From e8ddbbebff6cb114fcfc986cd9b43cbfbdfabe29 Mon Sep 17 00:00:00 2001
From: Gerry Chen <gerry.chen2015@gmail.com>
Date: Mon, 19 Dec 2022 20:14:12 -0500
Subject: [PATCH] Check type of CONTAINER constructor tparam This is a
 byproduct of the overload resolution problem when N=1, then it can be hard to
 differentiate between: NoiseModelFactorN(noise, key) NoiseModelFactorN(noise,
 {key})

---
 gtsam/nonlinear/NonlinearFactor.h | 7 ++++++-
 tests/testNonlinearFactor.cpp     | 1 +
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gtsam/nonlinear/NonlinearFactor.h b/gtsam/nonlinear/NonlinearFactor.h
index 33640f0b7..2f4b27d39 100644
--- a/gtsam/nonlinear/NonlinearFactor.h
+++ b/gtsam/nonlinear/NonlinearFactor.h
@@ -352,7 +352,12 @@ class NoiseModelFactorN : public NoiseModelFactor {
    * @param noiseModel Shared pointer to noise model.
    * @param keys A container of keys for the variables in this factor.
    */
-  template <typename CONTAINER = std::initializer_list<Key>>
+  template <typename CONTAINER = std::initializer_list<Key>,
+            // check that CONTAINER is a container of Keys:
+            typename T = typename std::decay<
+                decltype(*std::declval<CONTAINER>().begin())>::type,
+            typename std::enable_if<std::is_convertible<T, Key>::value,
+                                    bool>::type = true>
   NoiseModelFactorN(const SharedNoiseModel& noiseModel, CONTAINER keys)
       : Base(noiseModel, keys) {
     assert(keys.size() == N);
diff --git a/tests/testNonlinearFactor.cpp b/tests/testNonlinearFactor.cpp
index 349e2cd86..9c4b4cff1 100644
--- a/tests/testNonlinearFactor.cpp
+++ b/tests/testNonlinearFactor.cpp
@@ -376,6 +376,7 @@ TEST(NonlinearFactor, NoiseModelFactor1) {
   // Test constructors
   TestFactor1 tf2(noiseModel::Unit::Create(1), L(1));
   TestFactor1 tf3(noiseModel::Unit::Create(1), {L(1)});
+  TestFactor1 tf4(noiseModel::Unit::Create(1), gtsam::Symbol('L', 1));
 }
 
 /* ************************************************************************* */