[llvm] [STLExctras] Add out-of-line definition of friend operator== for C++20 (PR #72348)

[Utkarsh Saxena via llvm-commits]

https://github.com/usx95 created https://github.com/llvm/llvm-project/pull/72348

The last attempt at https://github.com/llvm/llvm-project/pull/72220 was reverted by https://github.com/llvm/llvm-project/commit/94d6699bf5eeb5aa4c50d1d90f8bf69b79201ceb because it breaks C++20 build in clang-17 and before.

This is a workaround of https://github.com/llvm/llvm-project/issues/70210 and unblocks https://github.com/llvm/llvm-project/pull/72213 which rectifies rewriting template operator and thus introduces new breakages.

Moving the function definition out of the class makes clang find a matching `operator!=` for the `operator==`. This makes clang not rewrite the `operator==` with reversed args. Hence, the ambiguity is resolved.

The final plan, when https://github.com/llvm/llvm-project/issues/70210 is fixed, is to move these back to inline definition or even convert to a member template operator. This should not be urgent and could even wait for a major clang release including https://github.com/llvm/llvm-project/pull/72213

>From b29aa485f2a541243d3764f4ed711ccc5a869519 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Wed, 15 Nov 2023 06:26:19 +0100
Subject: [PATCH] [STLExctras] Add out-of-line definition of friend operator==
 for C++20

---
 llvm/include/llvm/ADT/STLExtras.h | 40 +++++++++++++++++++++++--------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 18bc4d108b156bf..380d4d461c4e8db 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -1291,16 +1291,19 @@ class indexed_accessor_range_base {
   }
 
   /// Compare this range with another.
-  template <typename OtherT>
-  friend bool operator==(const indexed_accessor_range_base &lhs,
-                         const OtherT &rhs) {
-    return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
-  }
-  template <typename OtherT>
-  friend bool operator!=(const indexed_accessor_range_base &lhs,
-                         const OtherT &rhs) {
-    return !(lhs == rhs);
-  }
+  // FIXME: Make me a member function instead of friend when it works in C++20.
+  template <typename OtherT, typename DerivedT2, typename BaseT2, typename T2,
+            typename PointerT2, typename ReferenceT2>
+  friend bool
+  operator==(const indexed_accessor_range_base<DerivedT2, BaseT2, T2, PointerT2,
+                                               ReferenceT2> &lhs,
+             const OtherT &rhs);
+  template <typename OtherT, typename DerivedT2, typename BaseT2, typename T2,
+            typename PointerT2, typename ReferenceT2>
+  friend bool
+  operator!=(const indexed_accessor_range_base<DerivedT2, BaseT2, T2, PointerT2,
+                                               ReferenceT2> &lhs,
+             const OtherT &rhs);
 
   /// Return the size of this range.
   size_t size() const { return count; }
@@ -1364,6 +1367,23 @@ class indexed_accessor_range_base {
   /// The size from the owning range.
   ptrdiff_t count;
 };
+
+// FIXME: Make me a member function instead of friend when it works in C++20.
+template <typename OtherT, typename DerivedT2, typename BaseT2, typename T2,
+          typename PointerT2, typename ReferenceT2>
+bool operator==(const indexed_accessor_range_base<DerivedT2, BaseT2, T2,
+                                                  PointerT2, ReferenceT2> &lhs,
+                const OtherT &rhs) {
+  return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
+}
+
+template <typename OtherT, typename DerivedT2, typename BaseT2, typename T2,
+          typename PointerT2, typename ReferenceT2>
+bool operator!=(const indexed_accessor_range_base<DerivedT2, BaseT2, T2,
+                                                  PointerT2, ReferenceT2> &lhs,
+                const OtherT &rhs) {
+  return !(lhs == rhs);
+}
 } // end namespace detail
 
 /// This class provides an implementation of a range of