I use dub 0.9.25, DMD64 D Compiler v2.071.0. Create a minimal
project and write the following codes in main function.
```
import std.math;
long i = 0;
i += 3.5.floor;
```
Then run dub will get a warning and dmd failed.
```
source/app.d(4,7): Warning: long += double is performing
truncating conversion
dmd failed with exit code 1.
```
But build the app.d directly use dmd. It's OK. However if the
last line of code above is `i = 3.5.floor;`. An error occurs.
```
Error: cannot implicitly convert expression (floor(3.5)) of type
double to long
```
So what's the deal?
