Why does this `static foreach` lead to hidden usage of operator `~=` calls in some cases?

> static foreach(i; 0 .. cnt)

> onlineapp.d(9): Error: cannot use operator ~= in @nogc delegate onlineapp.xv!(myUDA("/")).__funcliteral2.__lambda1




> import std.traits;
> 
> private @safe pure nothrow @nogc
> {
>     enum xv(alias uda) = function()
>     {
>         enum cnt = 10;
> 
>         static foreach(i; 0 .. cnt)
>         {
>         }
>     };
> }
> 
> struct myUDA
> {
>     string path;
> }
> 
> @myUDA("/")
> void doSth(string path)
> {
> }
> 
> extern(C) void main()
> {
>     xv!(getUDAs!(doSth, myUDA))();
> }


Try: https://run.dlang.io/is/clQgeA

On Sunday, 7 July 2019 at 16:51:57 UTC, 0xEAB wrote:
> Why does this `static foreach` lead to hidden usage of operator `~=` calls in some cases?
>

probably same https://forum.dlang.org/post/qd9ee0$2eud$1@digitalmars.com

On Sunday, 7 July 2019 at 17:07:59 UTC, a11e99z wrote:
> On Sunday, 7 July 2019 at 16:51:57 UTC, 0xEAB wrote:
>> Why does this `static foreach` lead to hidden usage of operator `~=` calls in some cases?
>>
> probably same
oops! this one https://forum.dlang.org/post/eidpgqohllwmuumxwrbf@forum.dlang.org

On Sunday, 7 July 2019 at 16:51:57 UTC, 0xEAB wrote:
> Why does this `static foreach` lead to hidden usage of operator


Further notes by Dan (aka "Wild"):
> I added some small printfs to the compiler, http://ix.io/1NWM
> It seems like it lowers it into something weird

On Sunday, 7 July 2019 at 18:45:14 UTC, 0xEAB wrote:
> On Sunday, 7 July 2019 at 16:51:57 UTC, 0xEAB wrote:
>> Why does this `static foreach` lead to hidden usage of operator
>
>
> Further notes by Dan (aka "Wild"):
>> I added some small printfs to the compiler, http://ix.io/1NWM
>> It seems like it lowers it into something weird

Static foreach needs a tuple. When you have a range like "0 .. 10" it has to convert that to a tuple. It does this by generating some code and running it at CTFE. Guess this was done as it was a lot simpler to do, you don't have to worry about recreating ranges as that's already done by using CTFE.

What it ends up lowering to: https://github.com/dlang/dmd/blob/v2.087.0/src/dmd/cond.d#L274

     *     static foreach (x; {
     *         typeof({
     *             foreach (x; range) return x;
     *         }())[] __res;
     *         foreach (x; range) __res ~= x;
     *         return __res;
     *     }()) { ... }