Reinstantiate restriction to transparent inline methods

Reverts parts of #19922.

Fixes #20342, #20297

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

Author: odersky

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

@@ -11,7 +11,7 @@ import Constants.*
 import util.{Stats, SimpleIdentityMap, SimpleIdentitySet}
 import Decorators.*
 import Uniques.*
-import Flags.Method
+import Flags.{Method, Transparent}
 import inlines.Inlines
 import config.Printers.typr
 import Inferencing.*
@@ -108,7 +108,7 @@ 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 +128,12 @@ object ProtoTypes {
         case _ =>
           false
 
-      if Inlines.isInlineable(meth) then
+      if Inlines.isInlineable(meth) && meth.is(Transparent) then
         constrainResult(mt, wildApprox(pt))
         true
       else
         constFoldException(pt) || 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/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)
+      }
+    }