Hi Yvan,
Kyrill submitted a patch for that issue which was lost in the pile and approved today:
https://gcc.gnu.org/ml/gcc-patches/2015-12/msg00576.html
I'll backport it in our branch as soon as it is committed on trunk.
... I will apply that patch to our tree since the description and code change seems to exactly describe my observed behavior and I will provide you feedback on the results.
As expected that patch was the perfect cure for the problem I saw.
Robert