This bugzilla service is closed. All entries have been migrated to

Bug 1736

Summary: Column access of some IndexedView won't compile
Product: Eigen Reporter: Antoine Bussy <bussyantoine>
Component: Core - expression templatesAssignee: Nobody <eigen.nobody>
Severity: Compilation Problem CC: bussyantoine, chtz, gael.guennebaud, jacob.benoit.1
Priority: Normal Keywords: test-needed
Version: 3.4 (development)   
Hardware: All   
OS: All   
Bug Depends on:    
Bug Blocks: 814    

Description Antoine Bussy 2019-08-07 11:34:28 UTC

The following code won't compile on clang-7 (I also tried gcc-9 and clang-8 on

#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:

  CoeffReturnType coeff(Index index) const
    return coeff((m_xpr.rows() == 1 ? 0 : index),
                 (m_xpr.rows() == 1 ? index : 0));

  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?
Comment 1 Christoph Hertzberg 2019-08-07 17:18:03 UTC
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.
Comment 2 Antoine Bussy 2019-08-08 09:23:00 UTC
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:

CoeffReturnType coeff_impl(std::true_type, Index index) const
	return m_argImpl.coeff(m_linear_offset.value() + index); 

CoeffReturnType coeff_impl(std::false_type, Index index) const
	return coeff(RowsAtCompileTime == 1 ? 0 : index, RowsAtCompileTime == 1 ? index : 0);

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.
Comment 3 Gael Guennebaud 2019-09-11 13:41:52 UTC
Thank you for reporting this issue and suggesting a fix.
Comment 4 Nobody 2019-12-04 18:44:04 UTC
-- GitLab Migration Automatic Message --

This bug has been migrated to's GitLab instance and has been closed from further activity.

You can subscribe and participate further through the new bug through this link to our GitLab instance: