Comment (by Roland Haas):

This is the sympton caused by using an outdated stdc++ template library. In you fix you are basically re-introducing the compatibility hacks (more or less) that we have just removed from Cactus. A better fix for the compilation failure is to make sure that icpc uses a new enough g++ (and its STL library) by passing a -gxx-name option and the path to a new enough g++.

If on the other hand you really want an isnan that persists even when -Ofast is used (though you are defeating the Ofast purpose in that case and with Ofast there just may not be any NaN produce in a consitent way to begin with) you could use your change. But it addresses a different issue than the compilation failure I think.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2642/toolkit-fails-to-compile-on-sawtooth-idaho