4 fail_compilation/b19691e.d(16): Error: forward reference to template `this`
5 fail_compilation/b19691e.d(16): Error: constructor `b19691e.S2.this(int a, S1 s = "")` is not callable using argument types `(int, string)`
6 fail_compilation/b19691e.d(16): Error: forward reference to template `this`
9 // https://issues.dlang.org/show_bug.cgi?id=19691
22 this(int a
, S1 s
= ""){}