Add extension instances

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

@@ -859,21 +859,22 @@ object desugar {
    *     given object name extends parents { self => body' }
    *
    *  where every definition in `body` is expanded to an extension method
-   *  taking type parameters `tparams` and a leading parameter `(x: T)`.
-   *  See: makeExtensionDef
+   *  taking type parameters `tparams` and a leading paramter `(x: T)`.
+   *  See: collectiveExtensionBody
    */
   def moduleDef(mdef: ModuleDef)(implicit ctx: Context): Tree = {
     val impl = mdef.impl
     val mods = mdef.mods
     impl.constr match {
-      case DefDef(_, tparams, (vparams @ (vparam :: Nil)) :: givenParamss, _, _) =>
+      case DefDef(_, tparams, vparamss @ (vparam :: Nil) :: givenParamss, _, _) =>
+        // Transform collective extension
         assert(mods.is(Given))
         return moduleDef(
           cpy.ModuleDef(mdef)(
             mdef.name,
             cpy.Template(impl)(
               constr = emptyConstructor,
-              body = impl.body.map(makeExtensionDef(_, tparams, vparams, givenParamss)))))
+              body = collectiveExtensionBody(impl.body, tparams, vparamss))))
       case _ =>
     }
 
@@ -916,43 +917,40 @@ object desugar {
     }
   }
 
-  /** Given tpe parameters `Ts` (possibly empty) and a leading value parameter `(x: T)`,
-   *  map a method definition
+  /** Transform the statements of a collective extension
+   *   @param stats    the original statements as they were parsed
+   *   @param tparams  the collective type parameters
+   *   @param vparamss the collective value parameters, consisting
+   *                   of a single leading value parameter, followed by
+   *                   zero or more context parameter clauses
    *
-   *     def foo [Us] paramss ...
+   *  Note: It is already assured by Parser.checkExtensionMethod that all
+   *  statements conform to requirements.
    *
-   *  to
+   *  Each method in stats is transformed into an extension method. Example:
+   *
+   *    extension on [Ts](x: T)(using C):
+   *      def f(y: T) = ???
+   *      def g(z: T) = f(z)
    *
-   *     <extension> def foo[Ts ++ Us](x: T) parammss ...
+   *  is turned into
    *
-   *  If the given member `mdef` is not of this form, flag it as an error.
+   *    extension:
+   *      <extension> def f[Ts](x: T)(using C)(y: T) = ???
+   *      <extension> def g[Ts](x: T)(using C)(z: T) = f(z)
    */
-
-  def makeExtensionDef(mdef: Tree, tparams: List[TypeDef], leadingParams: List[ValDef],
-                       givenParamss: List[List[ValDef]])(using ctx: Context): Tree = {
-    mdef match {
-      case mdef: DefDef =>
-        if (mdef.mods.is(Extension)) {
-          ctx.error(NoExtensionMethodAllowed(mdef), mdef.sourcePos)
-          mdef
-        } else {
-          if (tparams.nonEmpty && mdef.tparams.nonEmpty) then
-            ctx.error(ExtensionMethodCannotHaveTypeParams(mdef), mdef.tparams.head.sourcePos)
-            mdef
-          else cpy.DefDef(mdef)(
+  def collectiveExtensionBody(stats: List[Tree],
+      tparams: List[TypeDef], vparamss: List[List[ValDef]])(using ctx: Context): List[Tree] =
+    for stat <- stats yield
+      stat match
+        case mdef: DefDef =>
+          cpy.DefDef(mdef)(
             tparams = tparams ++ mdef.tparams,
-            vparamss = leadingParams :: givenParamss ::: mdef.vparamss
+            vparamss = vparamss ::: mdef.vparamss,
           ).withMods(mdef.mods | Extension)
-        }
-      case mdef: Import =>
-        mdef
-      case mdef if !mdef.isEmpty => {
-        ctx.error(ExtensionCanOnlyHaveDefs(mdef), mdef.sourcePos)
-        mdef
-      }
-      case mdef => mdef
-    }
-  }
+        case mdef =>
+          mdef
+  end collectiveExtensionBody
 
   /** Transforms
    *

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

@@ -301,7 +301,7 @@ object Flags {
   /** A method that has default params */
   val (_, DefaultParameterized @ _, _) = newFlags(27, "<defaultparam>")
 
-  /** An extension method */
+  /** An extension method, or a collective extension instance */
   val (_, Extension @ _, _) = newFlags(28, "<extension>")
 
   /** An inferable (`given`) parameter */
@@ -499,14 +499,14 @@ object Flags {
 
   /** Flags that can apply to a module val */
   val RetainedModuleValFlags: FlagSet = RetainedModuleValAndClassFlags |
-    Override | Final | Method | Implicit | Given | Lazy |
+    Override | Final | Method | Implicit | Given | Lazy | Extension |
     Accessor | AbsOverride | StableRealizable | Captured | Synchronized | Erased
 
   /** Flags that can apply to a module class */
   val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | Enum
 
   /** Flags retained in export forwarders */
-  val RetainedExportFlags = Given | Implicit | Extension | Inline
+  val RetainedExportFlags = Given | Implicit | Inline
 
   /** Flags that apply only to classes */
   val ClassOnlyFlags = Sealed | Open | Abstract.toTypeFlags

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

@@ -931,7 +931,11 @@ object Parsers {
       lookahead.nextToken()
       if lookahead.isIdent && !lookahead.isIdent(nme.on) then
         lookahead.nextToken()
+      if lookahead.isNewLine then
+        lookahead.nextToken()
       lookahead.isIdent(nme.on)
+      || lookahead.token == LBRACE
+      || lookahead.token == COLON
 
 /* --------- OPERAND/OPERATOR STACK --------------------------------------- */
 
@@ -3470,6 +3474,23 @@ object Parsers {
       Template(constr, parents, Nil, EmptyValDef, Nil)
     }
 
+    def checkExtensionMethod(tparams: List[Tree],
+        vparamss: List[List[Tree]], stat: Tree): Unit = stat match {
+      case stat: DefDef =>
+        if stat.mods.is(Extension) && vparamss.nonEmpty then
+          syntaxError(i"no extension method allowed here since leading parameter was already given", stat.span)
+        else if !stat.mods.is(Extension) && vparamss.isEmpty then
+          syntaxError(i"an extension method is required here", stat.span)
+        else if tparams.nonEmpty && stat.tparams.nonEmpty then
+          syntaxError(i"extension method cannot have type parameters since some were already given previously",
+            stat.tparams.head.span)
+        else if stat.rhs.isEmpty then
+          syntaxError(i"extension method cannot be abstract", stat.span)
+      case EmptyTree =>
+      case stat =>
+        syntaxError(i"extension clause can only define methods", stat.span)
+    }
+
     /** GivenDef          ::=  [GivenSig] [‘_’ ‘<:’] Type ‘=’ Expr
      *                      |  [GivenSig] ConstrApps [TemplateBody]
      *  GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClauses} ‘as’
@@ -3516,22 +3537,27 @@ object Parsers {
       finalizeDef(gdef, mods1, start)
     }
 
-    /** ExtensionDef  ::=  [id] ‘on’ ExtParamClause {UsingParamClause} ExtMethods
+    /** ExtensionDef  ::=  [id] [‘on’ ExtParamClause {UsingParamClause}] TemplateBody
      */
     def extensionDef(start: Offset, mods: Modifiers): ModuleDef =
       in.nextToken()
       val name = if isIdent && !isIdent(nme.on) then ident() else EmptyTermName
       in.endMarkerScope(if name.isEmpty then nme.extension else name) {
-        if !isIdent(nme.on) then syntaxErrorOrIncomplete("`on` expected")
-        if isIdent(nme.on) then in.nextToken()
-        val tparams = typeParamClauseOpt(ParamOwner.Def)
-        val extParams = paramClause(0, prefix = true)
-        val givenParamss = paramClauses(givenOnly = true)
+        val (tparams, vparamss, extensionFlag) =
+          if isIdent(nme.on) then
+            in.nextToken()
+            val tparams = typeParamClauseOpt(ParamOwner.Def)
+            val extParams = paramClause(0, prefix = true)
+            val givenParamss = paramClauses(givenOnly = true)
+            (tparams, extParams :: givenParamss, Extension)
+          else
+            (Nil, Nil, EmptyFlags)
         possibleTemplateStart()
         if !in.isNestedStart then syntaxError("Extension without extension methods")
-        val templ = templateBodyOpt(makeConstructor(tparams, extParams :: givenParamss), Nil, Nil)
+        val templ = templateBodyOpt(makeConstructor(tparams, vparamss), Nil, Nil)
+        templ.body.foreach(checkExtensionMethod(tparams, vparamss, _))
         val edef = ModuleDef(name, templ)
-        finalizeDef(edef, addFlag(mods, Given), start)
+        finalizeDef(edef, addFlag(mods, Given | extensionFlag), start)
       }
 
 /* -------- TEMPLATES ------------------------------------------- */

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

@@ -220,4 +220,7 @@ class SymUtils(val self: Symbol) extends AnyVal {
   /** Is symbol a splice operation? */
   def isSplice(implicit ctx: Context): Boolean =
     self == defn.InternalQuoted_exprSplice || self == defn.QuotedType_splice
+
+  def isCollectiveExtensionClass(using Context): Boolean =
+    self.is(ModuleClass) && self.sourceModule.is(Extension, butNot = Method)
 }

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

@@ -205,7 +205,7 @@ trait ImportSuggestions:
         .alternatives
         .map(mbr => TermRef(site, mbr.symbol))
         .filter(ref =>
-          ref.symbol.is(Extension)
+          ref.symbol.isAllOf(ExtensionMethod)
           && isApplicableMethodRef(ref, argType :: Nil, WildcardType))
         .headOption
 

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

@@ -1096,7 +1096,8 @@ class Namer { typer: Typer =>
                     (StableRealizable, ExprType(path.tpe.select(sym)))
                   else
                     (EmptyFlags, mbr.info.ensureMethodic)
-                val mbrFlags = Exported | Method | Final | maybeStable | sym.flags & RetainedExportFlags
+                var mbrFlags = Exported | Method | Final | maybeStable | sym.flags & RetainedExportFlags
+                if sym.isAllOf(ExtensionMethod) then mbrFlags |= Extension
                 val forwarderName = checkNoConflict(alias, isPrivate = false, span)
                 ctx.newSymbol(cls, forwarderName, mbrFlags, mbrInfo, coord = span)
               }

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

@@ -170,7 +170,7 @@ object Nullables:
       case info :: infos1 =>
         if info.asserted.contains(ref) then true
         else if info.retracted.contains(ref) then false
-        else impliesNotNull(infos1)(ref)
+        else infos1.impliesNotNull(ref)
       case _ =>
         false
 

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

@@ -394,9 +394,9 @@ object RefChecks {
         overrideError("is erased, cannot override non-erased member")
       else if (other.is(Erased) && !member.isOneOf(Erased | Inline)) // (1.9.1)
         overrideError("is not erased, cannot override erased member")
-      else if (member.is(Extension) && !other.is(Extension)) // (1.9.2)
+      else if (member.isAllOf(ExtensionMethod) && !other.isAllOf(ExtensionMethod)) // (1.9.2)
         overrideError("is an extension method, cannot override a normal method")
-      else if (other.is(Extension) && !member.is(Extension)) // (1.9.2)
+      else if (other.isAllOf(ExtensionMethod) && !member.isAllOf(ExtensionMethod)) // (1.9.2)
         overrideError("is a normal method, cannot override an extension method")
       else if ((member.isInlineMethod || member.isScala2Macro) && other.is(Deferred) &&
                  member.extendedOverriddenSymbols.forall(_.is(Deferred))) // (1.10)

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

@@ -401,6 +401,19 @@ class Typer extends Namer
     if (name == nme.ROOTPKG)
       return tree.withType(defn.RootPackage.termRef)
 
+    /** Convert a reference `f` to an extension method in a collective extension
+     *  on parameter `x` to `x.f`
+     */
+    def extensionMethodSelect(xmethod: Symbol): untpd.Tree =
+      val leadParamName = xmethod.info.paramNamess.head.head
+      def isLeadParam(sym: Symbol) =
+        sym.is(Param) && sym.owner.owner == xmethod.owner && sym.name == leadParamName
+      def leadParam(ctx: Context): Symbol =
+        ctx.scope.lookupAll(leadParamName).find(isLeadParam) match
+          case Some(param) => param
+          case None => leadParam(ctx.outersIterator.dropWhile(_.scope eq ctx.scope).next)
+      untpd.cpy.Select(tree)(untpd.ref(leadParam(ctx).termRef), name)
+
     val rawType = {
       val saved1 = unimported
       val saved2 = foundUnderScala2
@@ -441,8 +454,14 @@ class Typer extends Namer
         errorType(new MissingIdent(tree, kind, name.show), tree.sourcePos)
 
     val tree1 = ownType match {
-      case ownType: NamedType if !prefixIsElidable(ownType) =>
-        ref(ownType).withSpan(tree.span)
+      case ownType: NamedType =>
+        val sym = ownType.symbol
+        if sym.isAllOf(ExtensionMethod)
+            && sym.owner.isCollectiveExtensionClass
+            && ctx.owner.isContainedIn(sym.owner)
+        then typed(extensionMethodSelect(sym), pt)
+        else if prefixIsElidable(ownType) then tree.withType(ownType)
+        else ref(ownType).withSpan(tree.span)
       case _ =>
         tree.withType(ownType)
     }

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1840,74 +1840,6 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         assertEquals("given x @ String", x.show)
       }
 
-  @Test def extensionMethodsNotAllowed =
-    checkMessagesAfter(RefChecks.name) {
-      """object Test {
-        |  extension on[T] (t: T) {
-        |    def (c: T).f: T = ???
-        |  }
-        |}
-      """.stripMargin
-    }
-      .expect { (ictx, messages) ⇒
-        implicit val ctx: Context = ictx
-        assertMessageCount(1, messages)
-        val errorMsg = messages.head.msg
-        val NoExtensionMethodAllowed(x) :: Nil = messages
-        assertEquals("No extension method allowed here, since collective parameters are given", errorMsg)
-        assertEquals("def (c: T) f: T = ???", x.show)
-      }
-
-  @Test def extensionMethodTypeParamsNotAllowed =
-    checkMessagesAfter(RefChecks.name) {
-      """object Test {
-        |  extension on[T] (t: T) {
-        |    def f[U](u: U): T = ???
-        |  }
-        |}
-      """.stripMargin
-    }
-      .expect { (ictx, messages) ⇒
-        implicit val ctx: Context = ictx
-        assertMessageCount(1, messages)
-        val errorMsg = messages.head.msg
-        val ExtensionMethodCannotHaveTypeParams(x) :: Nil = messages
-        assertEquals("Extension method cannot have type parameters since some were already given previously", errorMsg)
-        assertEquals("def f[U](u: U): T = ???", x.show)
-      }
-
-  @Test def extensionMethodCanOnlyHaveDefs =
-    checkMessagesAfter(RefChecks.name) {
-      """object Test {
-        |  extension on[T] (t: T) {
-        |    val v: T = t
-        |  }
-        |}
-      """.stripMargin
-    }
-      .expect { (ictx, messages) ⇒
-        implicit val ctx: Context = ictx
-        assertMessageCount(1, messages)
-        val errorMsg = messages.head.msg
-        val ExtensionCanOnlyHaveDefs(x) :: Nil = messages
-        assertEquals("Only methods allowed here, since collective parameters are given", errorMsg)
-        assertEquals("val v: T = t", x.show)
-      }
-
-  @Test def anonymousInstanceMustImplementAType =
-    checkMessagesAfter(RefChecks.name) {
-      """object Test {
-        |  extension on[T] (t: T) { }
-        |}
-      """.stripMargin
-    }
-      .expect { (ictx, messages) ⇒
-        implicit val ctx: Context = ictx
-        assertMessageCount(1, messages)
-        val errorMsg = messages.head.msg
-        assertEquals("anonymous instance must implement a type or have at least one extension method", errorMsg)
-      }
-
   @Test def typeSplicesInValPatterns =
     checkMessagesAfter(RefChecks.name) {
       s"""import scala.quoted._

docs/docs/internals/syntax.md

@@ -386,7 +386,8 @@ EnumDef           ::=  id ClassConstr InheritClauses EnumBody
 GivenDef          ::=  [GivenSig] [‘_’ ‘<:’] Type ‘=’ Expr
                     |  [GivenSig] ConstrApps [TemplateBody]
 GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘as’
-ExtensionDef      ::=  [id] ‘on’ ExtParamClause {WithParamsOrTypes} ExtMethods
+ExtensionDef      ::=  [id] [‘on’ ExtParamClause {UsingParamClause}]
+                       TemplateBody
 ExtMethods        ::=  [nl] ‘{’ ‘def’ DefDef {semi ‘def’ DefDef} ‘}’
 ExtParamClause    ::=  [DefTypeParamClause] ‘(’ DefParam ‘)’
 Template          ::=  InheritClauses [TemplateBody]                            Template(constr, parents, self, stats)