DMD different compiler behaviour on Linux and Windows

[Mike Parker]

On Thursday, 25 April 2019 at 20:18:28 UTC, Zans wrote:
> import std.stdio;
>
> void main()
> {
>     char[] mychars;
>     mychars ~= 'a';
>     long index = 0L;
>     writeln(mychars[index]);
> }
>
> Why would the code above compile perfectly on Linux (Ubuntu 
> 16.04), however it would produce the following error on Windows 
> 10:
>
> source\app.d(8,21): Error: cannot implicitly convert expression 
> index of type long to uint
>
> On both operating systems DMD version is 2.085.0.

DMD defaults to 64-bit output on 64-Bit Linux but always to 
32-bit output on Windows, If you compile with -m32 on Windows the 
error goes away. Reasons::

* Array indices are default typed as size_t, which is uint in 
32-bit and ulong in 64. long is not implicitly convertible to 
uint.

* On Linux, both 64- and 32-bit builds of DMD are available and 
the output for each defaults to the same. Only the 32-bit build 
is distributed on Windows.

* Historically, compiling 64-bit binaries on Windows required a 
separate installation of the Microsoft Build Tools ( or Visual 
Studio) and/or the Windows SDK. If 64-bit output were the 
default, DMD would not work out of the box. Recently, DMD has 
been shipping with the lld linker and some MinGW-based Windows 
libraries so that the additional installation is not required and 
64-bit compiles can work out of the box, but it’s still 
considered experimental.

When out-of-the-box 64-bit compilation is solid and 64-bit builds 
are distributed on Windows, the default behavior should be the 
same as on Linux.