The bugzilla search found this old thing with a similar title but different body:
https://issues.dlang.org/show_bug.cgi?id=10557

that was about default params, but this is about the thing as a whole. Notice the following:

---
void function(int) fp;
void delegate(int) dg;

static assert(is(typeof(fp) == __parameters)); // fails, but should succeed per spec

static assert(is(typeof(*fp) == __parameters)); // OK, this is how you extract in current system

static assert(is(typeof(dg) == __parameters)); // fails but should succeed per spec

// this is how you extract in current impl
static if(is(typeof(dg) FP == delegate))
        static assert(is(FP == __parameters)); // OK
else
        static assert(0);
---

(you will have to comment one or the other test to actually see the failure)


The spec says: https://dlang.org/spec/expression.html#IsExpression

===
__parameters	the parameter sequence of a function, delegate, or function pointer. This includes the parameter types, names, and default values.
===

The spec specifically states it should take a delegate or function pointer directly.


Has failed in all dmd versions I've tried.