SIP-23

bincompat-backward.whitelist.conf

@@ -207,6 +207,10 @@ filter {
     {
         matchName="scala.collection.immutable.Stream.scala$collection$immutable$Stream$$loop$4"
         problemName=MissingMethodProblem
+    },
+    {
+        matchName="scala.reflect.api.Types.LiteralType"
+        problemName=MissingMethodProblem
     }
   ]
 }

bincompat-forward.whitelist.conf

@@ -393,6 +393,18 @@ filter {
     {
         matchName="scala.reflect.runtime.Settings.ZirrefutableGeneratorPatterns"
         problemName=MissingMethodProblem
+    },
+    {
+        matchName="scala.reflect.api.Types$LiteralTypeApi"
+        problemName=MissingClassProblem
+    },
+    {
+        matchName="scala.reflect.api.Types.LiteralType"
+        problemName=MissingMethodProblem
+    },
+    {
+        matchName="scala.reflect.api.Types$LiteralTypeExtractor"
+        problemName=MissingClassProblem
     }
   ]
 }

spec/03-types.md

@@ -23,6 +23,7 @@ chapter: 3
                       |  SimpleType ‘#’ id
                       |  StableId
                       |  Path ‘.’ ‘type’
+                      |  Literal
                       |  ‘(’ Types ‘)’
   TypeArgs          ::=  ‘[’ Types ‘]’
   Types             ::=  Type {‘,’ Type}
@@ -102,16 +103,40 @@ forms.
 ### Singleton Types
 
 ```ebnf
-SimpleType  ::=  Path ‘.’ type
+SimpleType  ::=  Path ‘.’ ‘type’
 ```
 
 A singleton type is of the form $p.$`type`, where $p$ is a
 path pointing to a value expected to [conform](06-expressions.html#expression-typing)
 to `scala.AnyRef`. The type denotes the set of values
-consisting of `null` and the value denoted by $p$.
+consisting of `null` and the value denoted by $p$
+(i.e., the value $v$ for which `v eq p`).
 
-A _stable type_ is either a singleton type or a type which is
-declared to be a subtype of trait `scala.Singleton`.
+<!-- a pattern match/type test against a singleton type `p.type` desugars to `_ eq p` -->
+
+### Literal Types
+
+```ebnf
+SimpleType  ::=  Literal
+```
+
+A literal type `lit` is a special kind of singleton type which denotes the single literal value `lit`.
+Thus, the type ascription `1: 1` gives the most precise type to the literal value `1`:  the literal type `1`.
+
+At run time, an expression `e` is considered to have literal type `lit` if `e == lit`.
+Concretely, the result of `e.isInstanceOf[lit]` and `e match { case _ : lit => }` is determined by evaluating `e == lit`.
+
+<!-- TODO: use eq when we lift it up to Any -->
+
+<!-- TODO: relate to constant types, which trigger constant folding
+  ConstantType(1).deconst =:= LiteralType(1)
+  LiteralType(1).widen =:= IntClass.tpe
+-->
+
+
+### Stable Types
+A _stable type_ is a singleton type, a literal type,
+or a type that is declared to be a subtype of trait `scala.Singleton`.
 
 ### Type Projection
 

spec/06-expressions.md

@@ -76,8 +76,8 @@ $T$ forSome { type $t_1[\mathit{tps}\_1] >: L_1 <: U_1$; $\ldots$; type $t_n[\ma
 SimpleExpr    ::=  Literal
 ```
 
-Typing of literals is as described [here](01-lexical-syntax.html#literals); their
-evaluation is immediate.
+Typing of literals is described along with their [lexical syntax](01-lexical-syntax.html#literals);
+their evaluation is immediate.
 
 ## The _Null_ Value
 

spec/08-pattern-matching.md

@@ -337,9 +337,11 @@ A type pattern $T$ is of one of the following  forms:
   be used as type patterns, because they would match nothing in any case.
 
 * A singleton type `$p$.type`. This type pattern matches only the value
-  denoted by the path $p$ (that is, a pattern match involved a
-  comparison of the matched value with $p$ using method `eq` in class
-  `AnyRef`).
+  denoted by the path $p$ (the `eq` method is used to compare the matched value to $p$).
+
+* A literal type `$lit$`. This type pattern matches only the value
+  denoted by the literal $lit$ (the `==` method is used to compare the matched value to $lit$). <!-- SIP-23 -->
+
 * A compound type pattern `$T_1$ with $\ldots$ with $T_n$` where each $T_i$ is a
   type pattern. This type pattern matches all values that are matched by each of
   the type patterns $T_i$.

src/compiler/scala/tools/nsc/Global.scala

@@ -78,6 +78,9 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
 
   override def settings = currentSettings
 
+  // TODO: temporary flag to easily enable/disable SIP-23 (aka the type formerly known as 42.type)
+  override def sip23: Boolean = settings.Xexperimental.value
+
   /** Switch to turn on detailed type logs */
   var printTypings = settings.Ytyperdebug.value
 

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -339,6 +339,7 @@ self =>
     }
     private def inScalaRootPackage = inScalaPackage && currentPackage == "scala"
 
+
     def parseStartRule: () => Tree
 
     def parseRule[T](rule: this.type => T): T = {
@@ -675,11 +676,11 @@ self =>
 
     def isExprIntro: Boolean = isExprIntroToken(in.token)
 
-    def isTypeIntroToken(token: Token): Boolean = token match {
+    def isTypeIntroToken(token: Token): Boolean = (sip23 && isLiteralToken(token)) || (token match {
       case IDENTIFIER | BACKQUOTED_IDENT | THIS |
            SUPER | USCORE | LPAREN | AT => true
       case _ => false
-    }
+    })
 
     def isStatSeqEnd = in.token == RBRACE || in.token == EOF
 
@@ -932,31 +933,17 @@ self =>
        *                     |  SimpleType `#' Id
        *                     |  StableId
        *                     |  Path `.' type
+       *                     |  Literal
        *                     |  `(' Types `)'
        *                     |  WildcardType
        *  }}}
        */
       def simpleType(): Tree = {
         val start = in.offset
         simpleTypeRest(in.token match {
-          case LPAREN =>
-            atPos(start)(makeTupleType(inParens(types())))
-          case LBRACKET =>
-            atPos(start) {
-              val ts = typeParamClauseOpt(freshTypeName("typelambda"), null)
-              if (ts.isEmpty) {
-                syntaxError("missing type parameters", skipIt = false)
-                errorTypeTree
-              } else if (in.token == ARROW) {
-                in.skipToken()
-                makeTypeLambdaTypeTree(ts, typ())
-              } else {
-                syntaxError("`=>' expected", skipIt = false)
-                errorTypeTree
-              }
-            }
-          case USCORE =>
-            wildcardType(in.skipToken())
+          case LPAREN   => atPos(start)(makeTupleType(inParens(types())))
+          case USCORE   => wildcardType(in.skipToken())
+          case tok if sip23 && isLiteralToken(tok) => atPos(start){SingletonTypeTree(literal())} // SIP-23
           case _ =>
             path(thisOK = false, typeOK = true) match {
               case r @ SingletonTypeTree(_) => r
@@ -1038,7 +1025,15 @@ self =>
           else
             mkOp(infixType(InfixMode.RightOp))
         }
+        // SIP-23
+        def isNegatedLiteralType = sip23 && (
+          t match { // the token for `t` (Ident("-")) has already been read, thus `isLiteral` below is looking at next token (must be a literal)
+            case Ident(name) if isLiteral => name == nme.MINUS.toTypeName // TODO: OPT? lift out nme.MINUS.toTypeName?
+            case _ => false
+          }
+        )
         if (isIdent) checkRepeatedParam orElse asInfix
+        else if (isNegatedLiteralType) atPos(t.pos.start){SingletonTypeTree(literal(isNegated = true, start = t.pos.start))}
         else t
       }
 

src/compiler/scala/tools/nsc/backend/icode/TypeKinds.scala

@@ -383,6 +383,7 @@ trait TypeKinds { self: ICodes =>
     case ThisType(ArrayClass)            => ObjectReference
     case ThisType(sym)                   => REFERENCE(sym)
     case SingleType(_, sym)              => primitiveOrRefType(sym)
+    case LiteralType(_)                  => toTypeKind(t.underlying)
     case ConstantType(_)                 => toTypeKind(t.underlying)
     case TypeRef(_, sym, args)           => primitiveOrClassType(sym, args)
     case ClassInfoType(_, _, ArrayClass) => abort("ClassInfoType to ArrayClass!")

src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala

@@ -435,6 +435,7 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
             case ThisType(ArrayClass)               => ObjectReference // was introduced in 9b17332f11 to fix SI-999, but this code is not reached in its test, or any other test
             case ThisType(sym)                      => getClassBTypeAndRegisterInnerClass(sym)
             case SingleType(_, sym)                 => primitiveOrClassToBType(sym)
+            case LiteralType(_)                     => toTypeKind(t.underlying)
             case ConstantType(_)                    => toTypeKind(t.underlying)
             case RefinedType(parents, _)            => parents.map(toTypeKind(_).asClassBType).reduceLeft((a, b) => a.jvmWiseLUB(b))
           }

src/compiler/scala/tools/nsc/symtab/classfile/Pickler.scala

@@ -223,6 +223,8 @@ abstract class Pickler extends SubComponent {
         case SingleType(pre, sym) =>
           putType(pre)
           putSymbol(sym)
+        case LiteralType(value) =>
+          putConstant(value)
         case SuperType(thistpe, supertpe) =>
           putType(thistpe)
           putType(supertpe)
@@ -452,6 +454,7 @@ abstract class Pickler extends SubComponent {
         case NoType | NoPrefix                   =>
         case ThisType(sym)                       => writeRef(sym)
         case SingleType(pre, sym)                => writeRef(pre) ; writeRef(sym)
+        case LiteralType(value)                  => writeRef(value)
         case SuperType(thistpe, supertpe)        => writeRef(thistpe) ; writeRef(supertpe)
         case ConstantType(value)                 => writeRef(value)
         case TypeBounds(lo, hi)                  => writeRef(lo) ; writeRef(hi)

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -878,7 +878,7 @@ abstract class Erasure extends AddInterfaces
                     List(TypeTree(tp) setPos targ.pos)) setPos fn.pos,
                   List()) setPos tree.pos
               targ.tpe match {
-                case SingleType(_, _) | ThisType(_) | SuperType(_, _) =>
+                case SingleType(_, _) | LiteralType(_) | ThisType(_) | SuperType(_, _) =>
                   val cmpOp = if (targ.tpe <:< AnyValTpe) Any_equals else Object_eq
                   atPos(tree.pos) {
                     Apply(Select(qual, cmpOp), List(gen.mkAttributedQualifier(targ.tpe)))

src/compiler/scala/tools/nsc/transform/UnCurry.scala

@@ -322,7 +322,7 @@ abstract class UnCurry extends InfoTransform
         args.take(formals.length - 1) :+ (suffix setType formals.last)
       }
 
-      val args1 = if (isVarArgTypes(formals)) transformVarargs(formals.last.typeArgs.head) else args
+      val args1 = if (isVarArgTypes(formals)) transformVarargs(formals.last.typeArgs.head.widen) else args
 
       map2(formals, args1) { (formal, arg) =>
         if (!isByNameParamType(formal))

src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala

@@ -447,7 +447,16 @@ trait MatchTreeMaking extends MatchCodeGen with Debugging {
         //  - Scala's arrays are invariant (so we don't drop type tests unsoundly)
         if (extractorArgTypeTest) mkDefault
         else expectedTp match {
-          case SingleType(_, sym)                       => mkEqTest(gen.mkAttributedQualifier(expectedTp)) // SI-4577, SI-4897
+          case SingleType(_, sym)                       =>
+            val expected = gen.mkAttributedQualifier(expectedTp)
+            if (expectedTp <:< AnyRefTpe) mkEqTest(expected) // SI-4577, SI-4897
+            else mkEqualsTest(expected)
+          // TODO SIP-23: should we test equality for literal types with eq?
+          // Conceptually cleaner, as SingleType is tested using eq.
+          // In practice it doesn't really matter, since `equals` does the same thing as `eq` in the `AnyVal` subclasses of `Any`.
+          // Should revisit if we end up lifting `eq`'s definition to `Any`, as discussed here:
+          // https://groups.google.com/d/msg/scala-internals/jsVlJI4H5OQ/8emZWRmgzcoJ
+          case LiteralType(const)                       => mkEqualsTest(expTp(Literal(const)))
           case ThisType(sym) if sym.isModule            => and(mkEqualsTest(CODE.REF(sym)), mkTypeTest) // must use == to support e.g. List() == Nil
           case ConstantType(Constant(null)) if isAnyRef => mkEqTest(expTp(CODE.NULL))
           case ConstantType(const)                      => mkEqualsTest(expTp(Literal(const)))

src/compiler/scala/tools/nsc/typechecker/Implicits.scala

@@ -1160,6 +1160,7 @@ trait Implicits {
           // necessary only to compile typetags used inside the Universe cake
           case ThisType(thisSym) =>
             gen.mkAttributedThis(thisSym)
+          // TODO SIP-23: TypeTag for LiteralType
           case _ =>
             // if `pre` is not a PDT, e.g. if someone wrote
             //   implicitly[scala.reflect.macros.blackbox.Context#TypeTag[Int]]

src/compiler/scala/tools/nsc/typechecker/Namers.scala

@@ -825,7 +825,28 @@ trait Namers extends MethodSynthesis {
       }
     }
 
-    /** This method has a big impact on the eventual compiled code.
+    private def refersToSymbolLessAccessibleThan(tp: Type, sym: Symbol): Boolean = {
+      val accessibilityReference =
+        if (sym.isValue && sym.owner.isClass && sym.isPrivate)
+          sym.getterIn(sym.owner)
+        else sym
+
+      @tailrec def loop(tp: Type): Boolean = tp match {
+        case SingleType(pre, sym) =>
+          (sym isLessAccessibleThan accessibilityReference) || loop(pre)
+        case ThisType(sym) =>
+          sym isLessAccessibleThan accessibilityReference
+        case p: SimpleTypeProxy =>
+          loop(p.underlying)
+        case _ =>
+          false
+      }
+
+      loop(tp)
+    }
+
+    /**
+     * This method has a big impact on the eventual compiled code.
      *  At this point many values have the most specific possible
      *  type (e.g. in val x = 42, x's type is Int(42), not Int) but
      *  most need to be widened to avoid undesirable propagation of
@@ -838,35 +859,39 @@ trait Namers extends MethodSynthesis {
      *  value should not be widened, so it has a use even in situations
      *  whether it is otherwise redundant (such as in a singleton.)
      */
-    private def widenIfNecessary(sym: Symbol, tpe: Type, pt: Type): Type = {
-      val getter =
-        if (sym.isValue && sym.owner.isClass && sym.isPrivate)
-          sym.getter(sym.owner)
-        else sym
-      def isHidden(tp: Type): Boolean = tp match {
-        case SingleType(pre, sym) =>
-          (sym isLessAccessibleThan getter) || isHidden(pre)
-        case ThisType(sym) =>
-          sym isLessAccessibleThan getter
-        case p: SimpleTypeProxy =>
-          isHidden(p.underlying)
-        case _ =>
-          false
-      }
-      val shouldWiden = (
-           !tpe.typeSymbolDirect.isModuleClass // Infer Foo.type instead of "object Foo"
-        && (tpe.widen <:< pt)                  // Don't widen our way out of conforming to pt
-        && (   sym.isVariable
-            || sym.isMethod && !sym.hasAccessorFlag
-            || isHidden(tpe)
-           )
-      )
-      dropIllegalStarTypes(
+    private def widenIfNecessary(sym: Symbol, tpe: Type, pt: Type): Type =
+      if (sip23) { // SIP-23
+        // TODO: spec -- this is a crucial part of type inference
+        // NOTES:
+        //  - Can we widen less? (E.g., for local definitions.)
+        //  - Do we need to check tpe.deconst <:< pt?
+        //  - We don't need to call dropIllegalStarTypes on a ref to a module class, do we? Where would the stars be? In the prefix?
+
+        // We're inferring the result type of a stable symbol, and the type doesn't refer to a hidden symbol
+        val mayKeepSingletonType = sym.isStable && !refersToSymbolLessAccessibleThan(tpe, sym)
+
+        // (OPT: 99.99% of the time, pt will be WildcardType)
+        @inline def cannotWiden  = (pt ne WildcardType) && !(tpe.widen <:< pt)
+
+        // If the definition can keep its inferred singleton type,
+        // or widening would mean no longer conforming to the expected type,
+        // we must still deconst unless it's a final val. Otherwise, widen.
+        if (mayKeepSingletonType || cannotWiden) { if (sym.isFinal) tpe else tpe.deconst }
+        else tpe.widen
+      } else {
+        val shouldWiden = (
+             !tpe.typeSymbolDirect.isModuleClass // Infer Foo.type instead of "object Foo"
+          && (tpe.widen <:< pt)                  // Don't widen our way out of conforming to pt
+          && (   sym.isVariable
+              || sym.isMethod && !sym.hasAccessorFlag
+              || refersToSymbolLessAccessibleThan(tpe, sym)
+             )
+        )
         if (shouldWiden) tpe.widen
         else if (sym.isFinal) tpe    // "final val" allowed to retain constant type
         else tpe.deconst
-      )
-    }
+      }
+
     /** Computes the type of the body in a ValDef or DefDef, and
      *  assigns the type to the tpt's node.  Returns the type.
      */
@@ -876,7 +901,7 @@ trait Namers extends MethodSynthesis {
         case _ => defnTyper.computeType(tree.rhs, pt)
       }
 
-      val defnTpe = widenIfNecessary(tree.symbol, rhsTpe, pt)
+      val defnTpe = dropIllegalStarTypes(widenIfNecessary(tree.symbol, rhsTpe, pt))
       tree.tpt defineType defnTpe setPos tree.pos.focus
       tree.tpt.tpe
     }

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -5093,13 +5093,17 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       }
 
       def typedSingletonTypeTree(tree: SingletonTypeTree) = {
+        // SIP-23: don't require AnyRef for 1.type etc
+        val pt = if (sip23) WildcardType else AnyRefTpe
         val refTyped =
           context.withImplicitsDisabled {
-            typed(tree.ref, MonoQualifierModes | mode.onlyTypePat, AnyRefTpe)
+            typed(tree.ref, MonoQualifierModes | mode.onlyTypePat, pt)
           }
 
+        // .resultType unwraps NullaryMethodType (accessor of a path)
+        // .deconst unwraps the ConstantType to a LiteralType (for literal-based singleton types)
         if (!refTyped.isErrorTyped)
-          tree setType refTyped.tpe.resultType
+          tree setType refTyped.tpe.resultType.deconst
 
         if (treeInfo.admitsTypeSelection(refTyped)) tree
         else UnstableTreeError(refTyped)