Failure due to memcpy being called at compile time

[JG]

On Monday, 13 June 2022 at 19:59:16 UTC, Steven Schveighoffer 
wrote:
> On 6/13/22 3:48 PM, JG wrote:
>> Hi,
>> 
>> I reduced my code to the following.  Could anyone help me to 
>> discover why the line marked with //THIS LINE
>> 
>> causes memcpy to be called, and how can I avoid this?
>> 
>> 
>> ```d
>> import std;
>> 
>> struct ParseError { string msg; }
>> 
>> alias ParseErrorOr(T) = SumType!(ParseError,T);
>> auto parseErrorOr(T)(T x) { return ParseErrorOr!T(x); }
>> 
>> auto parserOr(I,fs...)(I i) {
>>    alias RetType = typeof(fs[0](I.init));
>>    auto cur = RetType(ParseError.init);
>>    foreach(f;fs) {
>>      if(cur.match!((ParseError e)=>false,_=>true)) { return 
>> cur; }
>>      cur = f(i); //THIS LINE
>>    }
>>    return cur;
>> }
>> 
>> auto parseNothing(I)(I i) {
>>      return parseErrorOr(tuple(i[0..0],i));
>> }
>> 
>> void main()
>> {
>>      enum a = 
>> parserOr!(string,parseNothing!string,parseNothing!string)("hello");
>> }
>> ```
>> 
>> 
>> 
>
> Happens in `moveEmplaceImpl` in `core.lifetime`. Somebody is 
> calling that.
>
> https://github.com/dlang/druntime/blob/v2.099.1/src/core/lifetime.d#L2192
>
> No stack trace though, that would actually be nice to have in 
> the CTFE interpreter.
>
> I imagine if you solved that call, you would get out to a place 
> where it tries to cast to the actual type and fail there 
> instead.
>
> -Steve

Thanks. It seems to be something to do with the variadic template 
since this
works:

```d
import std;

struct ParseError { string msg; }

alias ParseErrorOr(T) = SumType!(ParseError,T);
auto parseErrorOr(T)(T x) { return ParseErrorOr!T(x); }

auto parserOr(I,alias f, alias g)(I i) {
   auto cur = f(i);
   if(cur.match!((ParseError e)=>false,_=>true)) { return cur; }
   return g(i);
}

auto parseNothing(I)(I i) {
     return parseErrorOr(tuple(i[0..0],i));
}

void main()
{
     enum a = 
parserOr!(string,parseNothing!string,parseNothing!string)("hello");
}
```