[PATCH] D76084: [Sema][SVE] Reject subscripts on pointers to sizeless types

[Richard Sandiford via Phabricator via cfe-commits]

rsandifo-arm added a comment.

In D76084#1920103 <https://reviews.llvm.org/D76084#1920103>, @efriedma wrote:

> I'm not sure we actually want to reject this; let's discuss on the review for D76086 <https://reviews.llvm.org/D76086>, since subscripting is sort of a special case of arithmetic.


A silly corner case for this is:

  template<typename T>
  constexpr __SIZE_TYPE__ f(T *x) { return &x[1] - x; }
  typedef int arr1[f((int *)0) - 1];
  typedef int arr2[f((__SVInt8_t *)0) - 1];

which gives:

  a.cpp:2:48: warning: subtraction of pointers to type '__SVInt8_t' of zero size has undefined behavior [-Wpointer-arith]
  constexpr __SIZE_TYPE__ f(T *x) { return &x[1] - x; }
                                           ~~~~~ ^ ~
  a.cpp:4:18: note: in instantiation of function template specialization 'f<__SVInt8_t>' requested here
  typedef int arr2[f((__SVInt8_t *)0) - 1];
                   ^
  a.cpp:4:13: error: variable length array declaration not allowed at file scope
  typedef int arr2[f((__SVInt8_t *)0) - 1];
              ^

No real code would do anything like this of course.  But I think it shows that it's going to be difficult to divide cases that measure `sizeof` only during CodeGen from those that measure it during Sema.  With more and more things being accepted as constexprs, I think it's safer to treat any measure of `sizeof` as being at least potentially computable at compile time.

There's also the argument that this is what the spec says.  I realise that's a fairly weak argument though, since the spec could always be changed.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76084/new/

https://reviews.llvm.org/D76084