The standard does not specify whether a closure type (the type of a lambda expression) is trivial or not. It explicitly leaves this up to implementation, which makes it non-portable. I am afraid you cannot rely on your static_assert producing anything consistent.
Quoting C++14 (N4140) 5.1.2/3:
… An implementation may define the closure type differently from what is described below provided this does not alter the observable
behavior of the program other than by changing:
- the size and/or alignment of the closure type,
- whether the closure type is trivially copyable (Clause 9),
- whether the closure type is a standard-layout class (Clause 9), or
- whether the closure type is a POD class (Clause 9).
…
(Emphasis mine)
After parsing the double negation in that sentence, we can see that the implementation is allowed to decide whether the closure type is trivially copyable, standard layout, or POD.
Note that the same wording is also present in C++17 (N4659), [expr.prim.lambda.closure] 8.1.5.1/2.