From f351ec4c255841031c7d4099cc23c602b4d3289c Mon Sep 17 00:00:00 2001
From: Ondrej Lhotak <olhotak@uwaterloo.ca>
Date: Thu, 13 Jul 2023 21:07:51 -0400
Subject: [PATCH 1/2] allow nullability flow typing even in presence of pattern
 match

Nullability flow typing is conservatively disabled for mutable
variables to which a write occurs nested inside a Tree other than
some known ones, such as If and WhileDo. This is to prevent flow-sensitive
reasoning for variables that are captured and written to in a closure.
Pattern matches do not create a closure. This change enables nullability
flow typing even for mutable variables that are written to inside a pattern
match.
---
 compiler/src/dotty/tools/dotc/typer/Nullables.scala | 2 +-
 tests/explicit-nulls/pos/match-flow-typing.scala    | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)
 create mode 100644 tests/explicit-nulls/pos/match-flow-typing.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index 9104418d406f..f4748eb03849 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -445,7 +445,7 @@ object Nullables:
                 else candidates -= name
               case None =>
             traverseChildren(tree)
-          case _: (If | WhileDo | Typed) =>
+          case _: (If | WhileDo | Typed | Match | CaseDef) =>
             traverseChildren(tree)      // assignments to candidate variables are OK here ...
           case _ =>
             reachable = Set.empty       // ... but not here
diff --git a/tests/explicit-nulls/pos/match-flow-typing.scala b/tests/explicit-nulls/pos/match-flow-typing.scala
new file mode 100644
index 000000000000..cda8b25da5ff
--- /dev/null
+++ b/tests/explicit-nulls/pos/match-flow-typing.scala
@@ -0,0 +1,8 @@
+def m(): String = {
+  var x: String|Null = "foo"
+  1 match {
+    case 1 => x = x
+  }
+  if(x == null) "foo"
+  else x
+}

From 74f6851f0f7db62f1fcdb9e2362a6c2b5944aa85 Mon Sep 17 00:00:00 2001
From: Ondrej Lhotak <olhotak@uwaterloo.ca>
Date: Mon, 24 Jul 2023 15:33:19 -0400
Subject: [PATCH 2/2] additionally include writes under try/catch/finally

---
 compiler/src/dotty/tools/dotc/typer/Nullables.scala |  2 +-
 tests/explicit-nulls/pos/match-flow-typing.scala    | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index f4748eb03849..68e3c0f8ccd6 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -445,7 +445,7 @@ object Nullables:
                 else candidates -= name
               case None =>
             traverseChildren(tree)
-          case _: (If | WhileDo | Typed | Match | CaseDef) =>
+          case _: (If | WhileDo | Typed | Match | CaseDef | untpd.ParsedTry) =>
             traverseChildren(tree)      // assignments to candidate variables are OK here ...
           case _ =>
             reachable = Set.empty       // ... but not here
diff --git a/tests/explicit-nulls/pos/match-flow-typing.scala b/tests/explicit-nulls/pos/match-flow-typing.scala
index cda8b25da5ff..200af36a73e0 100644
--- a/tests/explicit-nulls/pos/match-flow-typing.scala
+++ b/tests/explicit-nulls/pos/match-flow-typing.scala
@@ -6,3 +6,16 @@ def m(): String = {
   if(x == null) "foo"
   else x
 }
+
+def m2(): String = {
+  var x: String|Null = "foo"
+  try {
+    x = x
+  } catch {
+    case e => x = x
+  } finally {
+    x = x
+  }
+  if(x == null) "foo"
+  else x
+}