How to attach function attributes to delegate type?

[Vijay Nayar]

On Wednesday, 27 February 2019 at 20:45:33 UTC, Alex wrote:
> On Wednesday, 27 February 2019 at 20:03:15 UTC, Q. Schroll 
> wrote:
>> For any type constructors like const, I can use ConstOf!T to 
>> get `T` with const attached. For a delegate/function type DG, 
>> e.g. int delegate(int), how can I get the @safe version of 
>> that type, i.e. int delegate(int) @safe?
>>
>> I tried
>>
>>     alias SafeOf(DG) = DG @safe;
>>
>> but it didn't compile.
>>
>> The case is not @safe-specific; it's the same for all function 
>> attributes.
>
> At https://p0nce.github.io/d-idioms/
>
> there is a demonstration for @nogc:
>
> ´´´
> import std.traits;
>
> // Casts @nogc out of a function or delegate type.
> auto assumeNoGC(T) (T t) if (isFunctionPointer!T || 
> isDelegate!T)
> {
>     enum attrs = functionAttributes!T | FunctionAttribute.nogc;
>     return cast(SetFunctionAttributes!(T, functionLinkage!T, 
> attrs)) t;
> }
> ´´´
>
> Didn't try this for other cases, however...

When I need particular attributes, such as for a comparator, but 
also need to pass in a function as a template argument, one 
approach I've started to adopt is to only call this function from 
within a wrapper that has all the properties I want.

For example:

import std.stdio;

class MyThing(T, alias LessF) {
   // This wrapper defines the conditions for a valid LessF.
   @safe @nogc
   private static bool less(in T t1, in T t2) {
     return LessF(t1, t2);
   }
}

void main()
{
   MyThing!(int, (a, b) => a < b) a;  // Compiles fine!
   MyThing!(int, (a, b) {
     writeln("Not @nogc!");  // Compiler error!
     return a < b;
   }) b;
}

The error looks like this, which is fairly readable too.

onlineapp.d(6): Error: `@nogc` function 
`onlineapp.main.MyThing!(int, (a, b)
{
writeln("Not @nogc!");
return a < b;
}
).MyThing.less` cannot call non- at nogc function