From 8231a48780fd692c120b038bb67330b9623bebc5 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 25 Jun 2023 16:37:26 +0200
Subject: [PATCH 1/2] Avoid crash in erasure when reference cannot be emitted

Fixes #17391
---
 compiler/src/dotty/tools/dotc/transform/Erasure.scala |  2 +-
 tests/neg/i17391/Bar.java                             |  7 +++++++
 tests/neg/i17391/test.scala                           | 11 +++++++++++
 3 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i17391/Bar.java
 create mode 100644 tests/neg/i17391/test.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/Erasure.scala b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
index f068a48cdba4..e20a16c9e60e 100644
--- a/compiler/src/dotty/tools/dotc/transform/Erasure.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
@@ -869,7 +869,7 @@ object Erasure {
 
           app(fun1)
         case t =>
-          if ownArgs.isEmpty then fun1
+          if ownArgs.isEmpty || t.isError then fun1
           else throw new MatchError(i"tree $tree has unexpected type of function $fun/$fun1: $t, was $origFunType, args = $ownArgs")
     end typedApply
 
diff --git a/tests/neg/i17391/Bar.java b/tests/neg/i17391/Bar.java
new file mode 100644
index 000000000000..6d432926bde2
--- /dev/null
+++ b/tests/neg/i17391/Bar.java
@@ -0,0 +1,7 @@
+package mypkg;
+
+class Base<E> {
+    public void retainAll(String x) {}
+}
+
+public class Bar<E> extends Base<E> {}
diff --git a/tests/neg/i17391/test.scala b/tests/neg/i17391/test.scala
new file mode 100644
index 000000000000..dbcc0ec39de8
--- /dev/null
+++ b/tests/neg/i17391/test.scala
@@ -0,0 +1,11 @@
+def test(): Unit =
+{
+  val x: Foo[mypkg.Bar[String]] = ???
+  val y: mypkg.Bar[String] = ???
+
+  y.retainAll("fd") // works
+  x.f.retainAll("fd"); // error
+
+}
+
+class Foo[T](val f: T)

From 996bb15289e61ba8d8305039052b0b6237c5e642 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 26 Jun 2023 22:17:38 +0200
Subject: [PATCH 2/2] Compute the original qualifier type before erasure

The original qualifier type was computed in the wrong context. For the logic
to make sense, it needs to compute the type before erasure and then erase that
type. That fixes #17391 by now compiling it correctly, the same way Scala 2 does.
---
 compiler/src/dotty/tools/dotc/transform/Erasure.scala | 4 +++-
 tests/{neg => pos}/i17391/Bar.java                    | 0
 tests/{neg => pos}/i17391/test.scala                  | 0
 3 files changed, 3 insertions(+), 1 deletion(-)
 rename tests/{neg => pos}/i17391/Bar.java (100%)
 rename tests/{neg => pos}/i17391/test.scala (100%)

diff --git a/compiler/src/dotty/tools/dotc/transform/Erasure.scala b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
index e20a16c9e60e..8e355175c176 100644
--- a/compiler/src/dotty/tools/dotc/transform/Erasure.scala
+++ b/compiler/src/dotty/tools/dotc/transform/Erasure.scala
@@ -764,7 +764,9 @@ object Erasure {
         val symIsPrimitive = sym.owner.isPrimitiveValueClass
 
         def originalQual: Type =
-          erasure(tree.qualifier.typeOpt.widen.finalResultType)
+          erasure(
+            inContext(preErasureCtx):
+              tree.qualifier.typeOpt.widen.finalResultType)
 
         if (qualIsPrimitive && !symIsPrimitive || qual.tpe.widenDealias.isErasedValueType)
           recur(box(qual))
diff --git a/tests/neg/i17391/Bar.java b/tests/pos/i17391/Bar.java
similarity index 100%
rename from tests/neg/i17391/Bar.java
rename to tests/pos/i17391/Bar.java
diff --git a/tests/neg/i17391/test.scala b/tests/pos/i17391/test.scala
similarity index 100%
rename from tests/neg/i17391/test.scala
rename to tests/pos/i17391/test.scala