Insert traits with implicit parameters as extra parents of classes

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

@@ -80,5 +80,10 @@ object TypeUtils {
       case self: TypeProxy =>
         self.underlying.companionRef
     }
+
+    /** Is this type a methodic type that takes implicit parameters (both old and new) at some point? */
+    def takesImplicitParams(using Context): Boolean = self.stripPoly match
+      case mt: MethodType => mt.isImplicitMethod || mt.resType.takesImplicitParams
+      case _ => false
   }
 }

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

@@ -1237,12 +1237,81 @@ class Namer { typer: Typer =>
         }
       }
 
+      /** Ensure that the first type in a list of parent types Ps points to a non-trait class.
+       *  If that's not already the case, add one. The added class type CT is determined as follows.
+       *  First, let C be the unique class such that
+       *  - there is a parent P_i such that P_i derives from C, and
+       *  - for every class D: If some parent P_j, j <= i derives from D, then C derives from D.
+       *  Then, let CT be the smallest type which
+       *  - has C as its class symbol, and
+       *  - for all parents P_i: If P_i derives from C then P_i <:< CT.
+       */
+      def ensureFirstIsClass(parents: List[Type]): List[Type] =
+
+        def realClassParent(sym: Symbol): ClassSymbol =
+          if !sym.isClass then defn.ObjectClass
+          else if !sym.is(Trait) then sym.asClass
+          else sym.info.parents match
+            case parentRef :: _ => realClassParent(parentRef.typeSymbol)
+            case nil => defn.ObjectClass
+
+        def improve(candidate: ClassSymbol, parent: Type): ClassSymbol =
+          val pcls = realClassParent(parent.classSymbol)
+          if (pcls derivesFrom candidate) pcls else candidate
+
+        parents match
+          case p :: _ if p.classSymbol.isRealClass => parents
+          case _ =>
+            val pcls = parents.foldLeft(defn.ObjectClass)(improve)
+            typr.println(i"ensure first is class $parents%, % --> ${parents map (_ baseType pcls)}%, %")
+            val first = TypeComparer.glb(defn.ObjectType :: parents.map(_.baseType(pcls)))
+            checkFeasibleParent(first, cls.srcPos, em" in inferred superclass $first") :: parents
+      end ensureFirstIsClass
+
+      /** If `parents` contains references to traits that have supertraits with implicit parameters
+       *  add those supertraits in linearization order unless they are already covered by other
+       *  parent types. For instance, in
+       *
+       *    class A
+       *    trait B(using I) extends A
+       *    trait C extends B
+       *    class D extends A, C
+       *
+       *  the class declaration of `D` is augmented to
+       *
+       *    class D extends A, B, C
+       *
+       *  so that an implicit `I` can be passed to `B`. See i7613.scala for more examples.
+       */
+      def addUsingTraits(parents: List[Type]): List[Type] =
+        lazy val existing = parents.map(_.classSymbol).toSet
+        def recur(parents: List[Type]): List[Type] = parents match
+          case parent :: parents1 =>
+            val psym = parent.classSymbol
+            val addedTraits =
+              if psym.is(Trait) then
+                psym.asClass.baseClasses.tail.iterator
+                  .takeWhile(_.is(Trait))
+                  .filter(p =>
+                    p.primaryConstructor.info.takesImplicitParams
+                    && !cls.superClass.isSubClass(p)
+                    && !existing.contains(p))
+                  .toList.reverse
+              else Nil
+            addedTraits.map(parent.baseType) ::: parent :: recur(parents1)
+          case nil =>
+            Nil
+        if cls.isRealClass then recur(parents) else parents
+      end addUsingTraits
+
       completeConstructor(denot)
       denot.info = tempInfo
 
       val parentTypes = defn.adjustForTuple(cls, cls.typeParams,
         defn.adjustForBoxedUnit(cls,
-          ensureFirstIsClass(parents.map(checkedParentType(_)), cls.span)
+          addUsingTraits(
+            ensureFirstIsClass(parents.map(checkedParentType(_)))
+          )
         )
       )
       typr.println(i"completing $denot, parents = $parents%, %, parentTypes = $parentTypes%, %")

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

@@ -103,8 +103,8 @@ class ReTyper extends Typer with ReChecking {
 
   override def completeAnnotations(mdef: untpd.MemberDef, sym: Symbol)(using Context): Unit = ()
 
-  override def ensureConstrCall(cls: ClassSymbol, parents: List[Tree])(using Context): List[Tree] =
-    parents
+  override def ensureConstrCall(cls: ClassSymbol, parent: Tree)(using Context): Tree =
+    parent
 
   override def handleUnexpectedFunType(tree: untpd.Apply, fun: Tree)(using Context): Tree = fun.tpe match {
     case mt: MethodType =>

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

@@ -2212,20 +2212,18 @@ class Typer extends Namer
      *  @param psym  Its type symbol
      *  @param cinfo The info of its constructor
      */
-    def maybeCall(ref: Tree, psym: Symbol, cinfo: Type): Tree = cinfo.stripPoly match {
+    def maybeCall(ref: Tree, psym: Symbol): Tree = psym.primaryConstructor.info.stripPoly match
       case cinfo @ MethodType(Nil) if cinfo.resultType.isImplicitMethod =>
         typedExpr(untpd.New(untpd.TypedSplice(ref)(using superCtx), Nil))(using superCtx)
       case cinfo @ MethodType(Nil) if !cinfo.resultType.isInstanceOf[MethodType] =>
         ref
       case cinfo: MethodType =>
-        if (!ctx.erasedTypes) { // after constructors arguments are passed in super call.
+        if !ctx.erasedTypes then // after constructors arguments are passed in super call.
           typr.println(i"constr type: $cinfo")
           report.error(ParameterizedTypeLacksArguments(psym), ref.srcPos)
-        }
         ref
       case _ =>
         ref
-    }
 
     val seenParents = mutable.Set[Symbol]()
 
@@ -2250,14 +2248,35 @@ class Typer extends Namer
       if (tree.isType) {
         checkSimpleKinded(result) // Not needed for constructor calls, as type arguments will be inferred.
         if (psym.is(Trait) && !cls.is(Trait) && !cls.superClass.isSubClass(psym))
-          result = maybeCall(result, psym, psym.primaryConstructor.info)
+          result = maybeCall(result, psym)
       }
       else checkParentCall(result, cls)
       checkTraitInheritance(psym, cls, tree.srcPos)
       if (cls is Case) checkCaseInheritance(psym, cls, tree.srcPos)
       result
     }
 
+    /** Augment `ptrees` to have the same class symbols as `parents`. Generate TypeTrees
+     *  or New trees to fill in any parents for which no tree exists yet.
+     */
+    def parentTrees(parents: List[Type], ptrees: List[Tree]): List[Tree] = parents match
+      case parent :: parents1 =>
+        val psym = parent.classSymbol
+        def hasSameParent(ptree: Tree) = ptree.tpe.classSymbol == psym
+        ptrees match
+          case ptree :: ptrees1 if hasSameParent(ptree) =>
+            ptree :: parentTrees(parents1, ptrees1)
+          case ptree :: ptrees1 if ptrees1.exists(hasSameParent) =>
+            ptree :: parentTrees(parents, ptrees1)
+          case _ =>
+            var added: Tree = TypeTree(parent).withSpan(cdef.nameSpan.focus)
+            if psym.is(Trait) && psym.primaryConstructor.info.takesImplicitParams then
+              // classes get a constructor separately using a different context
+              added = ensureConstrCall(cls, added)
+            added :: parentTrees(parents1, ptrees)
+      case _ =>
+        ptrees
+
     /** Checks if one of the decls is a type with the same name as class type member in selfType */
     def classExistsOnSelf(decls: Scope, self: tpd.ValDef): Boolean = {
       val selfType = self.tpt.tpe
@@ -2278,8 +2297,10 @@ class Typer extends Namer
 
     completeAnnotations(cdef, cls)
     val constr1 = typed(constr).asInstanceOf[DefDef]
-    val parentsWithClass = ensureFirstTreeIsClass(parents.mapconserve(typedParent).filterConserve(!_.isEmpty), cdef.nameSpan)
-    val parents1 = ensureConstrCall(cls, parentsWithClass)(using superCtx)
+    val parents0 = parentTrees(
+        cls.classInfo.declaredParents,
+        parents.mapconserve(typedParent).filterConserve(!_.isEmpty))
+    val parents1 = ensureConstrCall(cls, parents0)(using superCtx)
     val firstParentTpe = parents1.head.tpe.dealias
     val firstParent = firstParentTpe.typeSymbol
 
@@ -2348,52 +2369,23 @@ class Typer extends Namer
   protected def addAccessorDefs(cls: Symbol, body: List[Tree])(using Context): List[Tree] =
     ctx.compilationUnit.inlineAccessors.addAccessorDefs(cls, body)
 
-  /** Ensure that the first type in a list of parent types Ps points to a non-trait class.
-   *  If that's not already the case, add one. The added class type CT is determined as follows.
-   *  First, let C be the unique class such that
-   *  - there is a parent P_i such that P_i derives from C, and
-   *  - for every class D: If some parent P_j, j <= i derives from D, then C derives from D.
-   *  Then, let CT be the smallest type which
-   *  - has C as its class symbol, and
-   *  - for all parents P_i: If P_i derives from C then P_i <:< CT.
+  /** If this is a real class, make sure its first parent is a
+   *  constructor call. Cannot simply use a type. Overridden in ReTyper.
    */
-  def ensureFirstIsClass(parents: List[Type], span: Span)(using Context): List[Type] = {
-    def realClassParent(cls: Symbol): ClassSymbol =
-      if (!cls.isClass) defn.ObjectClass
-      else if (!cls.is(Trait)) cls.asClass
-      else cls.info.parents match {
-        case parentRef :: _ => realClassParent(parentRef.typeSymbol)
-        case nil => defn.ObjectClass
-      }
-    def improve(candidate: ClassSymbol, parent: Type): ClassSymbol = {
-      val pcls = realClassParent(parent.classSymbol)
-      if (pcls derivesFrom candidate) pcls else candidate
-    }
-    parents match {
-      case p :: _ if p.classSymbol.isRealClass => parents
-      case _ =>
-        val pcls = parents.foldLeft(defn.ObjectClass)(improve)
-        typr.println(i"ensure first is class $parents%, % --> ${parents map (_ baseType pcls)}%, %")
-        val first = TypeComparer.glb(defn.ObjectType :: parents.map(_.baseType(pcls)))
-        checkFeasibleParent(first, ctx.source.atSpan(span), em" in inferred superclass $first") :: parents
-    }
-  }
+  def ensureConstrCall(cls: ClassSymbol, parents: List[Tree])(using Context): List[Tree] = parents match
+    case parents @ (first :: others) =>
+      parents.derivedCons(ensureConstrCall(cls, first), others)
+    case parents =>
+      parents
 
-  /** Ensure that first parent tree refers to a real class. */
-  def ensureFirstTreeIsClass(parents: List[Tree], span: Span)(using Context): List[Tree] = parents match {
-    case p :: ps if p.tpe.classSymbol.isRealClass => parents
-    case _ => TypeTree(ensureFirstIsClass(parents.tpes, span).head).withSpan(span.focus) :: parents
-  }
-
-  /** If this is a real class, make sure its first parent is a
+ /** If this is a real class, make sure its first parent is a
    *  constructor call. Cannot simply use a type. Overridden in ReTyper.
    */
-  def ensureConstrCall(cls: ClassSymbol, parents: List[Tree])(using Context): List[Tree] = {
-    val firstParent :: otherParents = parents
-    if (firstParent.isType && !cls.is(Trait) && !cls.is(JavaDefined))
-      typed(untpd.New(untpd.TypedSplice(firstParent), Nil)) :: otherParents
-    else parents
-  }
+  def ensureConstrCall(cls: ClassSymbol, parent: Tree)(using Context): Tree =
+    if (parent.isType && !cls.is(Trait) && !cls.is(JavaDefined))
+      typed(untpd.New(untpd.TypedSplice(parent), Nil))
+    else
+      parent
 
   def localDummy(cls: ClassSymbol, impl: untpd.Template)(using Context): Symbol =
     newLocalDummy(cls, impl.span)

docs/docs/reference/other-new-features/trait-parameters.md

@@ -52,6 +52,36 @@ The correct way to write `E` is to extend both `Greeting` and
 class E extends Greeting("Bob"), FormalGreeting
 ```
 
+### Traits With Context Parameters
+
+This "explicit extension required" rule is relaxed if the missing trait contains only
+[context parameters](../contextual/using-clauses). In that case the trait reference is
+implicitly inserted as an additional parent with inferred arguments. For instance,
+here's a variant of greetings where the addressee is a context parameter of type
+`ImpliedName`:
+
+```scala
+case class ImpliedName(name: String):
+  override def toString = name
+
+trait ImpliedGreeting(using val iname: ImpliedName):
+   def msg = s"How are you, $iname"
+
+trait ImpliedFormalGreeting extends ImpliedGreeting:
+   override def msg = s"How do you do, $iname"
+
+class F(using iname: ImpliedName) extends ImpliedFormalGreeting
+```
+
+The definition of `F` in the last line is implicitly expanded to
+```scala
+class F(using iname: ImpliedName) extends
+   Object,
+   ImpliedGreeting(using iname),
+   ImpliedFormalGreeting(using iname)
+```
+Note the inserted reference to the super trait `ImpliedGreeting`, which was not mentioned explicitly.
+
 ## Reference
 
 For more information, see [Scala SIP 25](http://docs.scala-lang.org/sips/pending/trait-parameters.html).

tests/neg/i6060.scala

@@ -1,6 +1,6 @@
 class I1(i2: Int) {
   def apply(i3: Int) = 1
-  new I1(1)(2) {} // error: too many arguments in parent constructor
+  new I1(1)(2) {} // error: too many arguments in parent constructor // error
 }
 
 class I0(i1: Int) {

tests/neg/i7613.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg/i7613.scala:10:16 ----------------------------------------------------------------------------------
+10 |  new BazLaws[A] {}  // error // error
+   |                ^
+   |         no implicit argument of type Baz[A] was found for parameter x$1 of constructor BazLaws in trait BazLaws
+-- Error: tests/neg/i7613.scala:10:2 -----------------------------------------------------------------------------------
+10 |  new BazLaws[A] {}  // error // error
+   |  ^
+   |  no implicit argument of type Bar[A] was found for parameter x$1 of constructor BarLaws in trait BarLaws

tests/neg/i7613.scala

@@ -0,0 +1,11 @@
+trait Foo[A]
+trait Bar[A] extends Foo[A]
+trait Baz[A] extends Bar[A]
+
+trait FooLaws[A](using Foo[A])
+trait BarLaws[A](using Bar[A]) extends FooLaws[A]
+trait BazLaws[A](using Baz[A]) extends BarLaws[A]
+
+def instance[A](using Foo[A]): BazLaws[A] =
+  new BazLaws[A] {}  // error // error
+

tests/pos/reference/trait-parameters.scala

@@ -16,4 +16,13 @@ class E extends Greeting("Bob") with FormalGreeting
 
 // class D2 extends C with Greeting("Bill") // error
 
+case class ImpliedName(name: String):
+  override def toString = name
 
+trait ImpliedGreeting(using val iname: ImpliedName):
+   def msg = s"How are you, $iname"
+
+trait ImpliedFormalGreeting extends ImpliedGreeting:
+   override def msg = s"How do you do, $iname"
+
+class F(using iname: ImpliedName) extends ImpliedFormalGreeting

tests/run/i7613.check

@@ -0,0 +1,5 @@
+D: B1
+superD: B1
+E: B2
+F: B1
+F: B2

tests/run/i7613.scala

@@ -0,0 +1,29 @@
+trait Foo[A]
+trait Bar[A] extends Foo[A]
+trait Baz[A] extends Bar[A]
+
+trait FooLaws[A](using Foo[A])
+trait BarLaws[A](using Bar[A]) extends FooLaws[A]
+trait BazLaws[A](using Baz[A]) extends BarLaws[A]
+
+def instance1[A](using Baz[A]): BazLaws[A] =
+  new FooLaws[A] with BarLaws[A] with BazLaws[A] {}
+
+def instance2[A](using Baz[A]): BazLaws[A] =
+  new BazLaws[A] {}
+
+trait I:
+  def show(x: String): Unit
+class A
+trait B1(using I) extends A { summon[I].show("B1") }
+trait B2(using I) extends B1 { summon[I].show("B2") }
+trait C1 extends B1
+trait C2 extends B2
+class D(using I) extends A, C1
+class E(using I) extends D(using new I { def show(x: String) = println(s"superD: $x")}), C2
+class F(using I) extends A, C2
+
+@main def Test =
+  D(using new I { def show(x: String) = println(s"D: $x")})
+  E(using new I { def show(x: String) = println(s"E: $x")})
+  F(using new I { def show(x: String) = println(s"F: $x")})