Fix Singleton

Allow to constrain type variables to be singletons by a context bound
[X: Singleton] instead of an unsound supertype [X <: Singleton]. This
fixes the soundness hole of singletons.

Author: odersky

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

@@ -647,9 +647,9 @@ trait ConstraintHandling {
    * At this point we also drop the @Repeated annotation to avoid inferring type arguments with it,
    * as those could leak the annotation to users (see run/inferred-repeated-result).
    */
-  def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
+  def widenInferred(inst: Type, bound: Type, widen: Widen)(using Context): Type =
     def widenOr(tp: Type) =
-      if widenUnions then
+      if widen == Widen.Unions then
         val tpw = tp.widenUnion
         if tpw ne tp then
           if tpw.isTransparent() then
@@ -667,14 +667,10 @@ trait ConstraintHandling {
       val tpw = tp.widenSingletons(skipSoftUnions)
       if (tpw ne tp) && (tpw <:< bound) then tpw else tp
 
-    def isSingleton(tp: Type): Boolean = tp match
-      case WildcardType(optBounds) => optBounds.exists && isSingleton(optBounds.bounds.hi)
-      case _ => isSubTypeWhenFrozen(tp, defn.SingletonType)
-
     val wideInst =
-      if isSingleton(bound) then inst
+      if widen == Widen.None || bound.isSingletonBounded(frozen = true) then inst
       else
-        val widenedFromSingle = widenSingle(inst, skipSoftUnions = widenUnions)
+        val widenedFromSingle = widenSingle(inst, skipSoftUnions = widen == Widen.Unions)
         val widenedFromUnion = widenOr(widenedFromSingle)
         val widened = dropTransparentTraits(widenedFromUnion, bound)
         widenIrreducible(widened)
@@ -713,18 +709,18 @@ trait ConstraintHandling {
    *  The instance type is not allowed to contain references to types nested deeper
    *  than `maxLevel`.
    */
-  def instanceType(param: TypeParamRef, fromBelow: Boolean, widenUnions: Boolean, maxLevel: Int)(using Context): Type = {
+  def instanceType(param: TypeParamRef, fromBelow: Boolean, widen: Widen, maxLevel: Int)(using Context): Type = {
     val approx = approximation(param, fromBelow, maxLevel).simplified
     if fromBelow then
-      val widened = widenInferred(approx, param, widenUnions)
+      val widened = widenInferred(approx, param, widen)
       // Widening can add extra constraints, in particular the widened type might
       // be a type variable which is now instantiated to `param`, and therefore
       // cannot be used as an instantiation of `param` without creating a loop.
       // If that happens, we run `instanceType` again to find a new instantiation.
       // (we do not check for non-toplevel occurrences: those should never occur
       // since `addOneBound` disallows recursive lower bounds).
       if constraint.occursAtToplevel(param, widened) then
-        instanceType(param, fromBelow, widenUnions, maxLevel)
+        instanceType(param, fromBelow, widen, maxLevel)
       else
         widened
     else

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

@@ -59,10 +59,10 @@ class Definitions {
   private def enterCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef], decls: Scope) =
     newCompleteClassSymbol(owner, name, flags | Permanent | NoInits | Open, parents, decls).entered
 
-  private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
+  private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope): TypeSymbol =
     scope.enter(newPermanentSymbol(cls, name, flags, TypeBounds.empty))
 
-  private def enterTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
+  private def enterTypeParam(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope): TypeSymbol =
     enterTypeField(cls, name, flags | ClassTypeParamCreationFlags, scope)
 
   private def enterSyntheticTypeParam(cls: ClassSymbol, paramFlags: FlagSet, scope: MutableScope, suffix: String = "T0") =
@@ -538,9 +538,11 @@ class Definitions {
   @tu lazy val SingletonClass: ClassSymbol =
     // needed as a synthetic class because Scala 2.x refers to it in classfiles
     // but does not define it as an explicit class.
-    enterCompleteClassSymbol(
-      ScalaPackageClass, tpnme.Singleton, PureInterfaceCreationFlags | Final,
-      List(AnyType), EmptyScope)
+    val cls = enterCompleteClassSymbol(
+      ScalaPackageClass, tpnme.Singleton, PureInterfaceCreationFlags | Final | Erased,
+      List(AnyType))
+    enterTypeField(cls, tpnme.Self, Deferred, cls.info.decls.openForMutations)
+    cls
   @tu lazy val SingletonType: TypeRef = SingletonClass.typeRef
 
   @tu lazy val MaybeCapabilityAnnot: ClassSymbol =

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

@@ -3257,8 +3257,8 @@ object TypeComparer {
   def subtypeCheckInProgress(using Context): Boolean =
     comparing(_.subtypeCheckInProgress)
 
-  def instanceType(param: TypeParamRef, fromBelow: Boolean, widenUnions: Boolean, maxLevel: Int = Int.MaxValue)(using Context): Type =
-    comparing(_.instanceType(param, fromBelow, widenUnions, maxLevel))
+  def instanceType(param: TypeParamRef, fromBelow: Boolean, widen: Widen, maxLevel: Int = Int.MaxValue)(using Context): Type =
+    comparing(_.instanceType(param, fromBelow, widen: Widen, maxLevel))
 
   def approximation(param: TypeParamRef, fromBelow: Boolean, maxLevel: Int = Int.MaxValue)(using Context): Type =
     comparing(_.approximation(param, fromBelow, maxLevel))
@@ -3278,8 +3278,8 @@ object TypeComparer {
   def addToConstraint(tl: TypeLambda, tvars: List[TypeVar])(using Context): Boolean =
     comparing(_.addToConstraint(tl, tvars))
 
-  def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
-    comparing(_.widenInferred(inst, bound, widenUnions))
+  def widenInferred(inst: Type, bound: Type, widen: Widen)(using Context): Type =
+    comparing(_.widenInferred(inst, bound, widen: Widen))
 
   def dropTransparentTraits(tp: Type, bound: Type)(using Context): Type =
     comparing(_.dropTransparentTraits(tp, bound))

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

@@ -545,7 +545,7 @@ object TypeOps:
           val lo = TypeComparer.instanceType(
             tp.origin,
             fromBelow = variance > 0 || variance == 0 && tp.hasLowerBound,
-            widenUnions = tp.widenUnions)(using mapCtx)
+            tp.widenPolicy)(using mapCtx)
           val lo1 = apply(lo)
           if (lo1 ne lo) lo1 else tp
         case _ =>

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

@@ -44,8 +44,6 @@ import CaptureSet.{CompareResult, IdempotentCaptRefMap, IdentityCaptRefMap}
 import scala.annotation.internal.sharable
 import scala.annotation.threadUnsafe
 
-
-
 object Types extends TypeUtils {
 
   @sharable private var nextId = 0
@@ -330,6 +328,21 @@ object Types extends TypeUtils {
     /** Is this type a (possibly aliased) singleton type? */
     def isSingleton(using Context): Boolean = dealias.isInstanceOf[SingletonType]
 
+    /** Is this upper-bounded by a (possibly aliased) singleton type?
+     *  Overridden in TypeVar
+     */
+    def isSingletonBounded(frozen: Boolean)(using Context): Boolean = this.dealias.normalized match
+      case tp: SingletonType => tp.isStable
+      case tp: TypeRef =>
+        tp.name == tpnme.Singleton && tp.symbol == defn.SingletonClass
+        || tp.superType.isSingletonBounded(frozen)
+      case tp: TypeVar if !tp.isInstantiated =>
+        if frozen then tp frozen_<:< defn.SingletonType else tp <:< defn.SingletonType
+      case tp: HKTypeLambda => false
+      case tp: TypeProxy => tp.superType.isSingletonBounded(frozen)
+      case AndType(tpL, tpR) => tpL.isSingletonBounded(frozen) || tpR.isSingletonBounded(frozen)
+      case _ => false
+
     /** Is this type of kind `AnyKind`? */
     def hasAnyKind(using Context): Boolean = {
       @tailrec def loop(tp: Type): Boolean = tp match {
@@ -4924,7 +4937,11 @@ object Types extends TypeUtils {
    *  @param  creatorState     the typer state in which the variable was created.
    *  @param  initNestingLevel the initial nesting level of the type variable. (c.f. nestingLevel)
    */
-  final class TypeVar private(initOrigin: TypeParamRef, creatorState: TyperState | Null, val initNestingLevel: Int) extends CachedProxyType with ValueType {
+  final class TypeVar private(
+      initOrigin: TypeParamRef,
+      creatorState: TyperState | Null,
+      val initNestingLevel: Int,
+      precise: Boolean) extends CachedProxyType with ValueType {
     private var currentOrigin = initOrigin
 
     def origin: TypeParamRef = currentOrigin
@@ -5012,7 +5029,7 @@ object Types extends TypeUtils {
     }
 
     def typeToInstantiateWith(fromBelow: Boolean)(using Context): Type =
-      TypeComparer.instanceType(origin, fromBelow, widenUnions, nestingLevel)
+      TypeComparer.instanceType(origin, fromBelow, widenPolicy, nestingLevel)
 
     /** Instantiate variable from the constraints over its `origin`.
      *  If `fromBelow` is true, the variable is instantiated to the lub
@@ -5029,7 +5046,10 @@ object Types extends TypeUtils {
         instantiateWith(tp)
 
     /** Widen unions when instantiating this variable in the current context? */
-    def widenUnions(using Context): Boolean = !ctx.typerState.constraint.isHard(this)
+    def widenPolicy(using Context): Widen =
+      if precise then Widen.None
+      else if ctx.typerState.constraint.isHard(this) then Widen.Singletons
+      else Widen.Unions
 
     /** For uninstantiated type variables: the entry in the constraint (either bounds or
      *  provisional instance value)
@@ -5070,8 +5090,17 @@ object Types extends TypeUtils {
     }
   }
   object TypeVar:
-    def apply(using Context)(initOrigin: TypeParamRef, creatorState: TyperState | Null, nestingLevel: Int = ctx.nestingLevel) =
-      new TypeVar(initOrigin, creatorState, nestingLevel)
+    def apply(using Context)(
+        initOrigin: TypeParamRef,
+        creatorState: TyperState | Null,
+        nestingLevel: Int = ctx.nestingLevel,
+        precise: Boolean = false) =
+      new TypeVar(initOrigin, creatorState, nestingLevel, precise)
+
+  enum Widen:
+    case None        // no widening
+    case Singletons  // widen singletons but not unions
+    case Unions      // widen singletons and unions
 
   type TypeVars = SimpleIdentitySet[TypeVar]
 

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

@@ -2087,7 +2087,7 @@ class Namer { typer: Typer =>
             if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null)
         match
           case ctp: ConstantType if sym.isInlineVal => ctp
-          case tp => TypeComparer.widenInferred(tp, pt, widenUnions = true)
+          case tp => TypeComparer.widenInferred(tp, pt, Widen.Unions)
 
     // Replace aliases to Unit by Unit itself. If we leave the alias in
     // it would be erased to BoxedUnit.

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

@@ -701,6 +701,12 @@ object ProtoTypes {
       case FunProto((arg: untpd.TypedSplice) :: Nil, _) => arg.isExtensionReceiver
       case _ => false
 
+  object SingletonConstrained:
+    def unapply(tp: Type)(using Context): Option[Type] = tp.dealias match
+      case RefinedType(parent, tpnme.Self, TypeAlias(tp))
+      if parent.typeSymbol == defn.SingletonClass => Some(tp)
+      case _ => None
+
   /** Add all parameters of given type lambda `tl` to the constraint's domain.
    *  If the constraint contains already some of these parameters in its domain,
    *  make a copy of the type lambda and add the copy's type parameters instead.
@@ -713,26 +719,41 @@ object ProtoTypes {
     tl: TypeLambda, owningTree: untpd.Tree,
     alwaysAddTypeVars: Boolean,
     nestingLevel: Int = ctx.nestingLevel
-  ): (TypeLambda, List[TypeVar]) = {
+  ): (TypeLambda, List[TypeVar]) =
     val state = ctx.typerState
     val addTypeVars = alwaysAddTypeVars || !owningTree.isEmpty
     if (tl.isInstanceOf[PolyType])
       assert(!ctx.typerState.isCommittable || addTypeVars,
         s"inconsistent: no typevars were added to committable constraint ${state.constraint}")
       // hk type lambdas can be added to constraints without typevars during match reduction
+    val added = state.constraint.ensureFresh(tl)
+
+    def singletonConstrainedRefs(tp: Type): Set[TypeParamRef] = tp match
+      case tp: MethodType if tp.isContextualMethod =>
+        val ownBounds =
+          for case SingletonConstrained(ref: TypeParamRef) <- tp.paramInfos
+          yield ref
+        ownBounds.toSet ++ singletonConstrainedRefs(tp.resType)
+      case tp: LambdaType =>
+        singletonConstrainedRefs(tp.resType)
+      case _ =>
+        Set.empty
+
+    val singletonRefs = singletonConstrainedRefs(added)
+    def isSingleton(ref: TypeParamRef) = singletonRefs.contains(ref)
 
-    def newTypeVars(tl: TypeLambda): List[TypeVar] =
-      for paramRef <- tl.paramRefs
-      yield
-        val tvar = TypeVar(paramRef, state, nestingLevel)
+    def newTypeVars: List[TypeVar] =
+      for paramRef <- added.paramRefs yield
+        val tvar = TypeVar(paramRef, state, nestingLevel, precise = isSingleton(paramRef))
         state.ownedVars += tvar
         tvar
 
-    val added = state.constraint.ensureFresh(tl)
-    val tvars = if addTypeVars then newTypeVars(added) else Nil
+    val tvars = if addTypeVars then newTypeVars else Nil
     TypeComparer.addToConstraint(added, tvars)
+    for paramRef <- added.paramRefs do
+      if isSingleton(paramRef) then paramRef <:< defn.SingletonType
     (added, tvars)
-  }
+  end constrained
 
   def constrained(tl: TypeLambda, owningTree: untpd.Tree)(using Context): (TypeLambda, List[TypeVar]) =
     constrained(tl, owningTree,

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

@@ -237,6 +237,16 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         EmptyTreeNoError
   end synthesizedValueOf
 
+  val synthesizedSingleton: SpecialHandler = (formal, span) => formal match
+    case SingletonConstrained(tp) =>
+      if tp.isSingletonBounded(frozen = false) then
+        withNoErrors:
+          ref(defn.Compiletime_erasedValue).appliedToType(formal).withSpan(span)
+      else
+        withErrors(i"$tp is not a singleton")
+    case _ =>
+      EmptyTreeNoError
+
   /** Create an anonymous class `new Object { type MirroredMonoType = ... }`
    *  and mark it with given attachment so that it is made into a mirror at PostTyper.
    */
@@ -536,7 +546,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
                   val tparams = poly.paramRefs
                   val variances = childClass.typeParams.map(_.paramVarianceSign)
                   val instanceTypes = tparams.lazyZip(variances).map((tparam, variance) =>
-                    TypeComparer.instanceType(tparam, fromBelow = variance < 0, widenUnions = true)
+                    TypeComparer.instanceType(tparam, fromBelow = variance < 0, Widen.Unions)
                   )
                   val instanceType = resType.substParams(poly, instanceTypes)
                   // this is broken in tests/run/i13332intersection.scala,
@@ -738,6 +748,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     defn.MirrorClass          -> synthesizedMirror,
     defn.ManifestClass        -> synthesizedManifest,
     defn.OptManifestClass     -> synthesizedOptManifest,
+    defn.SingletonClass       -> synthesizedSingleton,
   )
 
   def tryAll(formal: Type, span: Span)(using Context): TreeWithErrors =

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

@@ -3321,8 +3321,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         val app1 = typed(app, if ctx.mode.is(Mode.Pattern) then pt else defn.TupleXXLClass.typeRef)
         if ctx.mode.is(Mode.Pattern) then app1
         else
-          val elemTpes = elems.lazyZip(pts).map((elem, pt) =>
-            TypeComparer.widenInferred(elem.tpe, pt, widenUnions = true))
+          val elemTpes = elems.lazyZip(pts).map: (elem, pt) =>
+            TypeComparer.widenInferred(elem.tpe, pt, Widen.Unions)
           val resTpe = TypeOps.nestedPairs(elemTpes)
           app1.cast(resTpe)
 

docs/_docs/reference/experimental/typeclasses.md

@@ -7,7 +7,7 @@ nightlyOf: https://docs.scala-lang.org/scala3/reference/experimental/typeclasses
 
 # Some Proposed Changes for Better Support of Type Classes
 
-Martin Odersky, 8.1.2024
+Martin Odersky, 8.1.2024, edited 5.4.2024
 
 A type class in Scala is a pattern where we define
 
@@ -27,6 +27,8 @@ under source version `future` if the additional experimental language import `mo
   scala compile -source:future -language:experimental.modularity
 ```
 
+It is intended to turn features described here into proposals under the Scala improvement process. A first installment is SIP 64, which covers some syntactic changes, names for context bounds, multiple context bounds and deferred givens. The order of exposition described in this note is different from the planned proposals of SIPs. This doc is not a guide on how to sequence details, but instead wants to present a vision of what is possible. For instance, we start here with a feature (Self types and `is` syntax) that has turned out to be controversial and that will probably be proposed only late in the sequence of SIPs.
+
 ## Generalizing Context Bounds
 
  The only place in Scala's syntax where the type class pattern is relevant is
@@ -54,6 +56,8 @@ requires that `Ordering` is a trait or class with a single type parameter (which
 
   trait Monoid extends SemiGroup:
     def unit: Self
+  object Monoid:
+    def unit[M](using m: Monoid { type Self = M}): M
 
   trait Functor:
     type Self[A]
@@ -129,14 +133,17 @@ We introduce a standard type alias `is` in the Scala package or in `Predef`, def
   infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
 ```
 
-This makes writing instance definitions quite pleasant. Examples:
+This makes writing instance definitions and using clauses quite pleasant. Examples:
 
 ```scala
   given Int is Ord ...
   given Int is Monoid ...
 
   type Reader = [X] =>> Env => X
   given Reader is Monad ...
+
+  object Monoid:
+    def unit[M](using m: M is Monoid): M
 ```
 
 (more examples will follow below)
@@ -682,7 +689,7 @@ With the improvements proposed here, the library can now be expressed quite clea
 
 ## Suggested Improvements unrelated to Type Classes
 
-The following improvements elsewhere would make sense alongside the suggested changes to type classes. But they are currently not part of this proposal or implementation.
+The following two improvements elsewhere would make sense alongside the suggested changes to type classes. But only the first (fixing singleton) forms a part of this proposal and is implemented.
 
 ### Fixing Singleton
 
@@ -704,7 +711,7 @@ Then, instead of using an unsound upper bound we can use a context bound:
 def f[X: Singleton](x: X) = ...
 ```
 
-The context bound would be treated specially by the compiler so that no using clause is generated at runtime.
+The context bound is treated specially by the compiler so that no using clause is generated at runtime (this is straightforward, using the erased definitions mechanism).
 
 _Aside_: This can also lead to a solution how to express precise type variables. We can introduce another special type class `Precise` and use it like this:
 

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -77,6 +77,6 @@ object Predef:
    *
    *  which is what is needed for a context bound `[A: TC]`.
    */
-  infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
+  infix type is[A <: AnyKind, B <: Any{type Self <: AnyKind}] = B { type Self = A }
 
 end Predef