Capturing non-type template parameter in lambda expression

Why it's forbidden to capture non-type template parameters in the lambda capture list?

The following code fails to compile on both GCC and clang:

template<int x>
auto foo()
{
    return [x]{return x;};
}

with an error message capture of non-variable 'x' on GCC and 'x' in capture list does not name a variable on clang.

... however, the following code compiles fine:

template<int x>
auto bar()
{
    return [y = x]{return y;};
}