dmd/compiler/test/fail_compilation/diag10768.d

/*
TEST_OUTPUT:
---
fail_compilation/diag10768.d(35): Error: cannot implicitly override base class method `diag10768.Frop.frop` with `diag10768.Foo.frop`; add `override` attribute
---
*/

struct CirBuff(T)
{

    CirBuff!T opAssign(R)(R)
    {}

    T[] toArray()
    {
        T[] ret; //  = new T[this.length];
        return ret;
    }
    alias toArray this;
}

class Bar(T=int)
{
    CirBuff!T _bar;
}

class Once
{
    Bar!Foo _foobar;
}

class Foo : Frop
{
    // override
    public int frop() { return 1; }
}

class Frop
{
    public int frop() { return 0; }
}