[Issue 15651] filter: only parameters or stack based variables can be inout

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=15651

--- Comment #4 from anonymous4 <dfj1esp02 at sneakemail.com> ---
It will need separation between collections and iterators
---
struct A
{
        int[][] a;
        inout(int[])[] f() inout
        {
                return a.filter(x=>x!=null).array;
        }
}

inout(FilterCollection) filter(inout int[][] r, bool function(const int[])
pred)
{
        return inout FilterCollection(r,pred);
}

struct FilterCollection
{
        int[][] r;
        bool function(const int[]) pred;
        FilterRange range() const
        {
                auto r=FilterRange(&this);
                r.next();
                return r;
        }
}

struct FilterRange
{
        const(FilterCollection)* src;
        long index;
        inout(int[]) front(ref inout FilterCollection src2)
        {
                assert(src==&src2);
                return src2.r[index];
        }
        void popFront()
        {
                index++;
                next();
        }
        package void next()
        {
                foreach(i,a;src.r[index..$])
                {
                        if(src.pred(a)){ index+=i; return; }
                }
                index=src.r.length;
        }
        bool empty()
        {
                return index==src.r.length;
        }
}

inout(int[])[] array(inout FilterCollection src)
{
        inout FilterCollection c=src;
        inout(int[])[] a;
        auto r=c.range;
        while(!r.empty)
        {
                //a~=r.front(c);
                inout int[] b=r.front(c);
                a~=b;
                r.popFront();
        }
        return a;
}

int main()
{
        A a;
        int[][4] b;
        b[0]=[1];
        b[3]=[2];
        a.a=b;
        int[][] c=a.f;
        assert(c.length==2);
        assert(c[0][0]==1);
        assert(c[1][0]==2);
        return 0;
}
---

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