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.

Author: olhotak

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

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
+}