Rewrite the if stat

Author: noti0na1

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

@@ -330,15 +330,16 @@ object Nullables with
     def computeAssignNullable()(given Context): tree.type = tree.lhs match
       case TrackedRef(ref) =>
         val rhstp = tree.rhs.typeOpt
-        if (rhstp.isNullType || (curCtx.explicitNulls && rhstp.isNullableUnion))
-          // If the type of rhs is nullable (`T|Null` or `Null`), then the nullability of the
-          // lhs variable is no longer trackable. We don't need to check whether the type `T`
-          // is correct here, as typer will check it.
-          tree.withNotNullInfo(NotNullInfo(Set(), Set(ref)))
-        else if (curCtx.explicitNulls && ref.isNullableUnion)
-          // If the initial type is nullable and the assigned value is non-null,
-          // we add it to the NotNull
-          tree.withNotNullInfo(NotNullInfo(Set(ref), Set()))
+        if curCtx.explicitNulls && ref.isNullableUnion then
+          if rhstp.isNullType || rhstp.isNullableUnion then
+            // If the type of rhs is nullable (`T|Null` or `Null`), then the nullability of the
+            // lhs variable is no longer trackable. We don't need to check whether the type `T`
+            // is correct here, as typer will check it.
+            tree.withNotNullInfo(NotNullInfo(Set(), Set(ref)))
+          else
+            // If the initial type is nullable and the assigned value is non-null,
+            // we add it to the NotNull.
+            tree.withNotNullInfo(NotNullInfo(Set(ref), Set()))
         else tree
       case _ => tree