link as hidden, vendor the include path

perception/point_cloud.cc

@@ -6,9 +6,9 @@
 #include <vector>
 
 #include "absl/container/flat_hash_map.h"
+#include <drake_vendor/nanoflann.hpp>
 #include <fmt/format.h>
 #include <fmt/ostream.h>
-#include <nanoflann.hpp>
 
 #include "drake/common/drake_assert.h"
 #include "drake/common/drake_throw.h"

tools/workspace/nanoflann_internal/package.BUILD.bazel

@@ -12,8 +12,8 @@ package(default_visibility = ["//visibility:public"])
 cc_library(
     name = "nanoflann",
     hdrs = ["include/nanoflann.hpp"],
-    includes = ["include"],
-    deps = [],
+    include_prefix = "drake_vendor",
+    strip_include_prefix = "include",
     linkstatic = 1,
 )
 

tools/workspace/nanoflann_internal/patches/namespace.patch

@@ -0,0 +1,21 @@
+Add a drake_vendor namespace ala tools/workspace/vendor_cxx
+
+This prevents link-time symbol conflicts in case code downstream of
+Drake wants to use a different build of nanoflann.
+
+--- include/nanoflann.hpp.orig
++++ include/nanoflann.hpp
+@@ -70,6 +70,7 @@
+ #endif
+ #endif
+
++inline namespace drake_vendor __attribute__ ((visibility ("hidden"))) {
+ namespace nanoflann
+ {
+ /** @addtogroup nanoflann_grp nanoflann C++ library for ANN
+@@ -2414,3 +2415,4 @@
+
+ /** @} */  // end of grouping
+ }  // namespace nanoflann
++}  // namespace drake_vendor
+\ No newline at end of file

tools/workspace/nanoflann_internal/repository.bzl

@@ -11,5 +11,8 @@ def nanoflann_internal_repository(
         commit = "v1.4.3",
         sha256 = "cbcecf22bec528a8673a113ee9b0e134f91f1f96be57e913fa1f74e98e4449fa",  # noqa
         build_file = ":package.BUILD.bazel",
+        patches = [
+            ":patches/namespace.patch",
+        ],
         mirrors = mirrors,
     )