[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:

> I recalled that the current standard wording doesn't actually require the result type of `Predicate` of `erase_if` for `flat_meow` to model _`boolean-testable`_. The boolean-testablity requirements seem only imposed for `erase_if` for sequence containers.
> 

I mainly referred to https://eel.is/c++draft/algorithms#requirements-6


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