Strange error

[Steven Schveighoffer]

On 3/21/21 3:18 AM, Jack Applegame wrote:
> Could someone please explain what is wrong with this code?
> 
> https://glot.io/snippets/fwxn2198kv
> 
> ```d
> import std.stdio;
> 
> struct Sample{
>    void function() func1;
>    void function() func2;
> }
> 
> void noth(Sample smpl)() {
>    smpl.func1(); // Error: expression __lambda1 is not a valid template 
> value argument
>    smpl.func2(); // Error: expression __lambda2 is not a valid template 
> value argument
> }
> 
> void main(){
>    enum s = Sample(
>      {writeln("Hello world1");},
>      {writeln("Hello world2");}
>    );
>    s.func1();
>    s.func2();
>    noth!(s)();
> }
> ```

You have the wrong lines attributed with the error message. The error 
message is in the assignment of s (line 15 and 16)

Essentially, you can't use lambda functions as compile-time values.

Valid template value parameters are [1]

"any type which can be statically initialized at compile time. Template 
value arguments can be integer values, floating point values, nulls, 
string values, array literals of template value arguments, associative 
array literals of template value arguments, or struct literals of 
template value arguments."

Now, one might argue that it's possible to statically initialize 
function pointers at compile time (i.e. you can do this as a static 
initialized function pointer at module scope), so possibly this is 
something that can be fixed. There might already be a bug report on it, 
or you can file one. It might end up with the spec being clarified to 
explicitly cover this case.

-Steve

[1] https://dlang.org/spec/template.html#template_value_parameter