[PATCH] D133574: [C2x] reject type definitions in offsetof

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D133574#3811496 <https://reviews.llvm.org/D133574#3811496>, @inclyc wrote:

> Emm, is it necessary to add a `LangOpts` check so that this change only applies to c2x? If clang was invoked without `-std=c2x`, should we just accept `offsetof` with definitions?

No, I think we should treat this as a defect report and have the behavior be the same in all versions of C.



================
Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:1650
+def err_type_defined_in_offsetof : Error<
+  "%0 cannot be defined in '__builtin_offsetof'">;
 
----------------
inclyc wrote:
> aaron.ballman wrote:
> > We might want this change, we might not -- can you test the diagnostic behavior when using `#include <stddef.h>`? Does it print `__builtin_offsetof` in the following example?
> > ```
> > #include <stddef.h>
> > 
> > int main() {
> >   return offsetof(struct S { int a; }, a);
> > }
> > ```
> > when executed with `clang -fsyntax-only -ffreestanding -std=c2x test.c`
> > 
> > If it prints the builtin name, I think we'll want to look at the builtin token to see if it was expanded from a macro named `offsetof` to improve the diagnostic quality.
> ```
> local/offsetofcc.c:4:26: error: 'struct S' cannot be defined in '__builtin_offsetof'
>   return offsetof(struct S { int a; }, a);
>                          ^
> 1 error generated.
> ```
> > If it prints the builtin name, I think we'll want to look at the builtin token to see if it was expanded from a macro named offsetof to improve the diagnostic quality.
> 
> OK
We have similar code for this here: 

https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaChecking.cpp#L13438


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