Mirrors can now be materialized with additional type member refinements.

Any additional type members will contribute to the implicit scope of any
searches involving the type of the summoned Mirror.  Typically such
types will be the singleton types of objects containing implicit
extension methods for the Mirror, or other related implicit definitions.

Pass through all extra refinements

Author: milessabin

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

@@ -884,14 +884,38 @@ trait Implicits { self: Typer =>
    *
    *     <parent> {
    *       MirroredMonoType = <monoType>
-   *       MirroredTypeConstrictor = <tycon>
-   *       MirroredLabel = <label> }
+   *       MirroredType = <mirroredType>
+   *       MirroredLabel = <label>
+   *       Scope = <scope> // if scope ne NoType
+   *     }
    */
-  private def mirrorCore(parent: Type, monoType: Type, mirroredType: Type, label: Name)(implicit ctx: Context) = {
-    parent
+  private def mirrorCore(parent: Type, monoType: Type, mirroredType: Type, label: Name, extras: List[(Name, Type)])(implicit ctx: Context) = {
+    val m = parent
       .refinedWith(tpnme.MirroredMonoType, TypeAlias(monoType))
       .refinedWith(tpnme.MirroredType, TypeAlias(mirroredType))
       .refinedWith(tpnme.MirroredLabel, TypeAlias(ConstantType(Constant(label.toString))))
+
+    extras.foldLeft(m) { case (m, (tpnme, tpe)) => m.refinedWith(tpnme, tpe) }
+  }
+
+  private def extraRefinements(formal: Type)(implicit ctx: Context): List[(Name, Type)] = {
+    def isMirrorMember(nme: Name): Boolean =
+      nme == tpnme.MirroredType ||
+      nme == tpnme.MirroredMonoType ||
+      nme == tpnme.MirroredLabel ||
+      nme == tpnme.MirroredElemTypes ||
+      nme == tpnme.MirroredElemLabels
+
+    @tailrec
+    def loop(tp: Type, acc: List[(Name, Type)]): List[(Name, Type)] = tp match {
+      case RefinedType(parent, nme, rhs) if !isMirrorMember(nme) =>
+        loop(parent, (nme, rhs) :: acc)
+      case RefinedType(parent, nme, rhs) =>
+        loop(parent, acc)
+      case other =>
+        acc
+    }
+    loop(formal, Nil)
   }
 
   /** A path referencing the companion of class type `clsType` */
@@ -916,12 +940,12 @@ trait Implicits { self: Typer =>
               val module = mirroredType.termSymbol
               val modulePath = pathFor(mirroredType).withSpan(span)
               if (module.info.classSymbol.is(Scala2x)) {
-                val mirrorType = mirrorCore(defn.Mirror_SingletonProxyType, mirroredType, mirroredType, module.name)
+                val mirrorType = mirrorCore(defn.Mirror_SingletonProxyType, mirroredType, mirroredType, module.name, extraRefinements(formal))
                 val mirrorRef = New(defn.Mirror_SingletonProxyType, modulePath :: Nil)
                 mirrorRef.cast(mirrorType)
               }
               else {
-                val mirrorType = mirrorCore(defn.Mirror_SingletonType, mirroredType, mirroredType, module.name)
+                val mirrorType = mirrorCore(defn.Mirror_SingletonType, mirroredType, mirroredType, module.name, extraRefinements(formal))
                 modulePath.cast(mirrorType)
               }
             }
@@ -943,7 +967,7 @@ trait Implicits { self: Typer =>
                   (mirroredType, elems)
               }
               val mirrorType =
-                mirrorCore(defn.Mirror_ProductType, monoType, mirroredType, cls.name)
+                mirrorCore(defn.Mirror_ProductType, monoType, mirroredType, cls.name, extraRefinements(formal))
                   .refinedWith(tpnme.MirroredElemTypes, TypeAlias(elemsType))
                   .refinedWith(tpnme.MirroredElemLabels, TypeAlias(TypeOps.nestedPairs(elemLabels)))
               val mirrorRef =
@@ -1024,7 +1048,7 @@ trait Implicits { self: Typer =>
             }
 
             val mirrorType =
-               mirrorCore(defn.Mirror_SumType, monoType, mirroredType, cls.name)
+               mirrorCore(defn.Mirror_SumType, monoType, mirroredType, cls.name, extraRefinements(formal))
                 .refinedWith(tpnme.MirroredElemTypes, TypeAlias(elemsType))
                 .refinedWith(tpnme.MirroredElemLabels, TypeAlias(TypeOps.nestedPairs(elemLabels)))
             val mirrorRef =

tests/pos/mirror-implicit-scope.scala

@@ -0,0 +1,27 @@
+import scala.deriving._
+
+object Test {
+  object K0 {
+    type Generic[T] = Mirror { type Scope = K0.type ; type MirroredType = T ; type MirroredElemTypes }
+    given Ops {
+      inline def (gen: Generic[T]) toRepr[T <: Product](t: T): gen.MirroredElemTypes = Tuple.fromProduct(t).asInstanceOf
+    }
+  }
+
+  object K1 {
+    type Generic[F[_]] = Mirror { type Scope = K1.type ; type MirroredType = F ; type MirroredElemTypes[_] }
+    given Ops {
+      inline def (gen: Generic[F]) toRepr[F[_] <: Product, T](t: F[T]): gen.MirroredElemTypes[T] = Tuple.fromProduct(t).asInstanceOf
+    }
+  }
+
+  case class ISB(i: Int, s: String, b: Boolean)
+  val v0 = the[K0.Generic[ISB]]
+  val v1 = v0.toRepr(ISB(23, "foo", true))
+  val v2: (Int, String, Boolean) = v1
+
+  case class Box[T](t: T)
+  val v3 = the[K1.Generic[Box]]
+  val v4 = v3.toRepr(Box(23))
+  val v5: Tuple1[Int] = v4
+}