[reland][libc] Add reverse_iterator comparisons

[gchatelet]

This is a reland of #86147 but with a proper base() function.

https://en.cppreference.com/w/cpp/iterator/reverse_iterator/operator_cmp

[llvmbot]

@llvm/pr-subscribers-libc

Author: Guillaume Chatelet (gchatelet)

Changes

https://en.cppreference.com/w/cpp/iterator/reverse_iterator/operator_cmp

---

Full diff: https://github.com/llvm/llvm-project/pull/86188.diff

1 Files Affected:

• (modified) libc/src/__support/CPP/iterator.h (+35)

diff --git a/libc/src/__support/CPP/iterator.h b/libc/src/__support/CPP/iterator.h
index c5bfb1912c7b74..b0fd5c9f22ae01 100644
--- a/libc/src/__support/CPP/iterator.h
+++ b/libc/src/__support/CPP/iterator.h
@@ -20,6 +20,7 @@ namespace cpp {
 template <typename T> struct iterator_traits;
 template <typename T> struct iterator_traits<T *> {
   using reference = T &;
+  using value_type = T;
 };
 
 template <typename Iter> class reverse_iterator {
@@ -27,6 +28,8 @@ template <typename Iter> class reverse_iterator {
 
 public:
   using reference = typename iterator_traits<Iter>::reference;
+  using value_type = typename iterator_traits<Iter>::value_type;
+  using iterator_type = Iter;
 
   LIBC_INLINE reverse_iterator() : current() {}
   LIBC_INLINE constexpr explicit reverse_iterator(Iter it) : current(it) {}
@@ -38,6 +41,38 @@ template <typename Iter> class reverse_iterator {
   LIBC_INLINE constexpr explicit reverse_iterator(const Other &it)
       : current(it) {}
 
+  LIBC_INLINE friend constexpr bool operator==(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() == rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator!=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() != rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator<(const reverse_iterator &lhs,
+                                              const reverse_iterator &rhs) {
+    return lhs.base() > rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator<=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() >= rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator>(const reverse_iterator &lhs,
+                                              const reverse_iterator &rhs) {
+    return lhs.base() < rhs.base();
+  }
+
+  LIBC_INLINE friend constexpr bool operator>=(const reverse_iterator &lhs,
+                                               const reverse_iterator &rhs) {
+    return lhs.base() <= rhs.base();
+  }
+
+  LIBC_INLINE constexpr iterator_type base() const { return current; }
+
   LIBC_INLINE constexpr reference operator*() const {
     Iter tmp = current;
     return *--tmp;

[gchatelet]

The fix 8cfdd0e is stacked on the original commit for easier review.

[nickdesaulniers]

Sorry I missed this in code review of #86147. I guess without calls to these methods (in tests or sources) we don't observe diagnostics at compile time?

[gchatelet]

Sorry I missed this in code review of #86147. I guess without calls to these methods (in tests or sources) we don't observe diagnostics at compile time?

Indeed.