From 640e6c8466eecadf4a888bbed61e9795fa320c99 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 4 Oct 2023 13:53:17 +0100
Subject: [PATCH] Fix the tparam bounds of exported inherited classes

When trying to export M2.F, seeing M1#F from the prefix M2 doesn't
change the bounds of F's T type parameter, which still refers to
M1.this.A, rather than M2.A.  So, we run asSeenFrom against that info,
when eta-expanding the class into a hk type lambda.
---
 .../src/dotty/tools/dotc/typer/Namer.scala     | 18 ++++++++++++++++--
 tests/pos/i18569.scala                         | 11 +++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 tests/pos/i18569.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 8ed881ca0d81..8797d016df98 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -1175,8 +1175,22 @@ class Namer { typer: Typer =>
             if mbr.isType then
               val forwarderName = checkNoConflict(alias.toTypeName, isPrivate = false, span)
               var target = pathType.select(sym)
-              if target.typeParams.nonEmpty then
-                target = target.EtaExpand(target.typeParams)
+              val tparams = target.typeParams
+              if tparams.nonEmpty then
+                // like `target = target.EtaExpand(target.typeParams)`
+                // except call `asSeenFrom` to fix class type parameter bounds
+                // e.g. in pos/i18569:
+                // `Test#F` should have `M2.A` or `Test.A` as bounds, not `M1#A`.
+                target = HKTypeLambda(tparams.map(_.paramName))(
+                  tl => tparams.map {
+                    case p: Symbol =>
+                      val info = p.info.asSeenFrom(pathType, sym.owner)
+                      HKTypeLambda.toPInfo(tl.integrate(tparams, info))
+                    case p =>
+                      val info = p.paramInfo
+                      HKTypeLambda.toPInfo(tl.integrate(tparams, info))
+                  },
+                  tl => tl.integrate(tparams, target.appliedTo(tparams.map(_.paramRef))))
               newSymbol(
                 cls, forwarderName,
                 Exported | Final,
diff --git a/tests/pos/i18569.scala b/tests/pos/i18569.scala
new file mode 100644
index 000000000000..1457f6fbe5b6
--- /dev/null
+++ b/tests/pos/i18569.scala
@@ -0,0 +1,11 @@
+trait M1:
+  trait A
+  trait F[T <: A]
+  type G[T <: A] = F[T]
+
+object M2 extends M1
+
+trait Test:
+  export M2.*
+  def y: F[A]
+  def z: G[A]