Support implicit arguments before extractor method

compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -1298,9 +1298,13 @@ object Objects:
           case select: Select =>
             eval(select.qualifier, thisV, klass)
 
-        val implicitValues = evalArgs(implicits.map(Arg.apply), thisV, klass)
-        // TODO: implicit values may appear before and/or after the scrutinee parameter.
-        val unapplyRes = call(receiver, funRef.symbol, TraceValue(scrutinee, summon[Trace]) :: implicitValues, funRef.prefix, superType = NoType, needResolve = true)
+        def implicitValuesBeforeScrutinee(fun: Tree): Contextual[List[ArgInfo]] = fun match
+          case Apply(f, implicitArgs) =>
+            implicitValuesBeforeScrutinee(f) ++ evalArgs(implicitArgs.map(Arg.apply), thisV, klass)
+          case _ => List()
+
+        val implicitValuesAfterScrtinee = evalArgs(implicits.map(Arg.apply), thisV, klass)
+        val unapplyRes = call(receiver, funRef.symbol, implicitValuesBeforeScrutinee(fun) ++ (TraceValue(scrutinee, summon[Trace]) :: implicitValuesAfterScrtinee), funRef.prefix, superType = NoType, needResolve = true)
 
         if fun.symbol.name == nme.unapplySeq then
           var resultTp = unapplyResTp

tests/init-global/neg/unapply-implicit-arg.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(i: Int) = i+1
+    def m2(i: Int) = i+2
+  }
+  def unapply(using f1: Foo)(i: Int): Option[Int] =
+    if i == 0 then Some(f1.m1(i)) else Some(f1.m2(i))
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = i2 match // error
+    case Bar(i) => i
+    case _ => 0
+}

tests/init-global/neg/unapply-implicit-arg2.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(i: Int) = i+1
+    def m2(i: Int) = i+2
+  }
+  def unapply(using f1: Foo)(i: Int): Option[Int] =
+    if i == 0 then Some(f1.m1(i1)) else Some(f1.m2(i2)) // error
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = i1 match
+    case Bar(i) => i
+    case _ => 0
+}

tests/init-global/neg/unapply-implicit-arg3.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(i: Int) = i + i1
+    def m2(i: Int) = i + i2 // error
+  }
+  def unapply(using f1: Foo)(i: Int): Option[Int] =
+    if i == 0 then Some(f1.m1(i)) else Some(f1.m2(i))
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = i1 match
+    case Bar(i) => i
+    case _ => 0
+}

tests/init-global/neg/unapplySeq-implicit-arg.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(seq: Seq[Int]) = 1 +: seq
+    def m2(seq: Seq[Int]) = 2 +: seq
+  }
+  def unapplySeq(using f1: Foo)(seqi: Seq[Int]): Option[Seq[Int]] =
+    if seqi(0) == 0 then Some(f1.m1(seqi)) else Some(f1.m2(seqi))
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = Seq(i2) match // error
+    case Bar(i) => i
+    case _ => 0
+}

tests/init-global/neg/unapplySeq-implicit-arg2.scala

@@ -0,0 +1,10 @@
+object Bar {
+  class Foo
+  def unapplySeq(using f1: Foo)(using f2: Foo)(seqi: Seq[Int])(using Foo): Option[Seq[Int]] =
+    Some(i1 +: seqi) // error
+  given Foo = new Foo
+  val i1: Int = Seq(0) match {
+    case Bar(i) => i
+    case _ => 0
+  }
+}

tests/init-global/neg/unapplySeq-implicit-arg3.scala

@@ -0,0 +1,12 @@
+object Bar {
+  class Foo {
+    def m(seq: Seq[Int]) = i1 +: seq // error
+  }
+  def unapplySeq(using f1: Foo)(seqi: Seq[Int])(using Foo): Option[Seq[Int]] =
+    Some(f1.m(seqi))
+  given Foo = new Foo
+  val i1: Int = Seq(0) match {
+    case Bar(i, _) => i
+    case _ => 0
+  }
+}

tests/init-global/pos/unapply-implicit-arg-pos.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(i: Int) = i + i1
+    def m2(i: Int) = i + 2
+  }
+  def unapply(using f1: Foo)(using f2: Foo)(i: Int)(using f3: Foo): Option[Int] =
+    if i == 0 then Some(f1.m1(i1) + f3.m1(i1)) else Some(f2.m2(i) + f3.m2(i))
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = i1 match
+    case Bar(i) => i
+    case _ => 0
+}

tests/init-global/pos/unapplySeq-implicit-arg-pos.scala

@@ -0,0 +1,14 @@
+object Bar {
+  class Foo {
+    def m1(seq: Seq[Int]) = 0 +: seq
+    def m2(seq: Seq[Int]) = i1 +: seq
+  }
+  def unapplySeq(using f1: Foo)(using f2: Foo)(seqi: Seq[Int])(using f3: Foo): Option[Seq[Int]] =
+    if seqi(0) == 0 then Some(f1.m1(seqi)) else Some(f2.m2(seqi))
+
+  given Foo = new Foo
+  val i1: Int = 0
+  val i2: Int = Seq(i1) match
+    case Bar(i) => i
+    case _ => 0
+}