(arrow) branch main updated: GH-49586: [C++][CI] StructToStructSubset test failure with libc++ 22.1.1 (#49682)

[kou]

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kou pushed a commit to branch main
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The following commit(s) were added to refs/heads/main by this push:
     new 35fb62e622 GH-49586: [C++][CI] StructToStructSubset test failure with 
libc++ 22.1.1 (#49682)
35fb62e622 is described below

commit 35fb62e6224617d5ae749533654e9f8c7a6250c7
Author: tadeja <tad...@users.noreply.github.com>
AuthorDate: Tue Apr 7 23:52:09 2026 +0200

    GH-49586: [C++][CI] StructToStructSubset test failure with libc++ 22.1.1 
(#49682)
    
    ### Rationale for this change
    Fixes #49586
    
    `std::multimap::extract(key)` does not guarantee returning the first 
element when there are duplicate keys.
    
    ### What changes are included in this PR?
    Replace extract(key) with lower_bound(key) plus erase(iterator). 
`lower_bound` guarantees first matching element, then read `it->second` and 
then `erase` the node (`extract` is not needed as the node isn't reused here).
    
    ### Are these changes tested?
    Yes, CI jobs are passing.
    
    ### Are there any user-facing changes?
    No.
    * GitHub Issue: #49586
    
    Authored-by: Tadeja Kadunc <tadeja.kad...@gmail.com>
    Signed-off-by: Sutou Kouhei <k...@clear-code.com>
---
 cpp/src/arrow/compute/kernels/scalar_cast_nested.cc | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc 
b/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
index 3ab42d89b6..392fd9fbb7 100644
--- a/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
+++ b/cpp/src/arrow/compute/kernels/scalar_cast_nested.cc
@@ -346,11 +346,13 @@ struct CastStruct {
     for (int out_field_index = 0; out_field_index < out_field_count; 
++out_field_index) {
       const auto& out_field = out_type.field(out_field_index);
 
-      // Take the first field with matching name, if any. Extract it from the 
map so it
-      // can't be reused.
-      auto maybe_in_field_index = in_fields.extract(out_field->name());
-      if (!maybe_in_field_index.empty()) {
-        fields_to_select[out_field_index] = maybe_in_field_index.mapped();
+      // Take the first field with matching name, if any. Erase it from the 
map so it
+      // can't be reused. Use lower_bound (which guarantees first-match) 
instead of
+      // find/extract (which do not guarantee first for duplicate keys).
+      auto it = in_fields.lower_bound(out_field->name());
+      if (it != in_fields.end() && it->first == out_field->name()) {
+        fields_to_select[out_field_index] = it->second;
+        in_fields.erase(it);
       } else if (out_field->nullable()) {
         fields_to_select[out_field_index] = kFillNullSentinel;
       } else {