Re: Template to retrieve compile-time enum member from run-time enum member?

[Simen Kjærås via Digitalmars-d-learn]

On Friday, 27 April 2018 at 13:27:45 UTC, Timoses wrote:
Bumped across another problem : /
```
import std.stdio;

enum menum { A, B, C }

void main()
{
   foo(menum.A);
}

void foo(menum e)
{
    writeln(instantiateWith!Temp(e));
}

auto instantiateWith(alias Fn, T)(T x)
    if (is(T == enum))
{
    switch (x)
    {
        import std.traits : EnumMembers;
        static foreach (e; EnumMembers!T)
            case e:
                return Fn!e;
        default:
            assert(false);
    }
}

template Temp(menum e)
{
    struct TempStruct { uint i; };
    TempStruct Temp;

    shared static this()
    {
        static if (e == menum.A)
            Temp.i = 3;
    }

}
```

now returns:
source\app.d(25,17): Error: mismatched function return type 
inference of `TempStruct` and `TempStruct`
source\app.d(12,33): Error: template instance 
`app.instantiateWith!(Temp, menum)` error instantiating
dmd failed with exit code 1.

It's the same return type... so why the error?

That's an unfortunate error message. The problem is TempStruct is 
defined inside the Temp template. In the same way that struct 
Foo(T) {} is different for Foo!int and Foo!string, TempStruct is 
a different type for Temp!(menum.A) and Temp!(menum.B).

The solution is to move TempStruct outside the template:

struct TempStruct { uint i; }

template Temp(menum e)
{
    TempStruct Temp;
    shared static this()
    {
        static if (e == menum.A)
            Temp.i = 3;
    }
}

--
  Simen