Refinements for context-bound companions

Add a config setting whether or not to use the type name for unary context bounds
as default name. It's on by default. If it is off, context bound companions are created
instead.

After fixing several problems, the test suite was verified to compile with the setting set to off.

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

@@ -10,7 +10,7 @@ import Annotations.Annotation
 import NameKinds.{UniqueName, ContextBoundParamName, ContextFunctionParamName, DefaultGetterName, WildcardParamName}
 import typer.{Namer, Checking}
 import util.{Property, SourceFile, SourcePosition, Chars}
-import config.Feature
+import config.{Feature, Config}
 import config.SourceVersion.*
 import collection.mutable.ListBuffer
 import reporting.*
@@ -230,7 +230,8 @@ object desugar {
       tdef: TypeDef,
       evidenceBuf: ListBuffer[ValDef],
       flags: FlagSet,
-      freshName: => TermName)(using Context): TypeDef =
+      freshName: untpd.Tree => TermName,
+      allParamss: List[ParamClause])(using Context): TypeDef =
 
     val evidenceNames = ListBuffer[TermName]()
 
@@ -241,11 +242,11 @@ object desugar {
           val evidenceName = bound match
             case ContextBoundTypeTree(_, _, ownName) if !ownName.isEmpty =>
               ownName
-            case _ if !isMember && cxbounds.tail.isEmpty && Feature.enabled(Feature.modularity) =>
+            case _ if !isMember && cxbounds.tail.isEmpty
+                && Feature.enabled(Feature.modularity) && Config.nameSingleContextBounds =>
               tdef.name.toTermName
             case _ =>
-              if isMember then inventGivenOrExtensionName(bound)
-              else freshName
+              freshName(bound)
           evidenceNames += evidenceName
           val evidenceParam = ValDef(evidenceName, bound, EmptyTree).withFlags(flags)
           evidenceParam.pushAttachment(ContextBoundParam, ())
@@ -257,9 +258,13 @@ object desugar {
         rhs
 
     val tdef1 = cpy.TypeDef(tdef)(rhs = desugarRhs(tdef.rhs))
-    if evidenceNames.nonEmpty && !evidenceNames.contains(tdef.name.toTermName) then
-      val witnessNamesAnnot = WitnessNamesAnnot(evidenceNames.toList).withSpan(tdef.span)
-      tdef1.withAddedAnnotation(witnessNamesAnnot)
+    if Feature.enabled(Feature.modularity)
+        && evidenceNames.nonEmpty
+        && !evidenceNames.contains(tdef.name.toTermName)
+        && !allParamss.nestedExists(_.name == tdef.name.toTermName)
+    then
+      tdef1.withAddedAnnotation:
+        WitnessNamesAnnot(evidenceNames.toList).withSpan(tdef.span)
     else
       tdef1
   end desugarContextBounds
@@ -268,7 +273,7 @@ object desugar {
     val DefDef(_, paramss, tpt, rhs) = meth
     val evidenceParamBuf = ListBuffer[ValDef]()
     var seenContextBounds: Int = 0
-    def freshName =
+    def freshName(unused: Tree) =
       seenContextBounds += 1 // Start at 1 like FreshNameCreator.
       ContextBoundParamName(EmptyTermName, seenContextBounds)
       // Just like with `makeSyntheticParameter` on nameless parameters of
@@ -280,7 +285,7 @@ object desugar {
       val iflag = if Feature.sourceVersion.isAtLeast(`future`) then Given else Implicit
       val flags = if isPrimaryConstructor then iflag | LocalParamAccessor else iflag | Param
       mapParamss(paramss) {
-        tparam => desugarContextBounds(tparam, evidenceParamBuf, flags, freshName)
+        tparam => desugarContextBounds(tparam, evidenceParamBuf, flags, freshName, paramss)
       }(identity)
 
     rhs match
@@ -326,9 +331,9 @@ object desugar {
 
     def getterParamss(n: Int): List[ParamClause] =
       mapParamss(takeUpTo(paramssNoRHS, n)) {
-        tparam => dropContextBounds(toDefParam(tparam, keepAnnotations = true))
+        tparam => dropContextBounds(toDefParam(tparam, KeepAnnotations.All))
       } {
-        vparam => toDefParam(vparam, keepAnnotations = true, keepDefault = false)
+        vparam => toDefParam(vparam, KeepAnnotations.All, keepDefault = false)
       }
 
     def defaultGetters(paramss: List[ParamClause], n: Int): List[DefDef] = paramss match
@@ -433,7 +438,14 @@ object desugar {
   private def addEvidenceParams(meth: DefDef, params: List[ValDef])(using Context): DefDef =
     if params.isEmpty then return meth
 
-    val boundNames = params.map(_.name).toSet
+    var boundNames = params.map(_.name).toSet
+    for mparams <- meth.paramss; mparam <- mparams do
+      mparam match
+        case tparam: TypeDef if tparam.mods.annotations.exists(WitnessNamesAnnot.unapply(_).isDefined) =>
+          boundNames += tparam.name.toTermName
+        case _ =>
+
+    //println(i"add ev params ${meth.name}, ${boundNames.toList}")
 
     def references(vdef: ValDef): Boolean =
       vdef.tpt.existsSubTree:
@@ -464,15 +476,26 @@ object desugar {
 
   @sharable private val synthetic = Modifiers(Synthetic)
 
-  private def toDefParam(tparam: TypeDef, keepAnnotations: Boolean): TypeDef = {
-    var mods = tparam.rawMods
-    if (!keepAnnotations) mods = mods.withAnnotations(Nil)
+  /** Which annotations to keep in derived parameters */
+  private enum KeepAnnotations:
+    case None, All, WitnessOnly
+
+  /** Filter annotations in `mods` according to `keep` */
+  private def filterAnnots(mods: Modifiers, keep: KeepAnnotations)(using Context) = keep match
+    case KeepAnnotations.None => mods.withAnnotations(Nil)
+    case KeepAnnotations.All => mods
+    case KeepAnnotations.WitnessOnly =>
+      mods.withAnnotations:
+        mods.annotations.filter:
+          case WitnessNamesAnnot(_) => true
+          case _ => false
+
+  private def toDefParam(tparam: TypeDef, keep: KeepAnnotations)(using Context): TypeDef =
+    val mods = filterAnnots(tparam.rawMods, keep)
     tparam.withMods(mods & EmptyFlags | Param)
-  }
 
-  private def toDefParam(vparam: ValDef, keepAnnotations: Boolean, keepDefault: Boolean)(using Context): ValDef = {
-    var mods = vparam.rawMods
-    if (!keepAnnotations) mods = mods.withAnnotations(Nil)
+  private def toDefParam(vparam: ValDef, keep: KeepAnnotations, keepDefault: Boolean)(using Context): ValDef = {
+    val mods = filterAnnots(vparam.rawMods, keep)
     val hasDefault = if keepDefault then HasDefault else EmptyFlags
     // Need to ensure that tree is duplicated since term parameters can be watched
     // and cloning a term parameter will copy its watchers to the clone, which means
@@ -495,8 +518,10 @@ object desugar {
 
   def typeDef(tdef: TypeDef)(using Context): Tree =
     val evidenceBuf = new ListBuffer[ValDef]
-    val result = desugarContextBounds(tdef, evidenceBuf,
-        (tdef.mods.flags.toTermFlags & AccessFlags) | Lazy | DeferredGivenFlags, EmptyTermName)
+    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 */
@@ -571,7 +596,7 @@ object desugar {
     // Annotations on class _type_ parameters are set on the derived parameters
     // but not on the constructor parameters. The reverse is true for
     // annotations on class _value_ parameters.
-    val constrTparams = impliedTparams.map(toDefParam(_, keepAnnotations = false))
+    val constrTparams = impliedTparams.map(toDefParam(_, KeepAnnotations.WitnessOnly))
     def defVparamss =
       if (originalVparamss.isEmpty) { // ensure parameter list is non-empty
         if (isCaseClass)
@@ -582,7 +607,7 @@ object desugar {
         report.error(CaseClassMissingNonImplicitParamList(cdef), namePos)
         ListOfNil
       }
-      else originalVparamss.nestedMap(toDefParam(_, keepAnnotations = true, keepDefault = true))
+      else originalVparamss.nestedMap(toDefParam(_, KeepAnnotations.All, keepDefault = true))
     val constrVparamss = defVparamss
       // defVparamss also needed as separate tree nodes in implicitWrappers below.
       // Need to be separate because they are `watch`ed in addParamRefinements.
@@ -608,7 +633,7 @@ object desugar {
             defDef(
               addEvidenceParams(
                 cpy.DefDef(ddef)(paramss = joinParams(constrTparams, ddef.paramss)),
-                evidenceParams(constr1).map(toDefParam(_, keepAnnotations = false, keepDefault = false)))))
+                evidenceParams(constr1).map(toDefParam(_, KeepAnnotations.None, keepDefault = false)))))
         case stat =>
           stat
       }
@@ -914,9 +939,9 @@ object desugar {
       }
       else {
         val defParamss = defVparamss.nestedMapConserve: param =>
-            // for named context bound parameters, we assume that they might have embedded types
+            // for context bound parameters, we assume that they might have embedded types
             // so they should be treated as tracked.
-            if param.hasAttachment(ContextBoundParam) && !param.name.is(ContextBoundParamName)
+            if param.hasAttachment(ContextBoundParam) && Feature.enabled(Feature.modularity)
             then param.withFlags(param.mods.flags | Tracked)
             else param
           match

compiler/src/dotty/tools/dotc/config/Config.scala

@@ -235,4 +235,11 @@ object Config {
    */
   inline val checkLevelsOnConstraints = false
   inline val checkLevelsOnInstantiation = true
+
+  /** If a type parameter `X` has a single context bounf `X: C`, should the
+   *  witness parameter be named `X`? This would prevent the creation of a
+   *  context bound companion.
+   */
+  inline val nameSingleContextBounds = true
 }
+

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

@@ -12,6 +12,7 @@ import Symbols.*
 import Scopes.*
 import Uniques.*
 import ast.Trees.*
+import Flags.ParamAccessor
 import ast.untpd
 import util.{NoSource, SimpleIdentityMap, SourceFile, HashSet, ReusableInstance}
 import typer.{Implicits, ImportInfo, SearchHistory, SearchRoot, TypeAssigner, Typer, Nullables}
@@ -399,7 +400,8 @@ object Contexts {
      *
      *  - as owner: The primary constructor of the class
      *  - as outer context: The context enclosing the class context
-     *  - as scope: The parameter accessors in the class context
+     *  - as scope: type parameters, the parameter accessors, and
+     *    the context bound companions in the class context,
      *
      *  The reasons for this peculiar choice of attributes are as follows:
      *
@@ -413,10 +415,11 @@ object Contexts {
      *    context see the constructor parameters instead, but then we'd need a final substitution step
      *    from constructor parameters to class parameter accessors.
      */
-    def superCallContext: Context = {
-      val locals = newScopeWith(owner.typeParams ++ owner.asClass.paramAccessors*)
-      superOrThisCallContext(owner.primaryConstructor, locals)
-    }
+    def superCallContext: Context =
+      val locals = owner.typeParams
+          ++ owner.asClass.unforcedDecls.filter: sym =>
+              sym.is(ParamAccessor) || sym.isContextBoundCompanion
+      superOrThisCallContext(owner.primaryConstructor, newScopeWith(locals*))
 
     /** The context for the arguments of a this(...) constructor call.
      *  The context is computed from the local auxiliary constructor context.

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

@@ -250,48 +250,42 @@ object NamerOps:
       rhsCtx.gadtState.addBound(psym, tr, isUpper = true)
     }
 
-  /** Create a context-bound companion for type symbol `tsym` unless it
-   *  would clash with another parameter. `tsym` is a context-bound symbol
-   *  that defined a set of witnesses with names `witnessNames`.
+  /** Create a context-bound companion for type symbol `tsym`, which has a context
+   *  bound that defines a set of witnesses with names `witnessNames`.
    *
-   *  @param paramSymss  If `tsym` is a type parameter, the other parameter symbols,
-   *                     including witnesses, of the method containing `tsym`.
-   *                     If `tsym` is an abstract type member, `paramSymss` is the
-   *                     empty list.
+   *  @param parans  If `tsym` is a type parameter, a list of parameter symbols
+   *                 that include all witnesses, otherwise the empty list.
    *
    *  The context-bound companion has as name the name of `tsym` translated to
    *  a term name. We create a synthetic val of the form
    *
-   *    val A: CBCompanion[witnessRef1 | ... | witnessRefN]
+   *    val A: `<context-bound-companion>`[witnessRef1 | ... | witnessRefN]
    *
    *  where
    *
-   *      CBCompanion is the <context-bound-companion> type created in Definitions
-   *      withnessRefK is a refence to the K'the witness.
+   *      <context-bound-companion> is the CBCompanion type created in Definitions
+   *      withnessRefK is a refence to the K'th witness.
    *
    *  The companion has the same access flags as the original type.
    */
-  def maybeAddContextBoundCompanionFor(tsym: Symbol, witnessNames: List[TermName], paramSymss: List[List[Symbol]])(using Context): Unit =
+  def addContextBoundCompanionFor(tsym: Symbol, witnessNames: List[TermName], params: List[Symbol])(using Context): Unit =
     val prefix = ctx.owner.thisType
     val companionName = tsym.name.toTermName
     val witnessRefs =
-      if paramSymss.nonEmpty then
-        if paramSymss.nestedExists(_.name == companionName) then Nil
-        else
-          witnessNames.map: witnessName =>
-            prefix.select(paramSymss.nestedFind(_.name == witnessName).get)
+      if params.nonEmpty then
+        witnessNames.map: witnessName =>
+            prefix.select(params.find(_.name == witnessName).get)
       else
-        witnessNames.map(prefix.select)
-    if witnessRefs.nonEmpty then
-      val cbtype = defn.CBCompanion.typeRef.appliedTo:
-        witnessRefs.reduce[Type](OrType(_, _, soft = false))
-      val cbc = newSymbol(
-          ctx.owner, companionName,
-          (tsym.flagsUNSAFE & AccessFlags) | Synthetic,
-          cbtype)
-      typr.println(i"contetx bpund companion created $cbc: $cbtype in ${ctx.owner}")
-      ctx.enter(cbc)
-  end maybeAddContextBoundCompanionFor
+        witnessNames.map(TermRef(prefix, _))
+    val cbtype = defn.CBCompanion.typeRef.appliedTo:
+      witnessRefs.reduce[Type](OrType(_, _, soft = false))
+    val cbc = newSymbol(
+        ctx.owner, companionName,
+        (tsym.flagsUNSAFE & (AccessFlags)).toTermFlags | Synthetic,
+        cbtype)
+    typr.println(s"context bound companion created $cbc for $witnessNames in ${ctx.owner}")
+    ctx.enter(cbc)
+  end addContextBoundCompanionFor
 
   /** Add context bound companions to all context-bound types declared in
    *  this class. This assumes that these types already have their
@@ -306,5 +300,5 @@ object NamerOps:
           if ann.symbol == defn.WitnessNamesAnnot then
             ann.tree match
               case ast.tpd.WitnessNamesAnnot(witnessNames) =>
-                maybeAddContextBoundCompanionFor(sym, witnessNames, Nil)
+                addContextBoundCompanionFor(sym, witnessNames, Nil)
 end NamerOps

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

@@ -88,7 +88,7 @@ class SymUtils:
     }
 
     def isContextBoundCompanion(using Context): Boolean =
-      self.is(Synthetic) && self.info.typeSymbol == defn.CBCompanion
+      self.is(Synthetic) && self.infoOrCompleter.typeSymbol == defn.CBCompanion
 
     /** Is this a case class for which a product mirror is generated?
     *  Excluded are value classes, abstract classes and case classes with more than one

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1124,6 +1124,7 @@ class TreeUnpickler(reader: TastyReader,
       })
       defn.patchStdLibClass(cls)
       NamerOps.addConstructorProxies(cls)
+      NamerOps.addContextBoundCompanions(cls)
       setSpan(start,
         untpd.Template(constr, mappedParents, self, lazyStats)
           .withType(localDummy.termRef))

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

@@ -430,11 +430,11 @@ class PlainPrinter(_ctx: Context) extends Printer {
     sym.isEffectiveRoot || sym.isAnonymousClass || sym.name.isReplWrapperName
 
   /** String representation of a definition's type following its name,
-   *  if symbol is completed, "?" otherwise.
+   *  if symbol is completed, ": ?" otherwise.
    */
   protected def toTextRHS(optType: Option[Type]): Text = optType match {
     case Some(tp) => toTextRHS(tp)
-    case None => "?"
+    case None => ": ?"
   }
 
   protected def decomposeLambdas(bounds: TypeBounds): (Text, TypeBounds) =