Reinstantiate restriction to transparent inline methods (#20371)

Reverts parts of #19922.

Fixes #20342 
Fixes #20297

The logic that we should ignore declared result types of inline methods
really only applies to transparent inlines.

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

@@ -11,8 +11,9 @@ import Constants.*
 import util.{Stats, SimpleIdentityMap, SimpleIdentitySet}
 import Decorators.*
 import Uniques.*
-import Flags.Method
+import Flags.{Method, Transparent}
 import inlines.Inlines
+import config.{Feature, SourceVersion}
 import config.Printers.typr
 import Inferencing.*
 import ErrorReporting.*
@@ -108,7 +109,7 @@ object ProtoTypes {
       res
 
     /** Constrain result with two special cases:
-     *   1. If `meth` is an inlineable method in an inlineable context,
+     *   1. If `meth` is a transparent inlineable method in an inlineable context,
      *      we should always succeed and not constrain type parameters in the expected type,
      *      because the actual return type can be a subtype of the currently known return type.
      *      However, we should constrain parameters of the declared return type. This distinction is
@@ -128,11 +129,30 @@ object ProtoTypes {
         case _ =>
           false
 
-      if Inlines.isInlineable(meth) then
-        constrainResult(mt, wildApprox(pt))
-        true
-      else
-        constFoldException(pt) || constrainResult(mt, pt)
+      constFoldException(pt) || {
+        if Inlines.isInlineable(meth) then
+          // Stricter behavisour in 3.4+: do not apply `wildApprox` to non-transparent inlines
+          // unless their return type is a MatchType. In this case there's no reason
+          // not to constrain type variables in the expected type. For transparent inlines
+          // we do not want to constrain type variables in the expected type since the
+          // actual return type might be smaller after instantiation. For inlines returning
+          // MatchTypes we do not want to constrain because the MatchType might be more
+          // specific after instantiation. TODO: Should we also use Wildcards for non-inline
+          // methods returning MatchTypes?
+          if Feature.sourceVersion.isAtLeast(SourceVersion.`3.4`) then
+            if meth.is(Transparent) || mt.resultType.isMatchAlias then
+              constrainResult(mt, wildApprox(pt))
+              // do not constrain the result type of transparent inline methods
+              true
+            else
+              constrainResult(mt, pt)
+          else
+            // Best-effort to fix https://github.com/scala/scala3/issues/9685 in the 3.3.x series
+            // while preserving source compatibility as much as possible
+            constrainResult(mt, wildApprox(pt)) || meth.is(Transparent)
+        else constrainResult(mt, pt)
+      }
+
     end constrainResult
   end Compatibility
 

tests/neg/i18123.check

@@ -0,0 +1,12 @@
+-- [E172] Type Error: tests/neg/i18123.scala:25:33 ---------------------------------------------------------------------
+25 |  (charClassIntersection.rep() | classItem.rep()) // error
+   |                                 ^^^^^^^^^^^^^^^
+   |No given instance of type pkg.Implicits.Repeater[pkg.RegexTree, V] was found.
+   |I found:
+   |
+   |    pkg.Implicits.Repeater.GenericRepeaterImplicit[T]
+   |
+   |But method GenericRepeaterImplicit in object Repeater does not match type pkg.Implicits.Repeater[pkg.RegexTree, V]
+   |
+   |where:    V is a type variable with constraint <: Seq[pkg.CharClassIntersection]
+   |.

tests/neg/i18123.scala

@@ -0,0 +1,25 @@
+// may not compile anymore in Scala 3.4+
+package pkg
+
+trait P[+T]
+
+extension [T](inline parse0: P[T])
+  inline def | [V >: T](inline other: P[V]): P[V] = ???
+
+extension [T](inline parse0: => P[T])
+  inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
+
+object Implicits:
+  trait Repeater[-T, R]
+  object Repeater:
+    implicit def GenericRepeaterImplicit[T]: Repeater[T, Seq[T]] = ???
+
+sealed trait RegexTree
+abstract class Node extends RegexTree
+class CharClassIntersection() extends Node
+
+def classItem: P[RegexTree] = ???
+def charClassIntersection: P[CharClassIntersection] = ???
+
+def x =
+  (charClassIntersection.rep() | classItem.rep()) // error

tests/pos/i18123.scala

@@ -7,7 +7,8 @@ extension [T](inline parse0: P[T])
   inline def | [V >: T](inline other: P[V]): P[V] = ???
 
 extension [T](inline parse0: => P[T])
-  inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
+  // transparent needed to make this compile in 3.4+
+  transparent inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
 
 object Implicits:
   trait Repeater[-T, R]

tests/pos/i19415.scala

@@ -0,0 +1,24 @@
+def Test = {
+  val left: Parser[String] = ???
+  val right: Parser[Int] = ???
+  val both = left && right
+
+  val works = both.map(Ior.Both.apply)
+  val fails = (left && right).map(Ior.Both.apply)
+}
+
+trait Parser[T]:
+  final def &&[T2](other: Parser[T2])(implicit zip: Zip[T, T2]): Parser[zip.Out] = ???
+  final def map[T2](f: T => T2): Parser[T2] = ???
+
+infix trait Ior[+A, +B]
+object Ior:
+  final case class Both[+A, +B](a: A, b: B) extends (A Ior B)
+
+trait Zip[In1, In2]:
+  type Out
+
+object Zip {
+  type Out[In1, In2, O] = Zip[In1, In2] { type Out = O }
+  implicit def zip2[_1, _2]: Zip.Out[_1, _2, (_1, _2)] = ???
+}

tests/pos/i19479.scala

@@ -0,0 +1,54 @@
+case class Person(id: Int)
+
+class GeodeContinuousSourceSpec {
+  summon[PdxEncoder[Person]]
+}
+
+trait PdxEncoder[A] {
+  def encode(a: A): Boolean
+}
+
+object PdxEncoder extends ObjectEncoder {
+  implicit def intEncoder: PdxEncoder[Int] = ???
+}
+
+trait ObjectEncoder {
+  given emptyTupleEncoder: PdxEncoder[EmptyTuple] = ???
+
+  given tupleEncoder[K <: String, H, T <: Tuple](using
+      m: ValueOf[K],
+      hEncoder: PdxEncoder[H],
+      tEncoder: PdxEncoder[T]
+  ): PdxEncoder[FieldType[K, H] *: T] = ???
+
+  given objectEncoder[A, Repr <: Tuple](using
+      gen: LabelledGeneric.Aux[A, Repr],
+      tupleEncoder: PdxEncoder[Repr]
+  ): PdxEncoder[A] = ???
+}
+
+import scala.deriving.Mirror
+
+private type FieldType[K, +V] = V & KeyTag[K, V]
+private type KeyTag[K, +V]
+private type ZipWith[T1 <: Tuple, T2 <: Tuple, F[_, _]] <: Tuple = (T1, T2) match {
+  case (h1 *: t1, h2 *: t2) => F[h1, h2] *: ZipWith[t1, t2, F]
+  case (EmptyTuple, ?)      => EmptyTuple
+  case (?, EmptyTuple)      => EmptyTuple
+  case _                    => Tuple
+}
+
+private trait LabelledGeneric[A] {
+  type Repr
+}
+
+private object LabelledGeneric {
+  type Aux[A, R] = LabelledGeneric[A] { type Repr = R }
+
+  transparent inline given productInst[A <: Product](using
+      m: Mirror.ProductOf[A]
+  ): LabelledGeneric.Aux[A, ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]] =
+    new LabelledGeneric[A] {
+      type Repr = Tuple & ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]
+    }
+}

tests/pos/i20297.scala

@@ -0,0 +1,20 @@
+sealed abstract class Kyo[+T, -S]
+opaque type <[+T, -S] >: T = T | Kyo[T, S]
+
+extension [T, S](v: T < S)
+  inline def map[U, S2](inline f: T => U < S2): U < (S & S2) = ???
+
+class Streams[V]
+object Streams:
+  def emitValue[V](v: V): Unit < Streams[V] = ???
+
+opaque type Stream[+T, V, -S] = T < (Streams[V] & S)
+object Stream:
+  extension [T, V, S](s: Stream[T, V, S])
+    def reemit[S2, V2](f: V => Unit < (Streams[V2] & S2)): Stream[T, V2, S & S2] = ???
+    def filter[S2](f: V => Boolean < S2): Stream[T, V, S & S2] = reemit { v =>
+      f(v).map {
+        case false => ()
+        case true  => Streams.emitValue(v)
+      }
+    }

tests/pos/i20342.scala

@@ -0,0 +1,15 @@
+class Repo[EC, E](using defaults: RepoDefaults[EC, E])
+trait RepoDefaults[EC, E]
+object RepoDefaults:
+  inline given genImmutableRepo[E: DbCodec]: RepoDefaults[E, E] = ???
+  inline given genRepo[EC: DbCodec, E: DbCodec]: RepoDefaults[EC, E] = ???
+
+trait DbCodec[E]
+
+case class PersonCreator(name: String)
+case class Person(id: Long)
+given DbCodec[Person] = ???
+given DbCodec[PersonCreator] = ???
+
+@main def Test =
+  val personRepo = Repo[PersonCreator, Person]