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

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang/Parse/Parser.h:2311
     case DeclSpecContext::DSC_association:
+    case DeclSpecContext::DSC_offsetof:
       return true;
----------------
Is this correct? I don't think we can deduce the type from `offsetof` through CTAD, can we?

https://godbolt.org/z/Kab6ahYe7

(This might be a good test case to add assuming we don't already have that coverage.)


================
Comment at: clang/include/clang/Sema/DeclSpec.h:2069
     case DeclaratorContext::Association:
+    case DeclaratorContext::OffsetOf:
       return true;
----------------
I don't think we can omit the identifier in this context -- this function is used to see whether you can do something like:
```
template <int>
void func(int) {
  try {
  } catch (int) {
  }
}
```
and `offsetof` seems quite different from that.


================
Comment at: clang/test/Sema/offsetof.c:79
+    int a;
+    struct B // no-error, struct B is not defined within __builtin_offsetof directly
+    {
----------------
I think this is defensible. The wording in the standard is "If the specified type defines a new type or if the specified member is a bit-field, the behavior is undefined." and the specified type in this case is `struct A`; that `struct A` happens to also define `struct B` is immaterial.

However, the intent behind the change to the rule is to support older implementations of `offsetof` to protect them from having to deal with a case like: `offsetof(struct S { int a, b }, b);` where `offsetof` is a macro and thus the comma between `a` and `b` is treated as a separator. So there's a part of me that wonders if we want to also support diagnosing this case. But then we'd have to look at the declarator context more recursively to see whether any of the contexts on the stack are an `offsetof` context and that might be tricky.

Thoughts?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D133574/new/

https://reviews.llvm.org/D133574