struct A{
	constexpr operator bool()const{ return true; }
};

int main(){
    auto f = [](auto v){ if constexpr(v){} };
    A a;
    f(a);
}


$ g++ -std=c++17 main.cpp
main.cpp: In lambda function:
main.cpp:6:40: error: 'v' is not a constant expression
     auto f = [](auto v){ if constexpr(v){} };
                                        ^
$ g++ --version
g++ (GCC) 8.0.1 20180216 (experimental)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


Known to work with GCC-7:

$ g++-7 --version
g++ (GCC) 7.3.1 20180216
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.