I think i was very missguided in today's call

Let me think if I can clarify the different scenario

for a thread_local of type S

If S is incomplete the program is ill-formed
If S has an incomplete destructor or constructor, or if S has a user-defined constructor or destructor, S is not trivial. In that case, thread_local in c++ will generate code to initialize and destroy the thread_local, but that will not happen if the variable is first accessed by a C TU

So my idea of an attribute to apply to the type is not helping (we don't care about incomplete types because that's ill-formed).

Another idea, brought up by Tom, was the idea of some syntax at point of use:

extern "C" thread_local S foo;

But the compiler still needs to create code for initialization and destruction. I guess in that case, the compiler can eagerly run the initialization, and that solves the not-yet-initialized problem C faces.

So Tom's solution is actually workable, and mine was just nonsense :)

But I think there is something else to think about here

void f() {

static thread_local S foo;

c_function(&foo); //#1

}

My understanding is that #1 is the issue here. Do we agree with that?

If that is the case, then I think we might have perfect information here

Indeed, c_function here is an extern "C" function (Are there cases where it isn't?).

A compiler could diagnose a call to c_function with a non-trivial thread_local object before initialization. I think. Has that been considered?

I understand this shifts the burden as a matter of QOI, unless we want to mandate it,

but it avoids burdening both languages and users with extra syntax complexity.

I might miss some important bits of informations so please do not take this as more than an unfiltered stream of consciousness

I think the proposal of no_destroy is generally useful yet orthogonal to C compat concerns.

Thanks,

Corentin