Allow context bounds in type declarations

Expand them to deferred givens

Author: odersky

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

@@ -237,12 +237,13 @@ object desugar {
 
     def desugarRhs(rhs: Tree): Tree = rhs match
       case ContextBounds(tbounds, cxbounds) =>
+        val isMember = flags.isAllOf(DeferredGivenFlags)
         for bound <- cxbounds do
           val evidenceName = bound match
             case ContextBoundTypeTree(_, _, ownName) if !ownName.isEmpty =>
               ownName
-            case _ if Config.nameSingleContextBounds && cxbounds.tail.isEmpty
-                && Feature.enabled(Feature.modularity) =>
+            case _ if Config.nameSingleContextBounds && !isMember
+                && cxbounds.tail.isEmpty && Feature.enabled(Feature.modularity) =>
               tdef.name.toTermName
             case _ =>
               freshName(bound)
@@ -262,7 +263,6 @@ object desugar {
   private def elimContextBounds(meth: DefDef, isPrimaryConstructor: Boolean)(using Context): DefDef =
     val DefDef(_, paramss, tpt, rhs) = meth
     val evidenceParamBuf = ListBuffer[ValDef]()
-
     var seenContextBounds: Int = 0
     def freshName(unused: Tree) =
       seenContextBounds += 1 // Start at 1 like FreshNameCreator.
@@ -492,6 +492,14 @@ object desugar {
         Apply(fn, params.map(refOfDef))
     }
 
+  def typeDef(tdef: TypeDef)(using Context): Tree =
+    val evidenceBuf = new ListBuffer[ValDef]
+    val result = desugarContextBounds(
+        tdef, evidenceBuf,
+        (tdef.mods.flags.toTermFlags & AccessFlags) | Lazy | DeferredGivenFlags,
+        inventGivenOrExtensionName, Nil)
+    if evidenceBuf.isEmpty then result else Thicket(result :: evidenceBuf.toList)
+
   /** The expansion of a class definition. See inline comments for what is involved */
   def classDef(cdef: TypeDef)(using Context): Tree = {
     val impl @ Template(constr0, _, self, _) = cdef.rhs: @unchecked
@@ -1425,7 +1433,7 @@ object desugar {
       case tree: TypeDef =>
         if (tree.isClassDef) classDef(tree)
         else if (ctx.mode.isQuotedPattern) quotedPatternTypeDef(tree)
-        else tree
+        else typeDef(tree)
       case tree: DefDef =>
         if (tree.name.isConstructorName) tree // was already handled by enclosing classDef
         else defDef(tree)

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3908,51 +3908,54 @@ object Parsers {
         argumentExprss(mkApply(Ident(nme.CONSTRUCTOR), argumentExprs()))
       }
 
-    /** TypeDef ::=  id [TypeParamClause] {FunParamClause} TypeBounds [‘=’ Type]
+    /** TypeDef ::=  id [TypeParamClause] {FunParamClause} TypeAndCtxBounds [‘=’ Type]
      */
     def typeDefOrDcl(start: Offset, mods: Modifiers): Tree = {
       newLinesOpt()
       atSpan(start, nameStart) {
         val nameIdent = typeIdent()
+        val tname = nameIdent.name.asTypeName
         val tparams = typeParamClauseOpt(ParamOwner.Type)
         val vparamss = funParamClauses()
+
         def makeTypeDef(rhs: Tree): Tree = {
           val rhs1 = lambdaAbstractAll(tparams :: vparamss, rhs)
           val tdef = TypeDef(nameIdent.name.toTypeName, rhs1)
           if (nameIdent.isBackquoted)
             tdef.pushAttachment(Backquoted, ())
           finalizeDef(tdef, mods, start)
         }
+
         in.token match {
           case EQUALS =>
             in.nextToken()
             makeTypeDef(toplevelTyp())
           case SUBTYPE | SUPERTYPE =>
-            val bounds = typeBounds()
-            if (in.token == EQUALS) {
-              val eqOffset = in.skipToken()
-              var rhs = toplevelTyp()
-              rhs match {
-                case mtt: MatchTypeTree =>
-                  bounds match {
-                    case TypeBoundsTree(EmptyTree, upper, _) =>
-                      rhs = MatchTypeTree(upper, mtt.selector, mtt.cases)
-                    case _ =>
-                      syntaxError(em"cannot combine lower bound and match type alias", eqOffset)
-                  }
-                case _ =>
-                  if mods.is(Opaque) then
-                    rhs = TypeBoundsTree(bounds.lo, bounds.hi, rhs)
-                  else
-                    syntaxError(em"cannot combine bound and alias", eqOffset)
-              }
-              makeTypeDef(rhs)
-            }
-            else makeTypeDef(bounds)
+            typeAndCtxBounds(tname) match
+              case bounds: TypeBoundsTree if in.token == EQUALS =>
+                val eqOffset = in.skipToken()
+                var rhs = toplevelTyp()
+                rhs match {
+                  case mtt: MatchTypeTree =>
+                    bounds match {
+                      case TypeBoundsTree(EmptyTree, upper, _) =>
+                        rhs = MatchTypeTree(upper, mtt.selector, mtt.cases)
+                      case _ =>
+                        syntaxError(em"cannot combine lower bound and match type alias", eqOffset)
+                    }
+                  case _ =>
+                    if mods.is(Opaque) then
+                      rhs = TypeBoundsTree(bounds.lo, bounds.hi, rhs)
+                    else
+                      syntaxError(em"cannot combine bound and alias", eqOffset)
+                }
+                makeTypeDef(rhs)
+              case bounds => makeTypeDef(bounds)
           case SEMI | NEWLINE | NEWLINES | COMMA | RBRACE | OUTDENT | EOF =>
-            makeTypeDef(typeBounds())
-          case _ if (staged & StageKind.QuotedPattern) != 0 =>
-            makeTypeDef(typeBounds())
+            makeTypeDef(typeAndCtxBounds(tname))
+          case _ if (staged & StageKind.QuotedPattern) != 0
+              || in.featureEnabled(Feature.modularity) && in.isColon =>
+            makeTypeDef(typeAndCtxBounds(tname))
           case _ =>
             syntaxErrorOrIncomplete(ExpectedTypeBoundOrEquals(in.token))
             return EmptyTree // return to avoid setting the span to EmptyTree

compiler/test/dotc/pos-test-pickling.blacklist

@@ -128,6 +128,7 @@ parsercombinators-givens.scala
 parsercombinators-givens-2.scala
 parsercombinators-arrow.scala
 hylolib-deferred-given
+hylolib-cb
 
 
 

docs/_docs/internals/syntax.md

@@ -450,7 +450,7 @@ PatDef            ::=  ids [‘:’ Type] [‘=’ Expr]
 DefDef            ::=  DefSig [‘:’ Type] [‘=’ Expr]                             DefDef(_, name, paramss, tpe, expr)
                     |  ‘this’ TypelessClauses [DefImplicitClause] ‘=’ ConstrExpr     DefDef(_, <init>, vparamss, EmptyTree, expr | Block)
 DefSig            ::=  id [DefParamClauses] [DefImplicitClause]
-TypeDef           ::=  id [TypeParamClause] {FunParamClause} TypeBounds         TypeDefTree(_, name, tparams, bound
+TypeDef           ::=  id [TypeParamClause] {FunParamClause} TypeAndCtxBounds   TypeDefTree(_, name, tparams, bound
                        [‘=’ Type]
 
 TmplDef           ::=  ([‘case’] ‘class’ | ‘trait’) ClassDef

tests/pos/deferredSummon.scala

@@ -9,11 +9,15 @@ trait A:
   given Ord[Elem] = deferred
   def foo = summon[Ord[Elem]]
 
+trait B:
+  type Elem: Ord
+  def foo = summon[Ord[Elem]]
+
 object Inst:
   given Ord[Int]:
     def less(x: Int, y: Int) = x < y
 
-object Test:
+object Test1:
   import Inst.given
   class C extends A:
     type Elem = Int
@@ -22,9 +26,22 @@ object Test:
   given A:
     type Elem = Int
 
-class D[T: Ord] extends A:
+class D1[T: Ord] extends B:
+  type Elem = T
+
+object Test2:
+  import Inst.given
+  class C extends B:
+    type Elem = Int
+  object E extends B:
+    type Elem = Int
+  given B:
+    type Elem = Int
+
+class D2[T: Ord] extends B:
   type Elem = T
 
 
 
 
+

tests/pos/dep-context-bounds.scala

@@ -0,0 +1,10 @@
+//> using options -language:experimental.modularity -source future
+trait A[X]:
+  type Self = X
+
+object Test2:
+  def foo[X: A as x](a: x.Self) = ???
+
+  def bar[X: A as x](a: Int) = ???
+
+  def baz[X: A as x](a: Int)(using String) = ???

tests/pos/hylolib-cb-extract.scala

@@ -0,0 +1,18 @@
+//> using options -language:experimental.modularity -source future
+package hylotest
+import compiletime.deferred
+
+trait Value[Self]
+
+/** A collection of elements accessible by their position. */
+trait Collection[Self]:
+
+  /** The type of the elements in the collection. */
+  type Element: Value
+
+class BitArray
+
+given Value[Boolean] {}
+
+given Collection[BitArray] with
+  type Element = Boolean

tests/pos/hylolib-cb/AnyCollection.scala

@@ -0,0 +1,66 @@
+package hylo
+
+/** A type-erased collection.
+  *
+  * A `AnyCollection` forwards its operations to a wrapped value, hiding its implementation.
+  */
+final class AnyCollection[Element] private (
+    val _start: () => AnyValue,
+    val _end: () => AnyValue,
+    val _after: (AnyValue) => AnyValue,
+    val _at: (AnyValue) => Element
+)
+
+object AnyCollection {
+
+  /** Creates an instance forwarding its operations to `base`. */
+  def apply[Base](using b: Collection[Base])(base: Base): AnyCollection[b.Element] =
+    // NOTE: This evidence is redefined so the compiler won't report ambiguity between `intIsValue`
+    // and `anyValueIsValue` when the method is called on a collection of `Int`s. None of these
+    // choices is even correct! Note also that the ambiguity is suppressed if the constructor of
+    // `AnyValue` is declared with a context bound rather than an implicit parameter.
+    given Value[b.Position] = b.positionIsValue
+
+    def start(): AnyValue =
+      AnyValue(base.startPosition)
+
+    def end(): AnyValue =
+      AnyValue(base.endPosition)
+
+    def after(p: AnyValue): AnyValue =
+      AnyValue(base.positionAfter(p.unsafelyUnwrappedAs[b.Position]))
+
+    def at(p: AnyValue): b.Element =
+      base.at(p.unsafelyUnwrappedAs[b.Position])
+
+    new AnyCollection[b.Element](
+      _start = start,
+      _end = end,
+      _after = after,
+      _at = at
+    )
+
+}
+
+given anyCollectionIsCollection[T](using tIsValue: Value[T]): Collection[AnyCollection[T]] with {
+
+  type Element = T
+  type Position = AnyValue
+
+  extension (self: AnyCollection[T]) {
+
+    def startPosition =
+      self._start()
+
+    def endPosition =
+      self._end()
+
+    def positionAfter(p: Position) =
+      self._after(p)
+
+    def at(p: Position) =
+      self._at(p)
+
+  }
+
+}

tests/pos/hylolib-cb/AnyValue.scala

@@ -0,0 +1,76 @@
+package hylo
+
+/** A wrapper around an object providing a reference API. */
+private final class Ref[T](val value: T) {
+
+  override def toString: String =
+    s"Ref($value)"
+
+}
+
+/** A type-erased value.
+  *
+  * An `AnyValue` forwards its operations to a wrapped value, hiding its implementation.
+  */
+final class AnyValue private (
+    private val wrapped: AnyRef,
+    private val _copy: (AnyRef) => AnyValue,
+    private val _eq: (AnyRef, AnyRef) => Boolean,
+    private val _hashInto: (AnyRef, Hasher) => Hasher
+) {
+
+  /** Returns a copy of `this`. */
+  def copy(): AnyValue =
+    _copy(this.wrapped)
+
+  /** Returns `true` iff `this` and `other` have an equivalent value. */
+  def eq(other: AnyValue): Boolean =
+    _eq(this.wrapped, other.wrapped)
+
+  /** Hashes the salient parts of `this` into `hasher`. */
+  def hashInto(hasher: Hasher): Hasher =
+    _hashInto(this.wrapped, hasher)
+
+  /** Returns the value wrapped in `this` as an instance of `T`. */
+  def unsafelyUnwrappedAs[T]: T =
+    wrapped.asInstanceOf[Ref[T]].value
+
+  /** Returns a textual description of `this`. */
+  override def toString: String =
+    wrapped.toString
+
+}
+
+object AnyValue {
+
+  /** Creates an instance wrapping `wrapped`. */
+  def apply[T](using Value[T])(wrapped: T): AnyValue =
+    def copy(a: AnyRef): AnyValue =
+      AnyValue(a.asInstanceOf[Ref[T]].value.copy())
+
+    def eq(a: AnyRef, b: AnyRef): Boolean =
+      a.asInstanceOf[Ref[T]].value `eq` b.asInstanceOf[Ref[T]].value
+
+    def hashInto(a: AnyRef, hasher: Hasher): Hasher =
+      a.asInstanceOf[Ref[T]].value.hashInto(hasher)
+
+    new AnyValue(Ref(wrapped), copy, eq, hashInto)
+
+}
+
+given anyValueIsValue: Value[AnyValue] with {
+
+  extension (self: AnyValue) {
+
+    def copy(): AnyValue =
+      self.copy()
+
+    def eq(other: AnyValue): Boolean =
+      self `eq` other
+
+    def hashInto(hasher: Hasher): Hasher =
+      self.hashInto(hasher)
+
+  }
+
+}