fixes #18371 (#18617)

compiler/vmops.nim

@@ -303,12 +303,15 @@ proc registerAdditionalOps*(c: PCtx) =
 
   proc getEffectList(c: PCtx; a: VmArgs; effectIndex: int) =
     let fn = getNode(a, 0)
+    var list = newNodeI(nkBracket, fn.info)
     if fn.typ != nil and fn.typ.n != nil and fn.typ.n[0].len >= effectListLen and
         fn.typ.n[0][effectIndex] != nil:
-      var list = newNodeI(nkBracket, fn.info)
       for e in fn.typ.n[0][effectIndex]:
         list.add opMapTypeInstToAst(c.cache, e.typ.skipTypes({tyRef}), e.info, c.idgen)
-      setResult(a, list)
+    else:
+      list.add newIdentNode(getIdent(c.cache, "UncomputedEffects"), fn.info)
+
+    setResult(a, list)
 
   registerCallback c, "stdlib.effecttraits.getRaisesListImpl", proc (a: VmArgs) =
     getEffectList(c, a, exceptionEffects)

lib/std/effecttraits.nim

@@ -11,7 +11,7 @@
 ## for Nim's macro system.
 ## **Since**: Version 1.4.
 ##
-## One can test for the existance of this standard module
+## One can test for the existence of this standard module
 ## via `defined(nimHasEffectTraitsModule)`.
 
 import macros