Bug 18587 – Context pointer not set for delegate template parameter when declared in struct body

struct S { int n; R!(() => n) a; } struct R(alias fn) { int get() { return fn(); } } unittest { auto s = S(3); assert(s.a.get() == 3); } The above code fails with 'Error: need 'this' for 'n' of type 'int''. It is clear that the intended semantics of the above code is similar to this: struct S { int n; auto a() { return R!(() => n)(); } } struct R(alias fn) { int get() { return fn(); } } unittest { auto s = S(3); assert(s.a.get() == 3); } However, the latter example works, while the former doesn't. This leads to unnecessary boilerplate and unreadable code.

Since structs in D are movable by simple blitting with no cleanup step, this is impossible. If we consider this code again: struct S { int n; R!(() => n) a; } struct R(alias fn) { int get() { return fn(); } } The layout of R would essentially be this: struct R(alias fn) { S* context; } Thus, the layout of S would essentially be this: struct S { int n; S* a.context; } That's an internal pointer, and runs afoul of this section of the GC spec[0]: > Do not have pointers in a struct instance that point back to the same instance. The trouble with this is if the instance gets moved in memory, the pointer will point back to where it came from, with likely disastrous results. [0]: https://dlang.org/spec/garbage.html