Relax comparison between Null and reference types in explicit nulls (scala#23308)

An up-to-date version of scala#19258 with merge conflicts resolved.

Author: noti0na1

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -176,19 +176,6 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
           cmpWithBoxed(cls1, cls2)
       else if cls2.isPrimitiveValueClass then
         cmpWithBoxed(cls2, cls1)
-      else if ctx.mode.is(Mode.SafeNulls) then
-        // If explicit nulls is enabled, and unsafeNulls is not enabled,
-        // we want to disallow comparison between Object and Null.
-        // If we have to check whether a variable with a non-nullable type has null value
-        // (for example, a NotNull java method returns null for some reasons),
-        // we can still cast it to a nullable type then compare its value.
-        //
-        // Example:
-        // val x: String = null.asInstanceOf[String]
-        // if (x == null) {} // error: x is non-nullable
-        // if (x.asInstanceOf[String|Null] == null) {} // ok
-        if cls1 == defn.NullClass || cls2 == defn.NullClass then cls1 == cls2
-        else cls1 == defn.NothingClass || cls2 == defn.NothingClass
       else if cls1 == defn.NullClass then
         cls1 == cls2 || cls2.derivesFrom(defn.ObjectClass)
       else if cls2 == defn.NullClass then

docs/_docs/reference/experimental/explicit-nulls.md

@@ -90,26 +90,10 @@ More details can be found in [safe initialization](../other-new-features/safe-in
 
 ## Equality
 
-We don't allow the double-equal (`==` and `!=`) and reference (`eq` and `ne`) comparison between
-`AnyRef` and `Null` anymore, since a variable with a non-nullable type cannot have `null` as value.
-`null` can only be compared with `Null`, nullable union (`T | Null`), or `Any` type.
-
-For some reason, if we really want to compare `null` with non-null values, we have to provide a type hint (e.g. `: Any`).
-
-```scala
-val x: String = ???
-val y: String | Null = ???
-
-x == null       // error: Values of types String and Null cannot be compared with == or !=
-x eq null       // error
-"hello" == null // error
-
-y == null       // ok
-y == x          // ok
-
-(x: String | Null) == null  // ok
-(x: Any) == null            // ok
-```
+We still allow the double-equal (`==` and `!=`), reference (`eq` and `ne`) comparison,
+and pattern matching between `Null` and reference types.
+Even if a type is non-nullable, we still need to consider the possibility of `null` value
+caused by the Java methods or uninitialized values.
 
 ## Java Interoperability and Flexible Types
 

tests/explicit-nulls/neg/equal1.scala

@@ -1,5 +1,8 @@
 // Test what can be compared for equality against null.
-class Foo {
+
+case class VC(x: Int) extends AnyVal
+
+def test =
   // Null itself
   val x0: Null = null
   x0 != x0
@@ -9,21 +12,21 @@ class Foo {
   null == null
   null != null
 
-  // Non-nullable types: error
+  // Non-nullable types: OK.
   val x1: String = "hello"
-  x1 != null  // error
-  x1 == null  // error
-  null == x1  // error
-  null != x1  // error
-  x1 == x0    // error
-  x0 != x1    // error
-  x1.asInstanceOf[String|Null] == null
-  x1.asInstanceOf[String|Null] == x0
+  x1 != null
+  x1 == null
+  null == x1
+  null != x1
+  x1 == x0
+  x0 != x1
+  x1.asInstanceOf[String | Null] == null
+  x1.asInstanceOf[String | Null] == x0
   x1.asInstanceOf[Any] == null
   x1.asInstanceOf[Any] == x0
 
   // Nullable types: OK
-  val x2: String|Null = null
+  val x2: String | Null = null
   x2 == null
   null == x2
   x2 == x0
@@ -41,4 +44,10 @@ class Foo {
   null == false // error
   'a' == null   // error
   null == 'b'   // error
-}
+
+  // Nullable value types: OK.
+  val x3: Int | Null = null
+  x3 == null
+  null == x3
+  x3 == x0
+  x3 != x0

tests/explicit-nulls/neg/equal2.scala

@@ -1,18 +1,17 @@
-// Test that we can't compare for equality `null` with classes.
-// This rule is for both regular classes and value classes.
+// Test that we can compare values of regular classes against null,
+// but not values of value classes.
 
 class Foo(x: Int)
 class Bar(x: Int) extends AnyVal
 
 class Test {
   locally {
     val foo: Foo = new Foo(15)
-    foo == null // error: Values of types Null and Foo cannot be compared
-    null == foo // error
-    foo != null // error
-    null != foo // error
+    foo == null
+    null == foo
+    foo != null
+    null != foo
 
-    // To test against null, make the type nullable.
     val foo2: Foo | Null = foo
     // ok
     foo2 == null

tests/explicit-nulls/neg/flow-match.scala

@@ -3,13 +3,11 @@
 object MatchTest {
   def f6(s: String | Null): String = s match {
     case s2 => s2 // error
-    case null => "other" // error
-    case s3 => s3
+    case s3 => s3 // OK since not null
   }
 
   def f7(s: String | Null): String = s match {
     case null => "other"
-    case null => "other" // error
-    case s3 => s3
+    case s3 => s3 // OK since not null
   }
-}
+}

tests/explicit-nulls/neg/flow-strip-null.scala

@@ -1,25 +1,21 @@
 // Test we are correctly striping nulls from nullable unions.
 
-class Foo {
+class Foo:
 
   class B1
   class B2
-  locally {
+
+  locally:
     val x: (Null | String) | Null |  (B1 | (Null | B2)) = ???
-    if (x != null) {
+    if x != null then
       val _: String | B1 | B2 = x // ok: can remove all nullable unions
-    }
-  }
 
-  locally {
+  locally:
     val x: (Null | String) & (Null | B1) = ???
-    if (x != null) {
+    if x != null then
       val _: String & B1 = x // ok: can remove null from embedded intersection
-    }
-  }
 
-  locally {
+  locally:
     val x: (Null | B1) & B2 = ???
-    if (x != null) {} // error: the type of x is not a nullable union, so we cannot remove the Null
-  }
-}
+    if x != null then
+      val _: B1 & B2 = x // error: the type of x is not a nullable union, so we cannot remove the Null

tests/explicit-nulls/unsafe-common/unsafe-equal.scala renamed to tests/explicit-nulls/pos/anyref-equal-nulls.scala

@@ -11,10 +11,10 @@ class S {
     null == s1
     null != s1
 
-    s2 == null // error
-    s2 != null // error
-    null == s2 // error
-    null != s2 // error
+    s2 == null
+    s2 != null
+    null == s2
+    null != s2
 
     s1 == s2
     s1 != s2
@@ -27,21 +27,21 @@ class S {
     null != n
 
     s1 == n
-    s2 == n // error
+    s2 == n
     n != s1
-    n != s2 // error
+    n != s2
   }
 
   locally {
-    ss1 == null // error
-    ss1 != null // error
-    null == ss1 // error
-    null != ss1 // error
-
-    ss1 == n // error
-    ss1 != n // error
-    n == ss1 // error
-    n != ss1 // error
+    ss1 == null
+    ss1 != null
+    null == ss1
+    null != ss1
+
+    ss1 == n
+    ss1 != n
+    n == ss1
+    n != ss1
 
     ss1 == ss2
     ss2 != ss1

tests/explicit-nulls/pos/pattern-matching.scala

@@ -0,0 +1,37 @@
+object Test:
+
+  def main(args: Array[String]): Unit =
+
+    val s: String = null.asInstanceOf[String]
+
+    val r1 = s match
+      case s: String => 100
+      case _ => 200
+    assert(r1 == 200)
+
+    val r2 = s match
+      case s: String => 100
+      case null => 200
+    assert(r2 == 200)
+
+    val r3 = s match
+      case null => 100
+      case _ => 200
+    assert(r3 == 100)
+
+    val s2: String | Null = null
+
+    val r4 = s2 match
+      case s2: String => 100
+      case _ => 200
+    assert(r4 == 200)
+
+    val r5 = s2 match
+      case s2: String => 100
+      case null => 200
+    assert(r5 == 200)
+
+    val r6 = s2 match
+      case null => 200
+      case s2: String => 100
+    assert(r6 == 200)