From 080668df42e2258b5c84f9630816f0a43bdcb562 Mon Sep 17 00:00:00 2001
From: Yuval Tassa <tassa@google.com>
Date: Tue, 23 Jun 2026 09:43:38 -0700
Subject: [PATCH] Add "(default)" to conflict warnings in user resolver.

When reporting conflicts between parent and child elements during attachment, the warning message now indicates if a value is derived from the default rather than being explicitly authored in the XML. This improves clarity in conflict resolution warnings.

PiperOrigin-RevId: 936724265
Change-Id: I68c677b19020c19bf0260f5b3b77fe6ca3661bb9
---
 src/user/user_resolver.cc       | 14 ++++++----
 test/user/user_resolver_test.cc | 47 +++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+), 5 deletions(-)

diff --git a/src/user/user_resolver.cc b/src/user/user_resolver.cc
index 621afc26..965887c6 100644
--- a/src/user/user_resolver.cc
+++ b/src/user/user_resolver.cc
@@ -32,7 +32,7 @@ namespace {
 // format a numeric value for conflict messages
 template <typename T>
 std::string fmtVal(T val) {
-  if constexpr (std::is_same_v<T, mjtNum>) {
+  if constexpr (std::is_floating_point_v<T>) {
     char buf[32];
     snprintf(buf, sizeof(buf), "%g", val);
     return buf;
@@ -87,8 +87,10 @@ struct Resolver {
 
     // "FIELD: parent has X, child has Y"
     auto prefix = [&]() {
-      return std::string(name) + ": parent has " + fmtVal(pval) +
-             ", child has " + fmtVal(cval);
+      std::string p_str = fmtVal(pval) + (parent_authored ? "" : " (default)");
+      std::string c_str = fmtVal(cval) + (child_authored ? "" : " (default)");
+      return std::string(name) + ": parent has " + p_str +
+             ", child has " + c_str;
     };
 
     // only child authored: adopt or keep
@@ -168,8 +170,10 @@ struct Resolver {
 
     // "FIELD: parent has X Y Z, child has X Y Z"
     auto prefix = [&]() {
-      return std::string(name) + ": parent has " + fmtArr(pval, N) +
-             ", child has " + fmtArr(cval, N);
+      std::string p_str = fmtArr(pval, N) + (parent_authored ? "" : " (default)");
+      std::string c_str = fmtArr(cval, N) + (child_authored ? "" : " (default)");
+      return std::string(name) + ": parent has " + p_str +
+             ", child has " + c_str;
     };
 
     // only child authored: adopt or keep
diff --git a/test/user/user_resolver_test.cc b/test/user/user_resolver_test.cc
index 7bbcf346..742d91a6 100644
--- a/test/user/user_resolver_test.cc
+++ b/test/user/user_resolver_test.cc
@@ -914,5 +914,52 @@ TEST_F(MujocoTest, AttachWarningBoundaryAndPreservation) {
   mj_deleteSpec(child2);
 }
 
+TEST_F(MujocoTest, AttachConflictWarningZFarDefaultMessage) {
+  mock_warning_handler.ExpectWarnings();
+
+  static constexpr char parent_xml[] = R"(
+    <mujoco>
+      <compiler conflict="warning"/>
+      <worldbody/>
+    </mujoco>
+  )";
+
+  static constexpr char child_xml[] = R"(
+    <mujoco>
+      <visual>
+        <map zfar="30"/>
+      </visual>
+      <worldbody/>
+    </mujoco>
+  )";
+
+  std::array<char, 1024> error;
+  mjSpec* parent =
+      mj_parseXMLString(parent_xml, nullptr, error.data(), error.size());
+  ASSERT_THAT(parent, NotNull()) << error.data();
+
+  mjSpec* child =
+      mj_parseXMLString(child_xml, nullptr, error.data(), error.size());
+  ASSERT_THAT(child, NotNull()) << error.data();
+
+  mjsBody* world = mjs_findBody(parent, "world");
+  mjsElement* attached =
+      mjs_attach(world->element, child->element, "child_", "");
+  ASSERT_THAT(attached, NotNull()) << "Error details: " << mjs_getError(parent);
+
+  // Since only the child authored 'zfar' and the parent relied on defaults,
+  // we warning-log and state that the parent has the default value.
+  EXPECT_TRUE(mjs_isWarning(parent));
+  EXPECT_EQ(mjs_numWarnings(parent), 1);
+  EXPECT_THAT(
+      mjs_getWarning(parent, 0),
+      HasSubstr("zfar: parent has 50 (default), child has "
+                "30, keeping parent value"));
+
+  mj_deleteSpec(parent);
+  mj_deleteSpec(child);
+}
+
 }  // namespace
 }  // namespace mujoco
+