fix #18282: consider Predef.eq/ne in nullability flow typing

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

@@ -115,6 +115,10 @@ object Nullables:
         testSym(tree.symbol, l)
       case Apply(Select(Literal(Constant(null)), _), r :: Nil) =>
         testSym(tree.symbol, r)
+      case Apply(Apply(op, l :: Nil), Literal(Constant(null)) :: Nil) =>
+        testPredefSym(op.symbol, l)
+      case Apply(Apply(op, Literal(Constant(null)) :: Nil), r :: Nil) =>
+        testPredefSym(op.symbol, r)
       case _ =>
         None
 
@@ -123,6 +127,13 @@ object Nullables:
       else if sym == defn.Any_!= || sym == defn.Object_ne then Some((operand, false))
       else None
 
+    private def testPredefSym(opSym: Symbol, operand: Tree)(using Context) =
+      if opSym.owner == defn.ScalaPredefModuleClass then
+        if opSym.name == nme.eq then Some((operand, true))
+        else if opSym.name == nme.ne then Some((operand, false))
+        else None
+      else None
+
   end CompareNull
 
   /** An extractor for null-trackable references */

tests/explicit-nulls/pos/flow-predef-eq.scala

@@ -0,0 +1,7 @@
+def f(s: String|Null): String = {
+  if(s eq null) "foo" else s
+}
+
+def f2(s: String|Null): String = {
+  if(s ne null) s else "foo"
+}