[res.on.functions]/2 states:
While undefined behavior is appropriate for the other cases specified in the earlier bullets, which describe failure to meet (runtime) semantic requirements, "instantiating a template component or evaluating a concept" with an incomplete type happens at compile-time, and could potentially be diagnosed. It would therefore be more appropriate to specify that programs which do so are ill-formed with no diagnostic required.-2- In particular, the effects are undefined in the following cases:
[…]
(2.5) — if an incomplete type ([basic.types]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.