Tweak assertion in withMods

Fixes #5037

Author: odersky

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -281,9 +281,10 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       else {
         if (ms.nonEmpty)
           for (m <- ms)
-            assert(flags.isAllOf(m.flags) ||
-                   m.isInstanceOf[Mod.Private] && !privateWithin.isEmpty,
-                   s"unaccounted modifier: $m in $this when adding $ms")
+            assert(flags.isAllOf(m.flags)
+                || m.isInstanceOf[Mod.Private] && !privateWithin.isEmpty
+                || m.isInstanceOf[Mod.Abstract] || m.isInstanceOf[Mod.Override] && flags.is(AbsOverride),
+                s"unaccounted modifier: $m in $this with flags ${flags.flagsString} when adding $ms")
         copy(mods = ms)
       }
 

tests/neg/i5037.scala

@@ -0,0 +1,3 @@
+class Foo {
+abstract override var // error
+}