From 2b0b1c5bc941ff67e6fa9d14a52bfcd9cd031401 Mon Sep 17 00:00:00 2001
From: Even Rouault <even.rouault@spatialys.com>
Date: Fri, 24 Jan 2025 18:07:54 +0100
Subject: [PATCH] Warper: add runtime warning when value changed to avoid
 colliding with dstnodata

---
 alg/gdalwarper.h       |  2 ++
 alg/gdalwarpkernel.cpp | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/alg/gdalwarper.h b/alg/gdalwarper.h
index 15eceb561022..f3dd6408d011 100644
--- a/alg/gdalwarper.h
+++ b/alg/gdalwarper.h
@@ -463,6 +463,8 @@ class CPL_DLL GDALWarpKernel
 
     GWKTieStrategy eTieStrategy;
 
+    bool bWarnedAboutDstNoDataReplacement = false;
+
     /*! @endcond */
 
     GDALWarpKernel();
diff --git a/alg/gdalwarpkernel.cpp b/alg/gdalwarpkernel.cpp
index 911a620e629f..527c15580e51 100644
--- a/alg/gdalwarpkernel.cpp
+++ b/alg/gdalwarpkernel.cpp
@@ -1561,6 +1561,19 @@ inline void AvoidNoData(const GDALWarpKernel *poWK, int iBand,
                     pDst[iDstOffset], std::numeric_limits<T>::max());
             }
         }
+
+        if (!poWK->bWarnedAboutDstNoDataReplacement)
+        {
+            const_cast<GDALWarpKernel *>(poWK)
+                ->bWarnedAboutDstNoDataReplacement = true;
+            CPLError(CE_Warning, CPLE_AppDefined,
+                     "Value %g in the source dataset has been changed to %g "
+                     "in the destination dataset to avoid being treated as "
+                     "NoData. To avoid this, select a different NoData value "
+                     "for the destination dataset.",
+                     poWK->padfDstNoDataReal[iBand],
+                     static_cast<double>(pDst[iDstOffset]));
+        }
     }
 }