[eigen] Matrix-Matrix Multiplication gives incorrect results (C++11, sometimes)

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Hi all,

I'm using dynamic row-major Eigen3 matrices. Within my code I'm generating a 
(n-1)xn matrix (Q_pre) and post-prefix it with a zero-row to get a nxn matrix 
Q. Afterwards I'll need to square it, which gives me QQ. (see 
'problematic.cpp' in [1])

However, the resulting matrix QQ has its "last" (bottom-right) entry 
incorrectly computed. (see 'OUT: problematic' in [1])

I composed a minimal example and compiled it with the same compiler (and 
options) just to discover that everything goes fine.

Am I missing something? Neither of the tested compilers (GCC 4.8.3, 4.9..1, 
Clang 3.4.2, 3.5.0) on a 64bit openSUSE 13.1 all with Eigen3 3.2.2 has a 
different behaviour. Nor does any of the compilers complain about anything when 
run with `-Wall -Wextra -Wpedantic -g -O0 -std=c++11`.

Kind regards,
Torbjörn Klatt

[1] https://gist.github.com/torbjoernk/70731289e5a8f054bc67

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