Why do my SFINAE expressions no longer work with GCC 8.2?

Question

I recently upgraded GCC to 8.2, and most of my SFINAE expressions have stopped working.

The following is somewhat simplified, but demonstrates the problem:

Answer 1

I am not a language lawyer, but cannot the following quote be somehow connected to the problem?

[temp.deduct.type/9]: If Pi is a pack expansion, then the pattern of Pi is compared with each remaining argument in the template argument list of A. Each comparison deduces template arguments for subsequent positions in the template parameter packs expanded by Pi.

It seems to me that since there is no remaining argument in the template argument list, then there no comparison of the pattern (which contains enable_if). If there is no comparison, then there is also no deduction and substitution occurs after deduction I believe. Consequently, if there is no substitution, no SFINAE is applied.

Please correct me if I am wrong. I am not sure whether this particular paragraph applies here, but there are more similar rules regarding pack expansion in [temp.deduct]. Also, this discussion can help someone more experienced to resolve the whole issue: https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/JwZiV2rrX1A.