This fails to compile with "error: non-constant condition for static assertion

     static_assert(std::char_traits<char>::length(cstr) == 2);"

```
#include <string>
static constexpr char cstr[3] = {'1', '2', '\0'};

constexpr int foobar() {
    static_assert(std::char_traits<char>::length(cstr) == 2);
    return 0;
}

int main() {}
```


But this compiles fine:
```
#include <string>
static constexpr char cstr[3] = {'1', '2', '\0'};

constexpr int foobar() {    
    return 0;
}

int main() { static_assert(std::char_traits<char>::length(cstr) == 2); }
```

As well as this:
```
#include <string>
static constexpr char cstr[3] = {'1', '2', '\0'};

int foobar() {    
    static_assert(std::char_traits<char>::length(cstr) == 2); 
    return 0;
}

int main() { }
```