfo)))
         case tp: MethodOrPoly =>
           tp.derivedLambdaType(resType = addInto(tp.resType))

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -266,7 +266,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       if !printDebug && appliedText(tp.asInstanceOf[HKLambda].resType).isEmpty =>
         // don't eta contract if the application would be printed specially
         toText(tycon)
-      case tp: RefinedType if defn.isFunctionOrPolyType(tp) && !printDebug =>
+      case tp: RefinedType if defn.isFunctionType(tp) && !printDebug =>
         toTextMethodAsFunction(tp.refinedInfo,
           isPure = Feature.pureFunsEnabled && !tp.typeSymbol.name.isImpureFunction)
       case tp: TypeRef =>
@@ -771,7 +771,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
   override protected def toTextCapturing(tp: Type, refsText: Text, boxText: Text): Text = tp match
     case tp: AppliedType if defn.isFunctionSymbol(tp.typeSymbol) && !printDebug =>
       boxText ~ toTextFunction(tp, refsText)
-    case tp: RefinedType if defn.isFunctionOrPolyType(tp) && !printDebug =>
+    case tp: RefinedType if defn.isFunctionType(tp) && !printDebug =>
       boxText ~ toTextMethodAsFunction(tp.refinedInfo, isPure = !tp.typeSymbol.name.isImpureFunction, refsText)
     case _ =>
       super.toTextCapturing(tp, refsText, boxText)

compiler/src/dotty/tools/dotc/transform/BetaReduce.scala

@@ -90,7 +90,7 @@ object BetaReduce:
         recur(expr, argss)
       case _ => None
     tree match
-      case Apply(Select(fn, nme.apply), args) if defn.isFunctionType(fn.tpe) =>
+      case Apply(Select(fn, nme.apply), args) if defn.isFunctionNType(fn.tpe) =>
         recur(fn, List(args)) match
           case Some(reduced) =>
             seq(bindingsBuf.result(), reduced).withSpan(tree.span)