Hi, The following code won't compile on clang-7 (I also tried gcc-9 and clang-8 on https://godbolt.org/): #include <Eigen/Core> #include <array> auto indexed_view_col() { auto const m = Eigen::Matrix4d::Random().eval(); auto ok = m(Eigen::all, Eigen::seq(1, Eigen::last)).col(0).eval(); auto ok2 = m(Eigen::all, Eigen::seq(0, 1)).col(0).eval(); auto err = m(Eigen::all, std::array<Eigen::Index,2>{0,1}).col(0).eval(); auto err2= m(Eigen::all, Eigen::Vector<Eigen::Index,2>(0, 1)).col(0).eval(); } The compilers don't seem to find the unary version of coeff(), and indeed, it isn't in IndexedView.h (whereas it is present in Block.h). I implemented it similarly to Block's: EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE CoeffReturnType coeff(Index index) const { return coeff((m_xpr.rows() == 1 ? 0 : index), (m_xpr.rows() == 1 ? index : 0)); } EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Scalar& coeffRef(Index index) { return coeffRef((m_xpr.rows() == 1 ? 0 : index), (m_xpr.rows() == 1 ? index : 0)); } and it seems to solve my problem. but I'm not familiar with Eigen's internals at all, so the root cause might be elsewhere?

The actual problem here is that `.coeff(Index)` is called -- even though only in a false-branch, i.e., never executed. With C++17 this would be trivial to workaround using if constexpr (ForwardLinearAccess) {...} Your patch should work of course (since it will never actually be called). But perhaps we should add assertions that the expression is a vector at compile time (actually, also to the corresponding Block::coeff methods) Ideally, m(X, Y).col(i) could be simplified to m(X,Y[i]) -- similar for any other block-operations.

Thanks for the answer, adding constexpr in CoreEvaluator.h is indeed much cleaner :) Concerning m(X,Y[i]), I can't really use this simplification in my code since m(X,Y) is the input of a function that works on matrices. For a non-C++17 workaround, it is possible to emulate the if constexpr with a small amount of boilerplate: EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE CoeffReturnType coeff_impl(std::true_type, Index index) const { return m_argImpl.coeff(m_linear_offset.value() + index); } EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE CoeffReturnType coeff_impl(std::false_type, Index index) const { return coeff(RowsAtCompileTime == 1 ? 0 : index, RowsAtCompileTime == 1 ? index : 0); } EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE CoeffReturnType coeff(Index index) const { return coeff_impl(std::bool_constant<ForwardLinearAccess>{}, index); } Of course, it would be necessary to reimplement std::bool_constant, but this is trivial.

Thank you for reporting this issue and suggesting a fix. https://bitbucket.org/eigen/eigen/commits/f4920c83f9c4/

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