Some gcc versions incorrectly complain:
tests/log_histogram_test.cc:87:22: error: ‘opts1’ is not a valid template argument for type ‘const log_histogram_options&’ because object ‘opts1’ has not external linkage
size_t hist_key<node<opts1>>(const node<opts1>& n) { return n.v; }
Apparently this is a bug in gcc:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52036Fixes#2307.
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