fix #13517 (#16681)

compiler/vm.nim

@@ -429,13 +429,13 @@ proc opConv(c: PCtx; dest: var TFullReg, src: TFullReg, desttyp, srctyp: PType):
         return true
     of tyUInt..tyUInt64:
       dest.ensureKind(rkInt)
-      case skipTypes(srctyp, abstractRange).kind
+      let styp = srctyp.skipTypes(abstractRange) # skip distinct types(dest type could do this too if needed)
+      case styp.kind
       of tyFloat..tyFloat64:
         dest.intVal = int(src.floatVal)
       else:
-        let srcDist = (sizeof(src.intVal) - srctyp.size) * 8
+        let srcDist = (sizeof(src.intVal) - styp.size) * 8
         let destDist = (sizeof(dest.intVal) - desttyp.size) * 8
-
         var value = cast[BiggestUInt](src.intVal)
         value = (value shl srcDist) shr srcDist
         value = (value shl destDist) shr destDist