Implement deferred givens

A definition like `given T = deferred` in a trait will be
expanded to an abstract given in the trait that is implemented
automatically in all classes inheriting the trait.

Author: odersky

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

@@ -240,6 +240,7 @@ class Definitions {
     @tu lazy val Compiletime_codeOf: Symbol = CompiletimePackageClass.requiredMethod("codeOf")
     @tu lazy val Compiletime_erasedValue  : Symbol = CompiletimePackageClass.requiredMethod("erasedValue")
     @tu lazy val Compiletime_uninitialized: Symbol = CompiletimePackageClass.requiredMethod("uninitialized")
+    @tu lazy val Compiletime_deferred     : Symbol = CompiletimePackageClass.requiredMethod("deferred")
     @tu lazy val Compiletime_error        : Symbol = CompiletimePackageClass.requiredMethod(nme.error)
     @tu lazy val Compiletime_requireConst : Symbol = CompiletimePackageClass.requiredMethod("requireConst")
     @tu lazy val Compiletime_constValue   : Symbol = CompiletimePackageClass.requiredMethod("constValue")

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

@@ -570,6 +570,7 @@ object Flags {
   val DeferredOrLazyOrMethod: FlagSet        = Deferred | Lazy | Method
   val DeferredOrTermParamOrAccessor: FlagSet = Deferred | ParamAccessor | TermParam           // term symbols without right-hand sides
   val DeferredOrTypeParam: FlagSet           = Deferred | TypeParam                           // type symbols without right-hand sides
+  val DeferredGivenFlags                     = Deferred | Given | HasDefault
   val EnumValue: FlagSet                     = Enum | StableRealizable                        // A Scala enum value
   val FinalOrInline: FlagSet                 = Final | Inline
   val FinalOrModuleClass: FlagSet            = Final | ModuleClass                            // A module class or a final class

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

@@ -452,6 +452,7 @@ object StdNames {
     val create: N               = "create"
     val currentMirror: N        = "currentMirror"
     val curried: N              = "curried"
+    val deferred: N             = "deferred"
     val definitions: N          = "definitions"
     val delayedInit: N          = "delayedInit"
     val delayedInitArg: N       = "delayedInit$body"

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

@@ -567,7 +567,13 @@ object Erasure {
           case Some(annot) =>
             val message = annot.argumentConstant(0) match
               case Some(c) =>
-                c.stringValue.toMessage
+                val addendum = tree match
+                  case tree: RefTree
+                  if tree.symbol == defn.Compiletime_deferred && tree.name != nme.deferred =>
+                    i".\nNote that `deferred` can only be used under its own name when implementing a given in a trait; `${tree.name}` is not accepted."
+                  case _ =>
+                    ""
+                (c.stringValue ++ addendum).toMessage
               case _ =>
                 em"""Reference to ${tree.symbol.showLocated} should not have survived,
                     |it should have been processed and eliminated during expansion of an enclosing macro or term erasure."""

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

@@ -922,10 +922,10 @@ trait Implicits:
 
 
   /** Search an implicit argument and report error if not found */
-  def implicitArgTree(formal: Type, span: Span)(using Context): Tree = {
+  def implicitArgTree(formal: Type, span: Span, where: => String = "")(using Context): Tree = {
     val arg = inferImplicitArg(formal, span)
     if (arg.tpe.isInstanceOf[SearchFailureType])
-      report.error(missingArgMsg(arg, formal, ""), ctx.source.atSpan(span))
+      report.error(missingArgMsg(arg, formal, where), ctx.source.atSpan(span))
     arg
   }
 

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

@@ -1827,6 +1827,18 @@ class Namer { typer: Typer =>
       case _ =>
         WildcardType
     }
+
+    // translate `given T = deferred` to an abstract given with HasDefault flag
+    if sym.is(Given) then
+      mdef.rhs match
+        case rhs: RefTree
+        if rhs.name == nme.deferred
+            && typedAheadExpr(rhs).symbol == defn.Compiletime_deferred
+            && sym.maybeOwner.is(Trait) =>
+          sym.resetFlag(Final)
+          sym.setFlag(Deferred | HasDefault)
+        case _ =>
+
     val mbrTpe = paramFn(checkSimpleKinded(typedAheadType(mdef.tpt, tptProto)).tpe)
     if (ctx.explicitNulls && mdef.mods.is(JavaDefined))
       JavaNullInterop.nullifyMember(sym, mbrTpe, mdef.mods.isAllOf(JavaEnumValue))

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

@@ -552,7 +552,7 @@ object RefChecks {
         overrideError("is an extension method, cannot override a normal method")
       else if (other.is(ExtensionMethod) && !member.is(ExtensionMethod)) // (1.3)
         overrideError("is a normal method, cannot override an extension method")
-      else if !other.is(Deferred)
+      else if (!other.is(Deferred) || other.isAllOf(Given | HasDefault))
             && !member.is(Deferred)
             && !other.name.is(DefaultGetterName)
             && !member.isAnyOverride

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

@@ -2569,12 +2569,17 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val ValDef(name, tpt, _) = vdef
     checkNonRootName(vdef.name, vdef.nameSpan)
     completeAnnotations(vdef, sym)
-    if (sym.isOneOf(GivenOrImplicit)) checkImplicitConversionDefOK(sym)
+    if sym.is(Implicit) then checkImplicitConversionDefOK(sym)
     if sym.is(Module) then checkNoModuleClash(sym)
     val tpt1 = checkSimpleKinded(typedType(tpt))
     val rhs1 = vdef.rhs match {
-      case rhs @ Ident(nme.WILDCARD) => rhs withType tpt1.tpe
-      case rhs => typedExpr(rhs, tpt1.tpe.widenExpr)
+      case rhs @ Ident(nme.WILDCARD) =>
+        rhs.withType(tpt1.tpe)
+      case rhs: RefTree
+      if rhs.name == nme.deferred && sym.isAllOf(DeferredGivenFlags, butNot = Param) =>
+        EmptyTree
+      case rhs =>
+        typedExpr(rhs, tpt1.tpe.widenExpr)
     }
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
     postProcessInfo(vdef1, sym)
@@ -2635,9 +2640,13 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
     if sym.isInlineMethod then rhsCtx.addMode(Mode.InlineableBody)
     if sym.is(ExtensionMethod) then rhsCtx.addMode(Mode.InExtensionMethod)
-    val rhs1 = PrepareInlineable.dropInlineIfError(sym,
-      if sym.isScala2Macro then typedScala2MacroBody(ddef.rhs)(using rhsCtx)
-      else typedExpr(ddef.rhs, tpt1.tpe.widenExpr)(using rhsCtx))
+    val rhs1 = ddef.rhs match
+      case Ident(nme.deferred) if sym.isAllOf(DeferredGivenFlags) =>
+        EmptyTree
+      case rhs =>
+        PrepareInlineable.dropInlineIfError(sym,
+          if sym.isScala2Macro then typedScala2MacroBody(ddef.rhs)(using rhsCtx)
+          else typedExpr(ddef.rhs, tpt1.tpe.widenExpr)(using rhsCtx))
 
     if sym.isInlineMethod then
       if StagingLevel.level > 0 then
@@ -2818,6 +2827,59 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         case None =>
           body
 
+    /** Implement givens that were declared with a `deferred` rhs.
+     *  The a given value matching the declared type is searched in a
+     *  context directly enclosing the current class, in which all given
+     *  parameters of the current class are also defined.
+     */
+    def implementDeferredGivens(body: List[Tree]): List[Tree] =
+      if cls.is(Trait) || ctx.isAfterTyper then body
+      else
+        def isGivenValue(mbr: TermRef) =
+          val dcl = mbr.symbol
+          if dcl.is(Method) then
+            report.error(
+              em"""Cannnot infer the implementation of the deferred ${dcl.showLocated}
+                  |since that given is parameterized. An implementing given needs to be written explicitly.""",
+              cdef.srcPos)
+            false
+          else true
+
+        def givenImpl(mbr: TermRef): ValDef =
+          val dcl = mbr.symbol
+          val target = dcl.info.asSeenFrom(cls.thisType, dcl.owner)
+          val constr = cls.primaryConstructor
+          val usingParamAccessors = cls.paramAccessors.filter(_.is(Given))
+          val paramScope = newScopeWith(usingParamAccessors*)
+          val searchCtx = ctx.outer.fresh.setScope(paramScope)
+          val rhs = implicitArgTree(target, cdef.span,
+              where = i"inferring the implementation of the deferred ${dcl.showLocated}"
+            )(using searchCtx)
+
+          val impl = dcl.copy(cls,
+            flags = dcl.flags &~ (HasDefault | Deferred) | Final | Override,
+            info = target,
+            coord = rhs.span).entered.asTerm
+
+          def anchorParams = new TreeMap:
+            override def transform(tree: Tree)(using Context): Tree = tree match
+              case id: Ident if usingParamAccessors.contains(id.symbol) =>
+                cpy.Select(id)(This(cls), id.name)
+              case _ =>
+                super.transform(tree)
+          ValDef(impl, anchorParams.transform(rhs))
+        end givenImpl
+
+        val givenImpls =
+          cls.thisType.implicitMembers
+            //.showing(i"impl def givens for $cls/$result")
+            .filter(_.symbol.isAllOf(DeferredGivenFlags, butNot = Param))
+            //.showing(i"impl def filtered givens for $cls/$result")
+            .filter(isGivenValue)
+            .map(givenImpl)
+        body ++ givenImpls
+    end implementDeferredGivens
+
     ensureCorrectSuperClass()
     completeAnnotations(cdef, cls)
     val constr1 = typed(constr).asInstanceOf[DefDef]
@@ -2839,9 +2901,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     else {
       val dummy = localDummy(cls, impl)
       val body1 =
-        addParentRefinements(
-          addAccessorDefs(cls,
-            typedStats(impl.body, dummy)(using ctx.inClassContext(self1.symbol))._1))
+        implementDeferredGivens(
+          addParentRefinements(
+            addAccessorDefs(cls,
+              typedStats(impl.body, dummy)(using ctx.inClassContext(self1.symbol))._1)))
 
       checkNoDoubleDeclaration(cls)
       val impl1 = cpy.Template(impl)(constr1, parents1, Nil, self1, body1)

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

@@ -102,7 +102,7 @@ i13842.scala
 # Position change under captureChecking
 boxmap-paper.scala
 
-# Function types print differnt after unpickling since test mispredicts Feature.preFundsEnabled
+# Function types print different after unpickling since test mispredicts Feature.preFundsEnabled
 caps-universal.scala
 
 # GADT cast applied to singleton type difference
@@ -124,6 +124,8 @@ i15525.scala
 parsercombinators-givens.scala
 parsercombinators-givens-2.scala
 parsercombinators-arrow.scala
+hylolib-deferred-given
+
 
 
 

library/src/scala/compiletime/package.scala

@@ -42,6 +42,19 @@ def erasedValue[T]: T = erasedValue[T]
 @compileTimeOnly("`uninitialized` can only be used as the right hand side of a mutable field definition")
 def uninitialized: Nothing = ???
 
+/** Used as the right hand side of a given in a trait, like this
+ *
+ *  ```
+ *  given T = deferred
+ *  ```
+ *
+ *  This signifies that the given will get a synthesized definition in all classes
+ *  that implement the enclosing trait and that do not contain an explicit overriding
+ *  definition of that given.
+ */
+@compileTimeOnly("`deferred` can only be used as the right hand side of a given definition in a trait")
+def deferred: Nothing = ???
+
 /** The error method is used to produce user-defined compile errors during inline expansion.
  *  If an inline expansion results in a call error(msgStr) the compiler produces an error message containing the given msgStr.
  *