Undefined behavior on lambda capturing constexpr by reference. The C++ standard provides the following example:

struct S

;
auto f() {
S x

;
constexpr S y

;
return [&](bool b)

;
}
auto g = f();
int m = g(false); // undefined behavior due to access of x.n outside its lifetime
int n = g(true); // OK, does not access y.n

gcc-7.2 replaces the reference to y.n with the constant 2, avoiding undefined behavior.