I believe that since C23 foo() is now a nullary function. As this is the last approved standard and it supersedes all previous standards, it is technically correct to say that de-jure this is what the (unqualified) C standard mandates.
None, but that is not my point. Before C23, fn() already meant the same thing as fn(void) in function definitions, which the situation under discussion here.
C23 changed what fn() means outside a function definition.
Oh, yeah, the codegen for the fn() itself would likely be the same, but the prototype of that definition is still a K&R function. https://godbolt.org/z/Psvae55Pr
Quote a different standard.
I believe that since C23 foo() is now a nullary function. As this is the last approved standard and it supersedes all previous standards, it is technically correct to say that de-jure this is what the (unqualified) C standard mandates.
Of course de-facto things are more nunanced.
C23 does not change anything in this situation, because we are talking about the definition of main(), not a forward declaration. More details here:
https://news.ycombinator.com/item?id=38729278#38732366
In what situation fn() doesn't mean fn(void) under C23?
None, but that is not my point. Before C23, fn() already meant the same thing as fn(void) in function definitions, which the situation under discussion here.
C23 changed what fn() means outside a function definition.
Oh, yeah, the codegen for the fn() itself would likely be the same, but the prototype of that definition is still a K&R function. https://godbolt.org/z/Psvae55Pr