"cannot inline default argument" of a mixin template alias parameter

[dlangBugzillaToGithub]

Nicholas Wilson (@thewilsonator) reported this on 2019-08-11T00:50:21Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20119

Description

struct S
{
    int data;
    int pos = 5;
private:  
    mixin template auxdispatch(alias F, alias indx)
    {
        auto auxdispatch(int x = indx) //Error here
        {
            return F(x);
        }
    }
    auto dispatcher(Func...)(int a)
    {
        alias Func0 = Func[0];
        return Func0!()(a);
    }
 public:   
    void outer()
    {
        void nested()(int a)
        {
            data += a;
        }
        
        mixin auxdispatch!(nested,pos);
        auxdispatch();
        import std.stdio;
        writeln(data);
    }
}

void main()
{
    S s;
    s.outer();
}

fails with "onlineapp.d(8): Error: cannot inline default argument this.pos"