[libcxx-commits] [libcxx] [libc++] Make sure `flat_{multi}map::key_compare` handle `boolean-testable` correctly (PR #132621)

[Hewill Kang via libcxx-commits]

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@@ -846,7 +846,7 @@ class flat_map {
         __compare_(std::forward<_CompArg>(__comp)...) {}
 
   _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); };
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hewillk wrote:

> But for other containers and container adaptors, there doesn't seem such requirement propagation. For `hive` it's quite weird that [[hive.operations]](https://eel.is/c++draft/hive.operations) propagates the requirements, but `erase_if` isn't in that subclause.

In any case, `std::erase_if` can be argued to be an _algorithm_. 
"When not otherwise constrained, the `Predicate` parameter is used whenever an _algorithm_ expects a function object..." should also apply IMHO.

https://github.com/llvm/llvm-project/pull/132621