[PATCH] D78938: Make LLVM build in C++20 mode

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone added inline comments.


================
Comment at: llvm/include/llvm/IR/BasicBlock.h:324-325
+    template <typename PHINodeU, typename BBIteratorU,
+              typename = std::enable_if_t<
+                  std::is_convertible<PHINodeU *, PHINodeT *>::value>>
     phi_iterator_impl(const phi_iterator_impl<PHINodeU, BBIteratorU> &Arg)
----------------
dblaikie wrote:
> BRevzin wrote:
> > dblaikie wrote:
> > > What tripped over/required this SFINAE?
> > There's somewhere which compared a const iterator to a non-const iterator, that ends up doing conversions in both directions under C++20 rules, one direction of which is perfectly fine and the other was a hard error. Need to make the non-const iterator not constructible from a const iterator.
> Is this true for all iterators? Or some quirk of how this one is written/used (that could be fixed/changed there instead)?
IMO there is a (much) bigger task hiding here, which is to audit every type in the codebase whose name contains the string "Iterator" and compare them to the C++20 Ranges `std::forward_iterator` concept. My impression is that the vast majority of real-world "iterator types" are not iterators according to C++20 Ranges, and that this can have arbitrarily weird effects when you mix them with the C++20 STL.

However, that is //massive// scope creep re this particular patch. I think the larger question of "do all our iterators need X / are all our iterators written wrong" should be scoped-outside-of this patch.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D78938/new/

https://reviews.llvm.org/D78938