A Nil Python generator returned from a def yields nothing

def gen(n):
    yield n
    yield n + 1
def mk():
    return gen(5)
print(list(mk()))     # [] ; CPython [5, 6]
x = mk()
print(next(x))        # compile error: no overload of next matches these arguments

g = gen(5); print(list(g)) at the same level works. A generator that is RETURNED does not survive, because the def's return kind is not a generator. The behaviour is the same on x86-64, i386, aarch64 and arm32.

When this is fixed, also check the str-literal argument temp in PyBuildGeneratorValue (bug-nilpy-a-generator-called-with-a-str-literal-segfaults). It is a local of the function that creates the generator, so an escaping generator would need its slot to hold its own reference.

Related: feature-nilpy-a-generator-as-a-first-class-value.