[libc++] Make sure flat_set::key_compare handle boolean-testable correctly

[hewillk]

No description provided.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Hewill Kang (hewillk)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/132622.diff

1 Files Affected:

• (modified) libcxx/include/__flat_set/flat_set.h (+1-1)

diff --git a/libcxx/include/__flat_set/flat_set.h b/libcxx/include/__flat_set/flat_set.h
index cc45db7565bd4..9074e9e9cf241 100644
--- a/libcxx/include/__flat_set/flat_set.h
+++ b/libcxx/include/__flat_set/flat_set.h
@@ -602,7 +602,7 @@ class flat_set {
 
 private:
   _LIBCPP_HIDE_FROM_ABI bool __is_sorted_and_unique(auto&& __key_container) const {
-    auto __greater_or_equal_to = [this](const auto& __x, const auto& __y) { return !__compare_(__x, __y); };
+    auto __greater_or_equal_to = [this](const auto& __x, const auto& __y) -> bool { return !__compare_(__x, __y); };
     return ranges::adjacent_find(__key_container, __greater_or_equal_to) == ranges::end(__key_container);
   }
 

[ldionne]

Thanks for the patch! Can you please add a test for this?