fixes #20958; fixes the return type of slice function [backport] (#20959)

* fixes #20958; fixes the return type of slice function

* add a testcase

compiler/semparallel.nim

@@ -402,6 +402,9 @@ proc transformSlices(g: ModuleGraph; idgen: IdGenerator; n: PNode): PNode =
     let op = n[0].sym
     if op.name.s == "[]" and op.fromSystem:
       result = copyNode(n)
+      var typ = newType(tyOpenArray, nextTypeId(g.idgen), result.typ.owner)
+      typ.add result.typ[0]
+      result.typ = typ
       let opSlice = newSymNode(createMagic(g, idgen, "slice", mSlice))
       opSlice.typ = getSysType(g, n.info, tyInt)
       result.add opSlice

tests/parallel/tsimple_array_checks.nim

@@ -61,3 +61,15 @@ maino() # Doesn't work outside a proc
 
 when true:
   main()
+
+block two:
+  proc f(a: openArray[int]) =
+    discard
+
+  proc main() =
+    var a: array[0..9, int] = [0,1,2,3,4,5,6,7,8,9]
+    parallel:
+      spawn f(a[0..2])
+
+
+  main()