import std;

struct W(T) {
    T value;
    auto hook(handlers...)() {
        return handlers[0](value);
    }
}

template f(handlers...) {
    auto ref f(T)(auto ref T value) {
        return value.hook!handlers;
    }
}

@nogc void main() {
    auto a = W!int(3);
    auto b = a.f!((_) => "yes");
}

Produces: Error: @nogc function D main cannot call non-@nogc function onlineapp.main.f!(W!int).f

Adding @nogc on function f says hook!((_) => "yes").hook closes over variable __this310

If you explicitly specify a type on the lambda that you pass to f then it compiles:

auto b = a.f!((int) => "yes");