From b71e262a8561760e70e69ae8b8fcd844705daf61 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Thu, 18 Nov 2021 22:02:15 -0500
Subject: [PATCH 1/4] Fix space in unsafe nulls

---
 .../src/dotty/tools/dotc/core/SymDenotations.scala   |  2 +-
 .../dotty/tools/dotc/transform/PatternMatcher.scala  |  7 ++++++-
 .../dotty/tools/dotc/transform/patmat/Space.scala    | 12 +++++-------
 compiler/src/dotty/tools/dotc/typer/Nullables.scala  |  7 +++++++
 compiler/src/dotty/tools/dotc/typer/Typer.scala      |  5 ++++-
 .../unsafe-common/unsafe-match-null.scala            | 12 ++++++++++++
 6 files changed, 35 insertions(+), 10 deletions(-)
 create mode 100644 tests/explicit-nulls/unsafe-common/unsafe-match-null.scala

diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 2f7e3debfa6f..5c3545bb2f65 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -813,7 +813,7 @@ object SymDenotations {
     /** Is this symbol a class of which `null` is a value? */
     final def isNullableClass(using Context): Boolean =
       if ctx.mode.is(Mode.SafeNulls) && !ctx.phase.erasedTypes
-      then symbol == defn.NullClass || symbol == defn.AnyClass
+      then symbol == defn.NullClass || symbol == defn.AnyClass || symbol == defn.MatchableClass
       else isNullableClassAfterErasure
 
     /** Is this symbol a class of which `null` is a value after erasure?
diff --git a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
index 5ba43253e8b3..fb4657f9d01a 100644
--- a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -13,6 +13,7 @@ import SymUtils._
 import Flags._, Constants._
 import Decorators._
 import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
+import typer.Nullables
 import config.Printers.patmatch
 import reporting._
 import dotty.tools.dotc.ast._
@@ -43,8 +44,12 @@ class PatternMatcher extends MiniPhase {
         case rt => tree.tpe
       val translated = new Translator(matchType, this).translateMatch(tree)
 
+      val engineCtx =
+        if tree.hasAttachment(Nullables.UnsafeNullsMatch)
+        then ctx.retractMode(Mode.SafeNulls) else ctx
+
       // check exhaustivity and unreachability
-      val engine = new patmat.SpaceEngine
+      val engine = new patmat.SpaceEngine()(using engineCtx)
       engine.checkExhaustivity(tree)
       engine.checkRedundancy(tree)
 
diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 13310299ed00..dcb946a45ec3 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -338,16 +338,13 @@ class SpaceEngine(using Context) extends SpaceLogic {
 
   override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type): Space = trace(s"atomic intersection: ${AndType(tp1, tp2).show}", debug) {
     // Precondition: !isSubType(tp1, tp2) && !isSubType(tp2, tp1).
-    if (!ctx.explicitNulls && (tp1.isNullType || tp2.isNullType)) {
+    if !ctx.mode.is(Mode.SafeNulls) && (tp1.isNullType || tp2.isNullType) then
       // Since projections of types don't include null, intersection with null is empty.
       Empty
-    }
-    else {
+    else
       val res = TypeComparer.provablyDisjoint(tp1, tp2)
-
-      if (res) Empty
+      if res then Empty
       else Typ(AndType(tp1, tp2), decomposed = true)
-    }
   }
 
   /** Return the space that represents the pattern `pat` */
@@ -549,7 +546,8 @@ class SpaceEngine(using Context) extends SpaceLogic {
 
   /** Is `tp1` a subtype of `tp2`?  */
   def isSubType(tp1: Type, tp2: Type): Boolean = trace(i"$tp1 <:< $tp2", debug, show = true) {
-    if tp1 == constantNullType && !ctx.explicitNulls then tp2 == constantNullType
+    if tp1 == constantNullType && !ctx.mode.is(Mode.SafeNulls)
+    then tp2 == constantNullType
     else adaptType(tp1, tp2) <:< tp2
   }
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index 0ea7207c7b2b..dc3957e78510 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -20,6 +20,13 @@ import ast.Trees.mods
 object Nullables:
   import ast.tpd._
 
+  /** An attachment that represents a match tree is created under Unsafe Nulls.
+   *  This is used to pass Unsafe Nulls information to PatternMatcher Phase,
+   *  so we don't get Match case Unreachable Warning when using `case null => ???`
+   *  on non-nullable type.
+   */
+  val UnsafeNullsMatch = Property.StickyKey[Unit]
+
   inline def unsafeNullsEnabled(using Context): Boolean =
     ctx.explicitNulls && !ctx.mode.is(Mode.SafeNulls)
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 02f3f6b4f164..c1a10c8f5846 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1499,7 +1499,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   }
 
   def typedMatch(tree: untpd.Match, pt: Type)(using Context): Tree =
-    tree.selector match {
+    val tree1 = tree.selector match {
       case EmptyTree =>
         if (tree.isInline) {
           checkInInlineContext("summonFrom", tree.srcPos)
@@ -1601,6 +1601,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             result
         }
     }
+    if Nullables.unsafeNullsEnabled then
+      tree1.putAttachment(Nullables.UnsafeNullsMatch, ())
+    tree1
 
   /** Special typing of Match tree when the expected type is a MatchType,
    *  and the patterns of the Match tree and the MatchType correspond.
diff --git a/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala b/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala
new file mode 100644
index 000000000000..4aa85d5ba9ae
--- /dev/null
+++ b/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala
@@ -0,0 +1,12 @@
+def test1 =
+  val s: String = ???
+  s match
+    case _: String =>
+    // under unsafeNulls, we should not get Match case Unreachable Warning
+    case null => // error
+
+def test2 =
+  val s: String | Null = ???
+  s match
+    case _: String =>
+    case null =>

From 60d7781b5f35c3fd802e6dd34c4e9d6c859b5aef Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Fri, 19 Nov 2021 01:04:44 -0500
Subject: [PATCH 2/4] Add patman test

---
 .../neg-patmat/{patmat1.scala => match-pat.scala}  | 14 +++++++-------
 .../neg-patmat/unsafe-match-null-pat.scala         | 14 ++++++++++++++
 .../unsafe-common/unsafe-match-null.scala          |  3 +--
 3 files changed, 22 insertions(+), 9 deletions(-)
 rename tests/explicit-nulls/neg-patmat/{patmat1.scala => match-pat.scala} (66%)
 create mode 100644 tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala

diff --git a/tests/explicit-nulls/neg-patmat/patmat1.scala b/tests/explicit-nulls/neg-patmat/match-pat.scala
similarity index 66%
rename from tests/explicit-nulls/neg-patmat/patmat1.scala
rename to tests/explicit-nulls/neg-patmat/match-pat.scala
index a73b1fca2e1e..fb7180d82e34 100644
--- a/tests/explicit-nulls/neg-patmat/patmat1.scala
+++ b/tests/explicit-nulls/neg-patmat/match-pat.scala
@@ -1,12 +1,9 @@
 class Foo {
+
   val s: String = ???
-  s match {
-    case s: String => 100 // warning: type test will always succeed
-    case _ => 200 // error: unreachable
-  }
 
   s match {
-    case s: String => 100 // warning: type test will always succeed
+    case s: String => 100
     case _ => 200 // error: unreachable
   }
 
@@ -15,20 +12,23 @@ class Foo {
   case object Cat extends Animal
 
   val a: Animal = ???
+
   a match {
     case Dog(name) => 100
     case Cat => 200
     case _ => 300 // error: unreachable
   }
 
-  val a2: Animal|Null = ???
+  val a2: Animal | Null = ???
+
   a2 match {
     case Dog(_) => 100
     case Cat => 200
     case _ => 300
   }
 
-  val a3: Animal|Null = ???
+  val a3: Animal | Null = ???
+
   a3 match {
     case Dog(_) => 100
     case Cat => 200
diff --git a/tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala b/tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala
new file mode 100644
index 000000000000..143a79d1e2f0
--- /dev/null
+++ b/tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala
@@ -0,0 +1,14 @@
+import scala.language.unsafeNulls
+
+def test1 =
+  val s: String = ???
+  s match
+    case _: String =>
+    // under unsafeNulls, we should not get Match case Unreachable Warning
+    case null => // ok
+
+def test2 =
+  val s: String | Null = ???
+  s match
+    case _: String =>
+    case null =>
diff --git a/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala b/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala
index 4aa85d5ba9ae..fc76de6ddd3a 100644
--- a/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala
+++ b/tests/explicit-nulls/unsafe-common/unsafe-match-null.scala
@@ -2,8 +2,7 @@ def test1 =
   val s: String = ???
   s match
     case _: String =>
-    // under unsafeNulls, we should not get Match case Unreachable Warning
-    case null => // error
+    case null => // error: Values of types Null and String cannot be compared
 
 def test2 =
   val s: String | Null = ???

From 37dcadafe69cfdff17bf30dd37c16808295b25d8 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Fri, 19 Nov 2021 03:42:35 -0500
Subject: [PATCH 3/4] Fix tests in Ycheck

---
 compiler/src/dotty/tools/dotc/transform/patmat/Space.scala  | 6 ------
 compiler/src/dotty/tools/dotc/typer/Typer.scala             | 2 +-
 compiler/test/dotty/tools/dotc/CompilationTests.scala       | 1 +
 .../{neg-patmat => pos-patmat}/unsafe-match-null-pat.scala  | 0
 4 files changed, 2 insertions(+), 7 deletions(-)
 rename tests/explicit-nulls/{neg-patmat => pos-patmat}/unsafe-match-null-pat.scala (100%)

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index dcb946a45ec3..7fbd6ba22178 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -330,12 +330,6 @@ class SpaceEngine(using Context) extends SpaceLogic {
 
   private val constantNullType     = ConstantType(Constant(null))
 
-  /** Does the given tree stand for the literal `null`? */
-  def isNullLit(tree: Tree): Boolean = tree match {
-    case Literal(Constant(null)) => true
-    case _ => false
-  }
-
   override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type): Space = trace(s"atomic intersection: ${AndType(tp1, tp2).show}", debug) {
     // Precondition: !isSubType(tp1, tp2) && !isSubType(tp2, tp1).
     if !ctx.mode.is(Mode.SafeNulls) && (tp1.isNullType || tp2.isNullType) then
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index c1a10c8f5846..112a5dd7fd18 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1601,7 +1601,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             result
         }
     }
-    if Nullables.unsafeNullsEnabled then
+    if Nullables.unsafeNullsEnabled && ctx.phase == Phases.typerPhase then
       tree1.putAttachment(Nullables.UnsafeNullsMatch, ())
     tree1
 
diff --git a/compiler/test/dotty/tools/dotc/CompilationTests.scala b/compiler/test/dotty/tools/dotc/CompilationTests.scala
index 88eab8d131e6..4ea8db553f54 100644
--- a/compiler/test/dotty/tools/dotc/CompilationTests.scala
+++ b/compiler/test/dotty/tools/dotc/CompilationTests.scala
@@ -247,6 +247,7 @@ class CompilationTests {
     aggregateTests(
       compileFilesInDir("tests/explicit-nulls/pos", explicitNullsOptions),
       compileFilesInDir("tests/explicit-nulls/pos-separate", explicitNullsOptions),
+      compileFilesInDir("tests/explicit-nulls/pos-patmat", explicitNullsOptions and "-Xfatal-warnings"),
       compileFilesInDir("tests/explicit-nulls/unsafe-common", explicitNullsOptions and "-language:unsafeNulls"),
     )
   }.checkCompile()
diff --git a/tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala b/tests/explicit-nulls/pos-patmat/unsafe-match-null-pat.scala
similarity index 100%
rename from tests/explicit-nulls/neg-patmat/unsafe-match-null-pat.scala
rename to tests/explicit-nulls/pos-patmat/unsafe-match-null-pat.scala

From d296dd4cd5b7072d9b53a46d338bc9d68ee23300 Mon Sep 17 00:00:00 2001
From: noti0na1 <noti0na1@users.noreply.github.com>
Date: Mon, 31 Jan 2022 13:26:43 -0500
Subject: [PATCH 4/4] Remove StickyKey

---
 .../src/dotty/tools/dotc/transform/PatternMatcher.scala    | 7 +------
 compiler/src/dotty/tools/dotc/typer/Nullables.scala        | 7 -------
 compiler/src/dotty/tools/dotc/typer/Typer.scala            | 5 +----
 3 files changed, 2 insertions(+), 17 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
index fb4657f9d01a..5ba43253e8b3 100644
--- a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -13,7 +13,6 @@ import SymUtils._
 import Flags._, Constants._
 import Decorators._
 import NameKinds.{PatMatStdBinderName, PatMatAltsName, PatMatResultName}
-import typer.Nullables
 import config.Printers.patmatch
 import reporting._
 import dotty.tools.dotc.ast._
@@ -44,12 +43,8 @@ class PatternMatcher extends MiniPhase {
         case rt => tree.tpe
       val translated = new Translator(matchType, this).translateMatch(tree)
 
-      val engineCtx =
-        if tree.hasAttachment(Nullables.UnsafeNullsMatch)
-        then ctx.retractMode(Mode.SafeNulls) else ctx
-
       // check exhaustivity and unreachability
-      val engine = new patmat.SpaceEngine()(using engineCtx)
+      val engine = new patmat.SpaceEngine
       engine.checkExhaustivity(tree)
       engine.checkRedundancy(tree)
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Nullables.scala b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
index dc3957e78510..0ea7207c7b2b 100644
--- a/compiler/src/dotty/tools/dotc/typer/Nullables.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Nullables.scala
@@ -20,13 +20,6 @@ import ast.Trees.mods
 object Nullables:
   import ast.tpd._
 
-  /** An attachment that represents a match tree is created under Unsafe Nulls.
-   *  This is used to pass Unsafe Nulls information to PatternMatcher Phase,
-   *  so we don't get Match case Unreachable Warning when using `case null => ???`
-   *  on non-nullable type.
-   */
-  val UnsafeNullsMatch = Property.StickyKey[Unit]
-
   inline def unsafeNullsEnabled(using Context): Boolean =
     ctx.explicitNulls && !ctx.mode.is(Mode.SafeNulls)
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 112a5dd7fd18..02f3f6b4f164 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1499,7 +1499,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   }
 
   def typedMatch(tree: untpd.Match, pt: Type)(using Context): Tree =
-    val tree1 = tree.selector match {
+    tree.selector match {
       case EmptyTree =>
         if (tree.isInline) {
           checkInInlineContext("summonFrom", tree.srcPos)
@@ -1601,9 +1601,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             result
         }
     }
-    if Nullables.unsafeNullsEnabled && ctx.phase == Phases.typerPhase then
-      tree1.putAttachment(Nullables.UnsafeNullsMatch, ())
-    tree1
 
   /** Special typing of Match tree when the expected type is a MatchType,
    *  and the patterns of the Match tree and the MatchType correspond.