https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70141
--- Comment #24 from Sori Lee <s.lee at dpmms dot cam.ac.uk> --- I retract my last report. I mistakenly thought my example compiled on clang, but rightly it didn't there either -- the U in the partial specialisation is in a non-deduced context, and fails to be deduced as discussed in https://stackoverflow.com/a/35877464. Apologies.
