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?

On Tuesday, 24 May 2016 at 23:47:25 UTC, mogu wrote:
> 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.
> ```
>

This is because dub is apparently compiling with warnings enabled. If you compile manually with -w, you will get the same result.

> 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?

Take a look at [1] and you'll see that floating point types are not implicitly convertible to integrals, so the error is appropriate.

As for the += situation, because one operand is a double, the other will be converted to a double for the operation. Because of the implicit conversion rules, it may seem that an error should occur because of the assignment part in the +=. This isn't something I've really considered before, but I assume the rule is ignored here because += would not be very useful if you had to cast the result back to the original type. Where would you place the cast? That's why you get the truncation warning when compiling with -w instead of an error about implicit conversions.

https://dlang.org/spec/type.html#usual-arithmetic-conversions

On Wednesday, 25 May 2016 at 03:07:31 UTC, Mike Parker wrote:
> This is because dub is apparently compiling with warnings ...

Thanks for your answer. And sorry for that this thread should be in Learn section now i think.