[Bug libstdc++/77802] [7 Regression] Boost Fiber doesn't compile

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77802

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
17.6.4.8 [res.on.functions] p2

In particular, the effects are undefined in the following cases:
...
— if an incomplete type (3.9) is used as a template argument when instantiating
a template component, unless specifically allowed for that component.