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#include <Eigen/Dense>

int main()
{
  Eigen::Matrix<double, Eigen::Dynamic, 1> v1, v2;
  v2 = v1.segment(0,2);
}


g++ -Wconversion -Werror -I /usr/local/include/eigen3 eigen_warning.cc -o test
In file included from /usr/local/include/eigen3/Eigen/Core:305:0,
from /usr/local/include/eigen3/Eigen/Dense:1,
from eigen_warning.cc:1:
/usr/local/include/eigen3/Eigen/src/Core/Block.h: In member function ‘void Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::init() [with XprType = Eigen::Matrix<double, -0x00000000000000001, 1>, int BlockRows = -0x00000000000000001, int BlockCols = 1, bool InnerPanel = false]’:
/usr/local/include/eigen3/Eigen/src/Core/Block.h:304:7: instantiated from ‘Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Block(XprType&, Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Index, Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Index, Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Index, Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Index) [with XprType = Eigen::Matrix<double, -0x00000000000000001, 1>, int BlockRows = -0x00000000000000001, int BlockCols = 1, bool InnerPanel = false, Eigen::Block<XprType, BlockRows, BlockCols, InnerPanel, true>::Index = long int]’
/usr/local/include/eigen3/Eigen/src/Core/VectorBlock.h:91:61: instantiated from ‘Eigen::VectorBlock<MatrixType, Size>::VectorBlock(VectorType&, Eigen::VectorBlock<MatrixType, Size>::Index, Eigen::VectorBlock<MatrixType, Size>::Index) [with VectorType = Eigen::Matrix<double, -0x00000000000000001, 1>, int Size = -0x00000000000000001, Eigen::VectorBlock<MatrixType, Size>::Index = long int]’
/usr/local/include/eigen3/Eigen/src/Core/VectorBlock.h:127:50: instantiated from ‘Eigen::DenseBase<Derived>::SegmentReturnType Eigen::DenseBase<Derived>::segment(Eigen::DenseBase<Derived>::Index, Eigen::DenseBase<Derived>::Index) [with Derived = Eigen::Matrix<double, -0x00000000000000001, 1>, Eigen::DenseBase<Derived>::SegmentReturnType = Eigen::VectorBlock<Eigen::Matrix<double, -0x00000000000000001, 1>, -0x00000000000000001>, Eigen::DenseBase<Derived>::Index = long int]’
eigen_warning.cc:6:22: instantiated from here
/usr/local/include/eigen3/Eigen/src/Core/Block.h:339:7: error: conversion to ‘int’ from ‘Eigen::Matrix<double, -0x00000000000000001, 1>::Index {aka long int}’ may alter its value [-Werror=conversion]
cc1plus: all warnings being treated as errors

make: *** [Wconversion] Error 1

ggael wrote:well you are asking a bit much by turning warnings into errors, but in this particular case we should declare m_outerStride as an Index to avoid this warning.