Does anyone can tell me why the following codes return TRUE. It confuses me.

```
> require(Rcpp)
Loading required package: Rcpp
Warning message:
package ‘Rcpp’ was built under R version 3.3.3 
> src12 <- '
+ #include <Rcpp.h>
+ using namespace Rcpp;
+ 
+ // [[Rcpp::plugins("cpp11")]]
+ 
+ // [[Rcpp::export]]
+ bool is_naFUN() {
+ 
+ LogicalVector y = {TRUE,FALSE};
+ bool x = is_na(all(y == NA_LOGICAL));
+ 
+ return x;
+ }
+ '
> sourceCpp(code = src12)
> is_naFUN()
[1] TRUE

```

Acturaly, it comes here. I'm learning the tutorial.
[rcppforeveryone-functions-related-to-logical-values](https://teuder.github.io/rcpp4everyone_en/210_rcpp_functions.html#functions-related-to-logical-values)
how to make a clear understanding about of NA\_LOGICAL in Rcpp? Thank you!