template <typename TF>
constexpr auto f(TF)
{
    return [](auto...) constexpr { return true; };
}   

// Compiles and works as intended.
template <typename T0>
void ok_0(T0)
{
    static_assert(f([](auto x) -> decltype(x){})(T0{}));
}

// Compiles and works as intended.
template <typename T0>
void ok_1(T0)
{
    constexpr auto a = f([](auto x) -> decltype(x){})(T0{});
    if constexpr(a) { }
}

// Compile-time error!
template <typename T0>
void fail_0(T0)
{
    if constexpr(f([](auto x) -> decltype(x){})(T0{})) { } 
}

---

Error:
prog.cc: In function 'void fail_0(T0)':
prog.cc:29:54: error: expression '<lambda>' is not a constant expression
     if constexpr(f([](auto x) -> decltype(x){})(T0{})) { }

Minimal example:
http://melpon.org/wandbox/permlink/NY4oKSlA76CkqPQ9

Related StackOverflow question:
http://stackoverflow.com/questions/40283001

P.S. the bug is still present even if 'constexpr' is explicitly specified for the lambdas passed to 'f'.