Pure function types -> and ?->

 1. Allow `->` and `?->` and function operators, treated like `=>` and `?=>`.
 2. under -Ycc treat `->` and `?->` as immutable function types, whereas `A => B`
    is an alias of `{*} A -> B` and `A ?=> B` is an alias of `{*} A ?-> B`.

Closures are unaffected, we still use `=>` for all closures where they are pure or not.
Improve printing of capturing types

Avoid explicit retains annotations also outside phase cc
Generate "Impure" function aliases

For every (possibly erased and/or context) function class
XFunctionN, generate an alias ImpureXFunctionN in the Scala package defined as

    type ImpureXFunctionN[...] = {*} XFunctionN[...]

Also:

 - Fix a bug in TypeComparer: glb has to test subCapture in a frozen state
 - Harden EventuallyCapturingType extractor to not crash on illegal capture sets
 - Cleanup transformation of inferred types

Author: odersky

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

@@ -69,13 +69,13 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   case class InterpolatedString(id: TermName, segments: List[Tree])(implicit @constructorOnly src: SourceFile)
     extends TermTree
 
-  /** A function type */
+  /** A function type or closure */
   case class Function(args: List[Tree], body: Tree)(implicit @constructorOnly src: SourceFile) extends Tree {
     override def isTerm: Boolean = body.isTerm
     override def isType: Boolean = body.isType
   }
 
-  /** A function type with `implicit`, `erased`, or `given` modifiers */
+  /** A function type or closure with `implicit`, `erased`, or `given` modifiers */
   class FunctionWithMods(args: List[Tree], body: Tree, val mods: Modifiers)(implicit @constructorOnly src: SourceFile)
     extends Function(args, body)
 
@@ -216,6 +216,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     case class Transparent()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Transparent)
 
     case class Infix()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Infix)
+
+    case class Impure()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Impure)
   }
 
   /** Modifiers and annotations for definitions

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

@@ -17,9 +17,13 @@ def retainedElems(tree: Tree)(using Context): List[Tree] = tree match
   case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
   case _ => Nil
 
+class IllegalCaptureRef(tpe: Type) extends Exception
+
 extension (tree: Tree)
 
-  def toCaptureRef(using Context): CaptureRef = tree.tpe.asInstanceOf[CaptureRef]
+  def toCaptureRef(using Context): CaptureRef = tree.tpe match
+    case ref: CaptureRef => ref
+    case tpe => throw IllegalCaptureRef(tpe)
 
   def toCaptureSet(using Context): CaptureSet =
     tree.getAttachment(Captures) match
@@ -59,20 +63,6 @@ extension (tp: Type)
 
   def isBoxedCapturing(using Context) = !tp.boxedCaptured.isAlwaysEmpty
 
-  def canHaveInferredCapture(using Context): Boolean = tp match
-    case tp: TypeRef if tp.symbol.isClass =>
-      !tp.symbol.isValueClass && tp.symbol != defn.AnyClass
-    case _: TypeVar | _: TypeParamRef =>
-      false
-    case tp: TypeProxy =>
-      tp.superType.canHaveInferredCapture
-    case tp: AndType =>
-      tp.tp1.canHaveInferredCapture && tp.tp2.canHaveInferredCapture
-    case tp: OrType =>
-      tp.tp1.canHaveInferredCapture || tp.tp2.canHaveInferredCapture
-    case _ =>
-      false
-
   def stripCapturing(using Context): Type = tp.dealiasKeepAnnots match
     case CapturingType(parent, _, _) =>
       parent.stripCapturing

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

@@ -56,6 +56,12 @@ sealed abstract class CaptureSet extends Showable:
       assert(v.isConst)
       Const(v.elems)
 
+  final def isUniversal(using Context) =
+    elems.exists {
+      case ref: TermRef => ref.symbol == defn.captureRoot
+      case _ => false
+    }
+
   /** Cast to variable. @pre: !isConst */
   def asVar: Var =
     assert(!isConst)

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

@@ -12,10 +12,22 @@ object CapturingType:
     else AnnotatedType(parent, CaptureAnnotation(refs, boxed))
 
   def unapply(tp: AnnotatedType)(using Context): Option[(Type, CaptureSet, Boolean)] =
-    if ctx.phase == Phases.checkCapturesPhase && tp.annot.symbol == defn.RetainsAnnot then
+    if ctx.phase == Phases.checkCapturesPhase then EventuallyCapturingType.unapply(tp)
+    else None
+
+end CapturingType
+
+object EventuallyCapturingType:
+
+  def unapply(tp: AnnotatedType)(using Context): Option[(Type, CaptureSet, Boolean)] =
+    if tp.annot.symbol == defn.RetainsAnnot then
       tp.annot match
         case ann: CaptureAnnotation => Some((tp.parent, ann.refs, ann.boxed))
-        case ann => Some((tp.parent, ann.tree.toCaptureSet, ann.tree.isBoxedCapturing))
+        case ann =>
+          try Some((tp.parent, ann.tree.toCaptureSet, ann.tree.isBoxedCapturing))
+          catch case ex: IllegalCaptureRef => None
     else None
 
-end CapturingType
+end EventuallyCapturingType
+
+

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

@@ -8,6 +8,7 @@ import Flags._, Scopes._, Decorators._, NameOps._, Periods._, NullOpsDecorator._
 import unpickleScala2.Scala2Unpickler.ensureConstructor
 import scala.collection.mutable
 import collection.mutable
+import Denotations.{SingleDenotation, staticRef}
 import util.{SimpleIdentityMap, SourceFile, NoSource}
 import typer.ImportInfo.RootRef
 import Comments.CommentsContext
@@ -88,7 +89,7 @@ class Definitions {
    *
    *  FunctionN traits follow this template:
    *
-   *      trait FunctionN[T0,...T{N-1}, R] extends Object {
+   *      trait FunctionN[-T0,...-T{N-1}, +R] extends Object {
    *        def apply($x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
    *
@@ -98,46 +99,65 @@ class Definitions {
    *
    *  ContextFunctionN traits follow this template:
    *
-   *      trait ContextFunctionN[T0,...,T{N-1}, R] extends Object {
+   *      trait ContextFunctionN[-T0,...,-T{N-1}, +R] extends Object {
    *        def apply(using $x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
    *
    *  ErasedFunctionN traits follow this template:
    *
-   *      trait ErasedFunctionN[T0,...,T{N-1}, R] extends Object {
+   *      trait ErasedFunctionN[-T0,...,-T{N-1}, +R] extends Object {
    *        def apply(erased $x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
    *
    *  ErasedContextFunctionN traits follow this template:
    *
-   *      trait ErasedContextFunctionN[T0,...,T{N-1}, R] extends Object {
+   *      trait ErasedContextFunctionN[-T0,...,-T{N-1}, +R] extends Object {
    *        def apply(using erased $x0: T0, ..., $x{N_1}: T{N-1}): R
    *      }
    *
    *  ErasedFunctionN and ErasedContextFunctionN erase to Function0.
+   *
+   *  EffXYZFunctionN afollow this template:
+   *
+   *      type EffXYZFunctionN[-T0,...,-T{N-1}, +R] = {*} XYZFunctionN[T0,...,T{N-1}, R]
    */
-  def newFunctionNTrait(name: TypeName): ClassSymbol = {
+  private def newFunctionNType(name: TypeName): Symbol = {
+    val impure = name.startsWith("Impure")
     val completer = new LazyType {
       def complete(denot: SymDenotation)(using Context): Unit = {
-        val cls = denot.asClass.classSymbol
-        val decls = newScope
         val arity = name.functionArity
-        val paramNamePrefix = tpnme.scala ++ str.NAME_JOIN ++ name ++ str.EXPAND_SEPARATOR
-        val argParamRefs = List.tabulate(arity) { i =>
-          enterTypeParam(cls, paramNamePrefix ++ "T" ++ (i + 1).toString, Contravariant, decls).typeRef
-        }
-        val resParamRef = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls).typeRef
-        val methodType = MethodType.companion(
-          isContextual = name.isContextFunction,
-          isImplicit = false,
-          isErased = name.isErasedFunction)
-        decls.enter(newMethod(cls, nme.apply, methodType(argParamRefs, resParamRef), Deferred))
-        denot.info =
-          ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: Nil, decls)
+        if impure then
+          val argParamNames = List.tabulate(arity)(tpnme.syntheticTypeParamName)
+          val argVariances = List.fill(arity)(Contravariant)
+          val underlyingName = name.asSimpleName.drop(6)
+          val underlyingClass = ScalaPackageVal.requiredClass(underlyingName)
+          denot.info = TypeAlias(
+            HKTypeLambda(argParamNames :+ "R".toTypeName, argVariances :+ Covariant)(
+              tl => List.fill(arity + 1)(TypeBounds.empty),
+              tl => CapturingType(underlyingClass.typeRef.appliedTo(tl.paramRefs),
+                CaptureSet.universal, boxed = false)
+            ))
+        else
+          val cls = denot.asClass.classSymbol
+          val decls = newScope
+          val paramNamePrefix = tpnme.scala ++ str.NAME_JOIN ++ name ++ str.EXPAND_SEPARATOR
+          val argParamRefs = List.tabulate(arity) { i =>
+            enterTypeParam(cls, paramNamePrefix ++ "T" ++ (i + 1).toString, Contravariant, decls).typeRef
+          }
+          val resParamRef = enterTypeParam(cls, paramNamePrefix ++ "R", Covariant, decls).typeRef
+          val methodType = MethodType.companion(
+            isContextual = name.isContextFunction,
+            isImplicit = false,
+            isErased = name.isErasedFunction)
+          decls.enter(newMethod(cls, nme.apply, methodType(argParamRefs, resParamRef), Deferred))
+          denot.info =
+            ClassInfo(ScalaPackageClass.thisType, cls, ObjectType :: Nil, decls)
       }
     }
-    val flags = Trait | NoInits
-    newPermanentClassSymbol(ScalaPackageClass, name, flags, completer)
+    if impure then
+      newPermanentSymbol(ScalaPackageClass, name, EmptyFlags, completer)
+    else
+      newPermanentClassSymbol(ScalaPackageClass, name, Trait | NoInits, completer)
   }
 
   private def newMethod(cls: ClassSymbol, name: TermName, info: Type, flags: FlagSet = EmptyFlags): TermSymbol =
@@ -211,7 +231,7 @@ class Definitions {
     val cls = ScalaPackageVal.moduleClass.asClass
     cls.info.decls.openForMutations.useSynthesizer(
       name =>
-        if (name.isTypeName && name.isSyntheticFunction) newFunctionNTrait(name.asTypeName)
+        if (name.isTypeName && name.isSyntheticFunction) newFunctionNType(name.asTypeName)
         else NoSymbol)
     cls
   }
@@ -1285,39 +1305,55 @@ class Definitions {
 
   @tu lazy val TupleType: Array[TypeRef] = mkArityArray("scala.Tuple", MaxTupleArity, 1)
 
+  /** Cached function types of arbitary arities.
+   *  Function types are created on demand with newFunctionNTrait, which is
+   *  called from a synthesizer installed in ScalaPackageClass.
+   */
   private class FunType(prefix: String):
     private var classRefs: Array[TypeRef] = new Array(22)
+
     def apply(n: Int): TypeRef =
       while n >= classRefs.length do
         val classRefs1 = new Array[TypeRef](classRefs.length * 2)
         Array.copy(classRefs, 0, classRefs1, 0, classRefs.length)
         classRefs = classRefs1
+      val funName = s"scala.$prefix$n"
       if classRefs(n) == null then
-        classRefs(n) = requiredClassRef(prefix + n.toString)
+        classRefs(n) =
+          if prefix.startsWith("Impure")
+          then staticRef(funName.toTypeName).symbol.typeRef
+          else requiredClassRef(funName)
       classRefs(n)
-
-  private val erasedContextFunType = FunType("scala.ErasedContextFunction")
-  private val contextFunType = FunType("scala.ContextFunction")
-  private val erasedFunType = FunType("scala.ErasedFunction")
-  private val funType = FunType("scala.Function")
-
-  def FunctionClass(n: Int, isContextual: Boolean = false, isErased: Boolean = false)(using Context): Symbol =
-    ( if isContextual && isErased then erasedContextFunType(n)
-      else if isContextual then contextFunType(n)
-      else if isErased then erasedFunType(n)
-      else funType(n)
-    ).symbol.asClass
+  end FunType
+
+  private def funTypeIdx(isContextual: Boolean, isErased: Boolean, isImpure: Boolean): Int =
+      (if isContextual then 1 else 0)
+    + (if isErased     then 2 else 0)
+    + (if isImpure     then 4 else 0)
+
+  private val funTypeArray: IArray[FunType] =
+    val arr = Array.ofDim[FunType](8)
+    val choices = List(false, true)
+    for contxt <- choices; erasd <- choices; impure <- choices do
+      var str = "Function"
+      if contxt then str = "Context" + str
+      if erasd then str = "Erased" + str
+      if impure then str = "Impure" + str
+      arr(funTypeIdx(contxt, erasd, impure)) = FunType(str)
+    IArray.unsafeFromArray(arr)
+
+  def FunctionSymbol(n: Int, isContextual: Boolean = false, isErased: Boolean = false, isImpure: Boolean = false)(using Context): Symbol =
+    funTypeArray(funTypeIdx(isContextual, isErased, isImpure))(n).symbol
 
   @tu lazy val Function0_apply: Symbol = Function0.requiredMethod(nme.apply)
-  @tu lazy val ContextFunction0_apply: Symbol = ContextFunction0.requiredMethod(nme.apply)
 
-  @tu lazy val Function0: Symbol = FunctionClass(0)
-  @tu lazy val Function1: Symbol = FunctionClass(1)
-  @tu lazy val Function2: Symbol = FunctionClass(2)
-  @tu lazy val ContextFunction0: Symbol = FunctionClass(0, isContextual = true)
+  @tu lazy val Function0: Symbol = FunctionSymbol(0)
+  @tu lazy val Function1: Symbol = FunctionSymbol(1)
+  @tu lazy val Function2: Symbol = FunctionSymbol(2)
+  @tu lazy val ContextFunction0: Symbol = FunctionSymbol(0, isContextual = true)
 
-  def FunctionType(n: Int, isContextual: Boolean = false, isErased: Boolean = false)(using Context): TypeRef =
-    FunctionClass(n, isContextual && !ctx.erasedTypes, isErased).typeRef
+  def FunctionType(n: Int, isContextual: Boolean = false, isErased: Boolean = false, isImpure: Boolean = false)(using Context): TypeRef =
+    FunctionSymbol(n, isContextual && !ctx.erasedTypes, isErased, isImpure).typeRef
 
   lazy val PolyFunctionClass = requiredClass("scala.PolyFunction")
   def PolyFunctionType = PolyFunctionClass.typeRef
@@ -1359,6 +1395,10 @@ class Definitions {
    */
   def isFunctionClass(cls: Symbol): Boolean = scalaClassName(cls).isFunction
 
+  /** Is a function class, or an impure function type alias */
+  def isFunctionSymbol(sym: Symbol): Boolean =
+    sym.isType && (sym.owner eq ScalaPackageClass) && sym.name.isFunction
+
   /** Is a function class where
    *    - FunctionN for N >= 0 and N != XXL
    */
@@ -1565,7 +1605,7 @@ class Definitions {
 
   def isSpecializableFunction(cls: ClassSymbol, paramTypes: List[Type], retType: Type)(using Context): Boolean =
     paramTypes.length <= 2
-    && (cls.derivesFrom(FunctionClass(paramTypes.length)) || isByNameFunctionClass(cls))
+    && (cls.derivesFrom(FunctionSymbol(paramTypes.length)) || isByNameFunctionClass(cls))
     && isSpecializableFunctionSAM(paramTypes, retType)
 
   /** If the Single Abstract Method of a Function class has this type, is it specializable? */

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

@@ -314,8 +314,8 @@ object Flags {
   /** A Scala 2x super accessor / an unpickled Scala 2.x class */
   val (SuperParamAliasOrScala2x @ _, SuperParamAlias @ _, Scala2x @ _) = newFlags(26, "<super-param-alias>", "<scala-2.x>")
 
-  /** A parameter with a default value */
-  val (_, HasDefault @ _, _) = newFlags(27, "<hasdefault>")
+  /** A parameter with a default value / an impure untpd.Function type */
+  val (_, HasDefault @ _, Impure @ _) = newFlags(27, "<hasdefault>", "<{*}>")
 
   /** An extension method, or a collective extension instance */
   val (Extension @ _, ExtensionMethod @ _, _) = newFlags(28, "<extension>")

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

@@ -196,20 +196,25 @@ object NameOps {
           else collectDigits(acc * 10 + d, idx + 1)
       collectDigits(0, suffixStart + 8)
 
-    /** name[0..suffixStart) == `str` */
-    private def isPreceded(str: String, suffixStart: Int) =
-      str.length == suffixStart && name.firstPart.startsWith(str)
+    private def isFunctionPrefix(suffixStart: Int, mustHave: String = ""): Boolean =
+      suffixStart >= 0
+      && {
+        val first = name.firstPart
+        var found = mustHave.isEmpty
+        def skip(idx: Int, str: String) =
+          if first.startsWith(str, idx) then
+            if str == mustHave then found = true
+            idx + str.length
+          else idx
+        skip(skip(skip(0, "Impure"), "Erased"), "Context") == suffixStart
+        && found
+      }
 
     /** Same as `funArity`, except that it returns -1 if the prefix
      *  is not one of "", "Context", "Erased", "ErasedContext"
      */
     private def checkedFunArity(suffixStart: Int): Int =
-      if suffixStart == 0
-         || isPreceded("Context", suffixStart)
-         || isPreceded("Erased", suffixStart)
-         || isPreceded("ErasedContext", suffixStart)
-      then funArity(suffixStart)
-      else -1
+      if isFunctionPrefix(suffixStart) then funArity(suffixStart) else -1
 
     /** Is a function name, i.e one of FunctionXXL, FunctionN, ContextFunctionN, ErasedFunctionN, ErasedContextFunctionN for N >= 0
      */
@@ -221,19 +226,14 @@ object NameOps {
      */
     def isPlainFunction: Boolean = functionArity >= 0
 
-    /** Is an context function name, i.e one of ContextFunctionN or ErasedContextFunctionN for N >= 0
-     */
-    def isContextFunction: Boolean =
+    /** Is a function name that contains `mustHave` as a substring */
+    private def isSpecificFunction(mustHave: String): Boolean =
       val suffixStart = functionSuffixStart
-      (isPreceded("Context", suffixStart) || isPreceded("ErasedContext", suffixStart))
-      && funArity(suffixStart) >= 0
+      isFunctionPrefix(suffixStart, mustHave) && funArity(suffixStart) >= 0
 
-    /** Is an erased function name, i.e. one of ErasedFunctionN, ErasedContextFunctionN for N >= 0
-      */
-    def isErasedFunction: Boolean =
-      val suffixStart = functionSuffixStart
-      (isPreceded("Erased", suffixStart) || isPreceded("ErasedContext", suffixStart))
-      && funArity(suffixStart) >= 0
+    def isContextFunction: Boolean = isSpecificFunction("Context")
+    def isErasedFunction: Boolean = isSpecificFunction("Erased")
+    def isImpureFunction: Boolean = isSpecificFunction("Impure")
 
     /** Is a synthetic function name, i.e. one of
      *    - FunctionN for N > 22

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

@@ -738,6 +738,8 @@ object StdNames {
     val XOR      : N = "^"
     val ZAND     : N = "&&"
     val ZOR      : N = "||"
+    val PUREARROW: N = "->"
+    val PURECTXARROW: N = "?->"
 
     // unary operators
     val UNARY_PREFIX: N = "unary_"

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

@@ -2410,7 +2410,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     case tp1: TypeVar if tp1.isInstantiated =>
       tp1.underlying & tp2
     case CapturingType(parent1, refs1, _) =>
-      if subCaptures(tp2.captureSet, refs1, frozenConstraint).isOK then
+      if subCaptures(tp2.captureSet, refs1, frozen = true).isOK then
         parent1 & tp2
       else
         tp1.derivedCapturingType(parent1 & tp2, refs1)