[libc++] Add _LIBCPP_NO_UNIQUE_ADDRESS to flat_{,multi}map::value_compare

[philnik777]

This breaks the ABI of flat_{,multi}map::value_compare, but this type has only been introduced in LLVM 20, so it should be very unlikely that we break anybody if we back-port this now.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

This breaks the ABI of flat_{,multi}map::value_compare, but this type has only been introduced in LLVM 20, so it should be very unlikely that we break anybody if we back-port this now.

---

Full diff: https://github.com/llvm/llvm-project/pull/137594.diff

2 Files Affected:

• (modified) libcxx/include/__flat_map/flat_map.h (+1-1)
• (modified) libcxx/include/__flat_map/flat_multimap.h (+1-1)

diff --git a/libcxx/include/__flat_map/flat_map.h b/libcxx/include/__flat_map/flat_map.h
index f5abfd0985280..f5e9756ff6a60 100644
--- a/libcxx/include/__flat_map/flat_map.h
+++ b/libcxx/include/__flat_map/flat_map.h
@@ -114,7 +114,7 @@ class flat_map {
 
   class value_compare {
   private:
-    key_compare __comp_;
+    _LIBCPP_NO_UNIQUE_ADDRESS key_compare __comp_;
     _LIBCPP_HIDE_FROM_ABI value_compare(key_compare __c) : __comp_(__c) {}
     friend flat_map;
 
diff --git a/libcxx/include/__flat_map/flat_multimap.h b/libcxx/include/__flat_map/flat_multimap.h
index ea77fb5d79bd2..15fcd7995ad0a 100644
--- a/libcxx/include/__flat_map/flat_multimap.h
+++ b/libcxx/include/__flat_map/flat_multimap.h
@@ -115,7 +115,7 @@ class flat_multimap {
 
   class value_compare {
   private:
-    key_compare __comp_;
+    _LIBCPP_NO_UNIQUE_ADDRESS key_compare __comp_;
     _LIBCPP_HIDE_FROM_ABI value_compare(key_compare __c) : __comp_(__c) {}
     friend flat_multimap;