Sample code adapted from https://adishavit.github.io/2016/Technical-Debt/ (https://godbolt.org/g/osKPbq):
void f(void (*f1)(int)) {
  f1(42);
}

//Converts any lambda to a captureless lambda that converts nicely to a function pointer for the lifetime of the temporary.
template <typename Lambda>
static auto callback(Lambda &&l)
{
  static auto* p = &l;
  p = &l;
  return [](auto... x){ return (*p)(x...); };
}

int main() {
  int x = 5;
  f(callback([=](int y){std::cout << x << ' ' << y;}));
}

Gives the error:
<source>: In instantiation of 'callback(Lambda&&)::<lambda(auto:1 ...)> [with auto:1 = {int}; Lambda = main()::<lambda(int)>]':
<source>:14:22:   required by substitution of 'template<class ... auto:1> callback(Lambda&&) [with Lambda = main()::<lambda(int)>]::<lambda(auto:1 ...)>::operator decltype (((const callback(Lambda&&) [with Lambda = main()::<lambda(int)>]::<lambda(auto:1 ...)>*)((const callback(Lambda&&) [with Lambda = main()::<lambda(int)>]::<lambda(auto:1 ...)>* const)0u))->operator()(static_cast<auto:1&>(callback::__lambda0<main::__lambda1>::_FUN::<unnamed>) ...)) (*)(auto:1 ...)() const [with auto:1 = {int}]'
<source>:19:54:   required from here
<source>:14:33: error: use of 'p' before deduction of 'auto'
return [](auto... x){ return (*p)(x...); };
~^~~
<source>:14:36: error: invalid use of 'auto'
return [](auto... x){ return (*p)(x...); };
~~~~^~~~~~

Making the lambda in callback non-generic(ie: [](int x){return (*p)(x);}) or explicitly calculating the type of p(std::add_pointer_t<decltype(l)>) makes the code compile successfully.