I managed to reproduce this with g++ (GCC) 8.1.1 20180712 from Fedora-28.
But it looks like a gcc SFINAE bug to me, it treats a substitution failure as error, even though an alternative substitution exists. All clang versions and gcc <= 8 compile this correctly.
it's not too easy to isolate though, I might give this another shot another day to involve the compiler guys.
@swesterfeld do you have gcc >= 8 and can weigh in on this?
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