aaron.ballman added a comment.
In D122895#3481564 <https://reviews.llvm.org/D122895#3481564>, @manojgupta
wrote:
> Some of our users are not very happy with the churn probably caused by this
> change where the declaration has the "void" argument but the later definition
> does not have explicit "void".
>
> void foo(void);
>
> void foo()
> {
> }
>
> GCC does not warn about this usage: https://godbolt.org/z/zPP8qjc98
>
> Any opinions?
Thank you for the feedback! I think it's debatable whether this is a bug or
not, but it's an interesting case that I may think about a bit further before
making any final decisions. My current thinking is...
We turned `-Wstrict-prototypes` into a pedantic warning specifically telling
the user about K&R C function usage everywhere it occurs, and from that
perspective `void f() {}` is correct to diagnose pedantically as the signature
is specifically a K&R C one. However, the composite type between `void f(void)`
(with a parameter list) and `void f() {}` (with an empty identifier list) is a
function with a prototype of `void(void)` (per C99 6.2.7p3). Thus, a call to
the function through the declaration can pass no arguments, and a call to the
function through the definition can pass no arguments, and so it also seems
reasonable to not diagnose.
(We added `-Wdeprecated-non-prototype` to cover the case where there's a
behavioral change to the code in C2x, which diagnoses `void f(int);` and `void
f(i) int i; {}` (this code will break in C2x), so despite the similarity with
your case, it's a different diagnostic entirely.)
FWIW, GCC's behavior isn't particularly interesting because we're knowingly
diverging from their strategy with the warning, so it's not too surprising that
our behavior is different.
I think what sells it for me not being a bug is that we issue that warning when
determining the function *type* when forming the declaration which happens
before doing function merging. At the point where we issue this warning, we
don't have enough information to know whether the function will eventually have
a composite type with a prototype or not (we don't even have enough information
to know whether it's for a function declaration or not; it could be for a
typedef, etc), so suppressing the diagnostic would be rather difficult. And
because it's a pedantic warning specifically, I think it's defensible that
we're issuing the diagnostic.
Is disabling the pedantic warning an option for your users?
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