Fix compilation of lenses with context bounds in Scala 3

Fixes #1259

Author: NTPape

core/shared/src/main/scala-3.x/monocle/internal/focus/features/GeneratorLoop.scala

@@ -14,6 +14,7 @@ import scala.quoted.Type
 
 private[focus] trait AllFeatureGenerators
     extends FocusBase
+    with SelectGeneratorBase
     with SelectFieldGenerator
     with SelectOnlyFieldGenerator
     with SomeGenerator

core/shared/src/main/scala-3.x/monocle/internal/focus/features/SelectGeneratorBase.scala

@@ -0,0 +1,45 @@
+package monocle.internal.focus.features
+
+import monocle.internal.focus.FocusBase
+import scala.annotation.tailrec
+
+private[focus] trait SelectGeneratorBase {
+  this: FocusBase =>
+
+  import this.macroContext.reflect._
+
+  def generateGetter(from: Term, fieldName: String): Term =
+    Select.unique(from, fieldName) // o.field
+
+  @tailrec
+  final def etaExpandIfNecessary(term: Term): Term = {
+    if (term.isExpr) {
+      term
+    } else {
+      val expanded: Term = term.etaExpand(Symbol.spliceOwner)
+
+      val implicits: List[Term] = expanded match {
+        case Block(List(DefDef(_, List(params), _, _)), _) =>
+          params.params.map {
+            case ValDef(_, t, _) =>
+              val typeRepr: TypeRepr = t.tpe.dealias
+              Implicits.search(typeRepr) match {
+                case success: ImplicitSearchSuccess => success.tree
+                case _ => report.errorAndAbort(
+                  s"Couldn't find assumed implicit for ${typeRepr.show}. Neither " +
+                    s"multiple (non-implicit) parameter sets nor default arguments for implicits are supported."
+                )
+              }
+            case other => report.errorAndAbort(
+              s"Expected a value definition as parameter but found $other."
+            )
+          }
+        case other => report.errorAndAbort(
+          s"Expected code block with eta expanded function but found $other."
+        )
+      }
+
+      etaExpandIfNecessary(Apply(term, implicits))
+    }
+  }
+}

core/shared/src/main/scala-3.x/monocle/internal/focus/features/selectfield/SelectFieldGenerator.scala

@@ -1,27 +1,25 @@
 package monocle.internal.focus.features.selectfield
 
 import monocle.internal.focus.FocusBase
+import monocle.internal.focus.features.SelectGeneratorBase
 import monocle.Lens
-import scala.quoted.Quotes
 
 private[focus] trait SelectFieldGenerator {
-  this: FocusBase =>
+  this: FocusBase with SelectGeneratorBase =>
 
   import macroContext.reflect._
 
   def generateSelectField(action: FocusAction.SelectField): Term = {
     import action.{fieldName, fromType, fromTypeArgs, toType}
 
-    def generateGetter(from: Term): Term =
-      Select.unique(from, fieldName) // o.field
-
     def generateSetter(from: Term, to: Term): Term =
-      Select.overloaded(from, "copy", fromTypeArgs, NamedArg(fieldName, to) :: Nil) // o.copy(field = value)
+      // o.copy(field = value)(implicits)*
+      etaExpandIfNecessary(Select.overloaded(from, "copy", fromTypeArgs, NamedArg(fieldName, to) :: Nil))
 
     (fromType.asType, toType.asType) match {
       case ('[f], '[t]) =>
         '{
-          Lens.apply[f, t]((from: f) => ${ generateGetter('{ from }.asTerm).asExprOf[t] })((to: t) =>
+          Lens.apply[f, t]((from: f) => ${ generateGetter('{ from }.asTerm, fieldName).asExprOf[t] })((to: t) =>
             (from: f) => ${ generateSetter('{ from }.asTerm, '{ to }.asTerm).asExprOf[f] }
           )
         }.asTerm

core/shared/src/main/scala-3.x/monocle/internal/focus/features/selectonlyfield/SelectOnlyFieldGenerator.scala

@@ -1,27 +1,25 @@
 package monocle.internal.focus.features.selectonlyfield
 
 import monocle.internal.focus.FocusBase
+import monocle.internal.focus.features.SelectGeneratorBase
 import monocle.Iso
-import scala.quoted.Quotes
 
 private[focus] trait SelectOnlyFieldGenerator {
-  this: FocusBase =>
+  this: FocusBase with SelectGeneratorBase =>
 
   import macroContext.reflect._
 
   def generateSelectOnlyField(action: FocusAction.SelectOnlyField): Term = {
     import action.{fieldName, fromType, fromTypeArgs, fromCompanion, toType}
 
-    def generateGetter(from: Term): Term =
-      Select.unique(from, fieldName) // o.field
-
     def generateReverseGet(to: Term): Term =
-      Select.overloaded(fromCompanion, "apply", fromTypeArgs, List(to)) // Companion.apply(value)
+      // Companion.apply(value)(implicits)*
+      etaExpandIfNecessary(Select.overloaded(fromCompanion, "apply", fromTypeArgs, List(to)))
 
     (fromType.asType, toType.asType) match {
       case ('[f], '[t]) =>
         '{
-          Iso.apply[f, t]((from: f) => ${ generateGetter('{ from }.asTerm).asExprOf[t] })((to: t) =>
+          Iso.apply[f, t]((from: f) => ${ generateGetter('{ from }.asTerm, fieldName).asExprOf[t] })((to: t) =>
             ${ generateReverseGet('{ to }.asTerm).asExprOf[f] }
           )
         }.asTerm

…l/ContextBoundCompilationIssueSpec.scala …l/ContextBoundCompilationIssueSpec.scalamacro/src/test/scala-2.x/monocle/macros/internal/ContextBoundCompilationIssueSpec.scala renamed to macro/src/test/scala/monocle/macros/internal/ContextBoundCompilationIssueSpec.scala macro/src/test/scala-2.x/monocle/macros/internal/ContextBoundCompilationIssueSpec.scala renamed to macro/src/test/scala/monocle/macros/internal/ContextBoundCompilationIssueSpec.scala

@@ -9,18 +9,16 @@ class ContextBoundCompilationIssueSpec extends DisciplineSuite {
   private trait Foo[T]
   private trait Bar[T]
 
-  private case class A[T: Foo](s: A.S[T]) {
-    val lens: Lens[A.S[T], Bar[T]] = GenLens[A.S[T]](_.bar)
+  private case class A[T: Foo](s: S[T]) {
+    val lens: Lens[S[T], Bar[T]] = GenLens[S[T]](_.bar)
   }
 
-  private object A {
-    case class S[T: Foo](bar: Bar[T])
-  }
+  private case class S[T: Foo](bar: Bar[T])
 
   private case object FooImpl extends Foo[Unit]
   private case object BarImpl extends Bar[Unit]
 
-  private val a: A[Unit] = A(A.S(BarImpl)(FooImpl))(FooImpl)
+  private val a: A[Unit] = A(S(BarImpl)(FooImpl))(FooImpl)
 
   test("context.bound.compilation") {
     assertEquals(a.lens.get(a.s), BarImpl)