From 36ae3d544578432c1a6c58f972ac7690e1ecc937 Mon Sep 17 00:00:00 2001
From: Gerhard Raven <gerhard.raven@nikhef.nl>
Date: Mon, 22 Sep 2025 20:05:59 +0200
Subject: [PATCH] Make TrackCompactVertex' decayProducts satisfy
 std::ranges::viewable_range

---
 .../include/TrackKernel/TrackCompactVertex.h  | 31 +++++++++++++------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/Tr/TrackKernel/include/TrackKernel/TrackCompactVertex.h b/Tr/TrackKernel/include/TrackKernel/TrackCompactVertex.h
index 023b23ae857..24b67b5374d 100644
--- a/Tr/TrackKernel/include/TrackKernel/TrackCompactVertex.h
+++ b/Tr/TrackKernel/include/TrackKernel/TrackCompactVertex.h
@@ -230,27 +230,38 @@ namespace LHCb {
       friend auto decayProducts( const TrackCompactVertex& vtx ) {
         struct children_t {
           TrackCompactVertex const* vtx;
-          struct Sentinel {};
-          struct Proxy {
-            bool                      operator!=( Sentinel ) const { return i != vtx->numChildren(); }
-            TrackCompactVertex const* vtx;
-            unsigned                  i;
-            Proxy&                    operator++() {
+          struct iterator_t {
+            using difference_type [[maybe_unused]]   = std::ptrdiff_t;
+            using value_type [[maybe_unused]]        = iterator_t;
+            using iterator_concept [[maybe_unused]]  = std::input_iterator_tag;
+            using iterator_category [[maybe_unused]] = std::input_iterator_tag;
+
+            TrackCompactVertex const* vtx = nullptr;
+            unsigned                  i   = -1;
+            iterator_t&               operator++() {
               ++i;
               return *this;
             }
+            iterator_t operator++( int ) {
+              auto i = *this;
+              ++*this;
+              return i;
+            }
             const auto& operator*() const { return *this; }
+            bool        operator==( std::default_sentinel_t ) const { return !vtx || i == vtx->numChildren(); }
             // hack - hack -- client code sofar only needs mag2 of a vector....
             // so instead we need another customization point which default to returning threeMomentum(x).mag2()...
             auto threeMomentum() const {
               return LHCb::LinAlg::Vec<SIMDWrapper::scalar::float_v, 3>{ 0, 0, vtx->daughters.P( i ) };
             }
           };
-          Proxy    begin() const { return { vtx, 0 }; }
-          Sentinel end() const { return {}; }
-          Proxy    operator[]( unsigned i ) { return { vtx, i }; }
-          auto     size() const { return vtx->numChildren(); }
+          static_assert( std::input_iterator<iterator_t> );
+          iterator_t              begin() { return { vtx, 0 }; }
+          std::default_sentinel_t end() { return {}; }
+          iterator_t              operator[]( unsigned i ) { return { vtx, i }; }
+          auto                    size() const { return vtx->numChildren(); }
         };
+        static_assert( std::ranges::viewable_range<children_t> );
         return children_t{ &vtx };
       }
 
-- 
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