[PATCH] D134529: [C++20][Clang] P2468R2 The Equality Operator You Are Looking For

[Utkarsh Saxena via Phabricator via cfe-commits]

usaxena95 added a comment.

In D134529#3852990 <https://reviews.llvm.org/D134529#3852990>, @erichkeane wrote:

> Note that @BertalanD  noticed an issue with what I expect to be this patch: https://godbolt.org/z/Wjb9rsEYG
>
> Can someone more knowledgable about this paper please make sure it is an intended change?  It seems to me that the reversing behavior of the equality operators is a little awkward here (note that commenting out != makes that 'work' again).

This is intentional. Example from https://eel.is/c++draft/over.match.oper#4

  struct A {};
  template<typename T> bool operator==(A, T);     // #1
  bool a1 = 0 == A();                             // OK, calls reversed #1
  template<typename T> bool operator!=(A, T);
  bool a2 = 0 == A();                             // error, #1 is not a rewrite target

Adding a matching `operator!=` disables reversing arguments (C++17 mode). The LHS in this case should provide a `operator==`.

The diagnostic might not be the most helpful at this point is. You can choose to

- Remove `operator!=`. Allows assuming `x==y` is equivalent to `y==x` and therefore uses the same function with reversed arguments.
- Write `s == c` instead of `c == s`.


Repository:
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https://reviews.llvm.org/D134529