From 3f544e15b798094d36c5ce3758bf08db20ce0abc Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 22 Aug 2022 17:52:22 +0200
Subject: [PATCH 1/2] Fix swapKey

Fixes #15845.

Curiously this code was never exercised before, despite the many millions of lines of code we compile. It's even more
curious that the faulty behavior could be triggered by such a simple example.
---
 .../dotty/tools/dotc/core/OrderingConstraint.scala    |  4 +++-
 tests/pos/i15845/State.scala                          | 11 +++++++++++
 tests/pos/i15845/state/Config.scala                   |  4 ++++
 3 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i15845/State.scala
 create mode 100644 tests/pos/i15845/state/Config.scala

diff --git a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
index a304fa9f5705..ed04145c1d02 100644
--- a/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
+++ b/compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala
@@ -498,7 +498,9 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     (this.typeVarOfParam(tl.paramRefs(0)) ne that.typeVarOfParam(tl.paramRefs(0)))
 
   def subst(from: TypeLambda, to: TypeLambda)(using Context): OrderingConstraint =
-    def swapKey[T](m: ArrayValuedMap[T]) = m.remove(from).updated(to, m(from).nn)
+    def swapKey[T](m: ArrayValuedMap[T]) =
+      val info = m(from)
+      if info == null then m else m.remove(from).updated(to, info)
     var current = newConstraint(swapKey(boundsMap), swapKey(lowerMap), swapKey(upperMap))
     def subst[T <: Type](x: T): T = x.subst(from, to).asInstanceOf[T]
     current.foreachParam {(p, i) =>
diff --git a/tests/pos/i15845/State.scala b/tests/pos/i15845/State.scala
new file mode 100644
index 000000000000..ed907b7e899e
--- /dev/null
+++ b/tests/pos/i15845/State.scala
@@ -0,0 +1,11 @@
+package config
+
+import state._
+
+object State {
+
+  object Info {
+    def apply(): Info = Info(Config(), Seq.empty)
+  }
+  case class Info(cfg: AnyRef, allTypes: Seq[AnyRef])
+}
diff --git a/tests/pos/i15845/state/Config.scala b/tests/pos/i15845/state/Config.scala
new file mode 100644
index 000000000000..4b0e11d7d191
--- /dev/null
+++ b/tests/pos/i15845/state/Config.scala
@@ -0,0 +1,4 @@
+package config
+package state
+
+case class Config(simulations: Seq[(String, Any)] = Seq.empty)

From 6a74d4dea8ee03bbabe2de9f4e244d1d33bb528b Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 22 Aug 2022 18:05:58 +0200
Subject: [PATCH 2/2] Fix test directory

---
 tests/pos/i15845/{ => config}/State.scala        | 0
 tests/pos/i15845/{ => config}/state/Config.scala | 0
 2 files changed, 0 insertions(+), 0 deletions(-)
 rename tests/pos/i15845/{ => config}/State.scala (100%)
 rename tests/pos/i15845/{ => config}/state/Config.scala (100%)

diff --git a/tests/pos/i15845/State.scala b/tests/pos/i15845/config/State.scala
similarity index 100%
rename from tests/pos/i15845/State.scala
rename to tests/pos/i15845/config/State.scala
diff --git a/tests/pos/i15845/state/Config.scala b/tests/pos/i15845/config/state/Config.scala
similarity index 100%
rename from tests/pos/i15845/state/Config.scala
rename to tests/pos/i15845/config/state/Config.scala