From 9000ebfc7855ccc142ed1bebd00a8cd6153f6b2e Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 28 Jun 2023 11:04:20 +0100
Subject: [PATCH] Avoid GADT casting with ProtoTypes

---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 13 +++++++++++--
 tests/pos/i18062.scala                          | 14 ++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 tests/pos/i18062.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 1c00c276376e..a8ba3c31655c 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -4560,7 +4560,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           AnnotatedType(conj, Annotation(defn.UncheckedStableAnnot, tree.symbol.span))
         else conj
       else pt
-    gadts.println(i"insert GADT cast from $tree to $target")
-    tree.cast(target)
+    if target.existsPart(_.isInstanceOf[ProtoType]) then
+      // we want to avoid embedding a SelectionProto in a Conversion, as the result type
+      // as it might end up within a GADT cast type, e.g. tests/pos/i15867.scala
+      // so we just bail - in that example, a GADT cast will be insert on application, so it compiles.
+      // but tests/pos/i18062.scala is an example with a polymorphic method, which requires type variables to
+      // be applied to the tree and then constrained before they match the prototype.
+      // so rather than try to handle all that before calling adapt, let's just bail on this side.
+      tree
+    else
+      gadts.println(i"insert GADT cast from $tree to $target")
+      tree.cast(target)
   end insertGadtCast
 }
diff --git a/tests/pos/i18062.scala b/tests/pos/i18062.scala
new file mode 100644
index 000000000000..48863c4349c7
--- /dev/null
+++ b/tests/pos/i18062.scala
@@ -0,0 +1,14 @@
+trait CB[X] { def get: X }
+
+trait WrapperConvert[F[_], G[_]]:
+  def conv[X](fx: F[X]): G[X]
+
+object WrapperConvert:
+  implicit def id[F[_]]: WrapperConvert[F, F] = new WrapperConvert[F, F]:
+    def conv[X](fx: F[X]): F[X] = fx
+
+transparent inline given convertX[F[_], X](using wc: WrapperConvert[F, CB]): Conversion[F[X], X] =
+  new Conversion[F[X], X]:
+    def apply(fx: F[X]) = wc.conv(fx).get
+
+def test(cb: CB[Int], x: Int): Int = cb + x