Define nested ref function is not allowed with current dmd, but should be.
----
int n;
void main()
{
    ref int f(){ return n; }            // NG
//  nothrow ref int f(){ return n; }    // OK
//  auto ref int f(){ return n; }       // OK
    f() = 1;
    assert(n == 1);
----

See parse.c, calling route parseDeclDefs -> parseDeclaration changed with dmd 2.020(Fixing issue 2345), but did not changed parseStatement -> parseDeclaration from git brame.

It seems to me that this is a parser problem.