Fix compilation of test_Icosphere with Eigen 3.4

[traversaro]

While debugging a build failure in the dartsim conda-forge package (see conda-forge/dartsim-feedstock#16), I noticed the following build failure when building dart with Eigen 3.4 :

(dart_new_eigen) traversaro@IITICUBLAP257:~/dart/build_new_eigen$ ninja test_Icosphere
[1/2] Building CXX object unittests/unit/CMakeFiles/test_Icosphere.dir/test_Icosphere.cpp.o
FAILED: unittests/unit/CMakeFiles/test_Icosphere.dir/test_Icosphere.cpp.o
/home/traversaro/mambaforge/envs/dart_new_eigen/bin/c++ -DBOOST_ALL_NO_LIB -DBOOST_FILESYSTEM_DYN_LINK -DBOOST_SYSTEM_DYN_LINK -DBOOST_TEST_DYN_LINK -I/home/traversaro/dart/unittests -I/home/traversaro/dart -I/home/traversaro/dart/build_new_eigen -isystem /home/traversaro/dart/unittests/gtest -isystem /home/traversaro/dart/unittests/gtest/include -isystem /home/traversaro/mambaforge/envs/dart_new_eigen/include/eigen3 -fvisibility-inlines-hidden -std=c++17 -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem /home/traversaro/mambaforge/envs/dart_new_eigen/include -Wall -Wextra -fPIC -O3 -DNDEBUG -MD -MT unittests/unit/CMakeFiles/test_Icosphere.dir/test_Icosphere.cpp.o -MF unittests/unit/CMakeFiles/test_Icosphere.dir/test_Icosphere.cpp.o.d -o unittests/unit/CMakeFiles/test_Icosphere.dir/test_Icosphere.cpp.o -c /home/traversaro/dart/unittests/unit/test_Icosphere.cpp
In file included from /home/traversaro/dart/dart/math/Icosphere.hpp:104,
                 from /home/traversaro/dart/unittests/unit/test_Icosphere.cpp:35:
/home/traversaro/dart/dart/math/detail/Icosphere-impl.hpp: In instantiation of 'static std::pair<std::vector<typename dart::math::TriMesh<S>::Vector3>, std::vector<typename dart::math::TriMesh<S>::Triangle> > dart::math::Icosphere<S_>::computeIcosahedron(dart::math::Icosphere<S_>::S) [with S_ = double; typename dart::math::TriMesh<S>::Triangle = Eigen::Matrix<long unsigned int, 3, 1, 0, 3, 1>; typename dart::math::TriMesh<S>::Vector3 = Eigen::Matrix<double, 3, 1>; dart::math::Icosphere<S_>::S = double]':
/home/traversaro/dart/dart/math/detail/Icosphere-impl.hpp:133:67:   required from 'void dart::math::Icosphere<S_>::build() [with S_ = double]'
/home/traversaro/dart/dart/math/detail/Icosphere-impl.hpp:109:3:   required from 'dart::math::Icosphere<S_>::Icosphere(dart::math::Icosphere<S_>::S, std::size_t) [with S_ = double; dart::math::Icosphere<S_>::S = double; std::size_t = long unsigned int]'
/home/traversaro/dart/unittests/unit/test_Icosphere.cpp:50:59:   required from here
/home/traversaro/dart/dart/math/detail/Icosphere-impl.hpp:77:24: error: converting to 'Eigen::Matrix<double, 3, 1>' from initializer list would use explicit constructor 'Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols>::Matrix(const std::initializer_list<std::initializer_list<typename Eigen::internal::traits<Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols> >::Scalar> >&) [with _Scalar = double; int _Rows = 3; int _Cols = 1; int _Options = 0; int _MaxRows = 3; int _MaxCols = 1; typename Eigen::internal::traits<Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols> >::Scalar = double]'
   77 |   std::vector<Vector3> vertices = {{{-x, 0, z},
      |                        ^~~~~~~~
/home/traversaro/dart/dart/math/detail/Icosphere-impl.hpp:90:32: error: converting to 'Eigen::Matrix<long unsigned int, 3, 1, 0, 3, 1>' from initializer list would use explicit constructor 'Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols>::Matrix(const std::initializer_list<std::initializer_list<typename Eigen::internal::traits<Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols> >::Scalar> >&) [with _Scalar = long unsigned int; int _Rows = 3; int _Cols = 1; int _Options = 0; int _MaxRows = 3; int _MaxCols = 1; typename Eigen::internal::traits<Eigen::Matrix<_Scalar, _Rows, _Cols, _Options, _MaxRows, _MaxCols> >::Scalar = long unsigned int]'
   90 |   static std::vector<Triangle> triangles
      |                                ^~~~~~~~~
ninja: build stopped: subcommand failed.

Building exactly the same code with exactly the same dependency except for changing Eigen from 3.4.0 to 3.3.9 was working fine, so I am quite sure that this problem is related to Eigen 3.4.0 . I am not sure about the exact nature of the error as I am not familiar with initializations of std containers of Eigen vectors, but according to my tests it seems that the error is fixed by removing a pair of curly bracket solved the problem while ensuring that everything still compiles fine on Eigen 3.3.9 , and this is proposed in this PR.

However, things to check accurately:

• Make sure that all relevant Eigen versions/compilers pairs are covered by CI, eitherwise we may break them.
• Check if the format that I used for the initializer list after the change still make sense

---

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• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[traversaro]

Check if the format that I used for the initializer list after the change still make sense

I double checked with clang-format9 and it seems correct.

[jslee02]

Yeah, I see the same error building on archilinux. Thanks for the fix!