[Issue 21537] Pointer to function pointer cannot be converted to pointer to const function pointer with lower attributes

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21537

--- Comment #6 from moonlightsentinel at disroot.org ---
Right, I've fixed the example. Conversions including const class references are
accepted but both conversion including FP and delegates are rejected on
nightly:

For reference:

class A {}
class B : A {}

alias SysFP = void function();
alias SafeFP = void function() @safe;

alias SysDG = void delegate();
alias SafeDG = void delegate() @safe;

void main()
{
    {
        A a;
        B b;
        a = b; // Fine
    }{
        const(A)* ap;
        const(B)* bp;
        ap = bp; // Accepted
    }{
        SafeFP safeFp;
        SysFP sysFp;
        sysFp = safeFp; // Fine
    }{
        const(SafeFP)* safeFpPtr;
        const(SysFP)* sysFpPtr;
        sysFpPtr = safeFpPtr; // Error: cannot implicitly convert expression
`safeFpPtr` of type `const(void function() @safe)*` to `const(void
function())*`
    }{
        SafeDG safeDg;
        SysDG sysDg;
        sysDg = safeDg; // Fine
    }{
        const(SafeDG)* safeDgPtr;
        const(SysDG)* sysDgPtr;
        sysDgPtr = safeDgPtr; // Error: cannot implicitly convert expression
`safeDgPtr` of type `const(void delegate() @safe)*` to `const(void
delegate())*`
    }
}

--