N: yield from is not implemented

Repro

def inner():
    yield 1

def outer():
    yield from inner()

print(list(outer()))

pascal26:4: error: undefined variable (from)

The diagnostic is the informative part: from is being parsed as an expression after yield, so yield from is not recognised as a construct at all — this is a missing form in the statement grammar, not a lowering that fails. Whatever fixes it is in compiler/pyparser.inc, at whatever handles yield.

Why this is a bug and not a divergence

CLAUDE.md's NilPy charter: if code works on CPython, it must work on NilPy — one direction only. Generator delegation is ordinary Python, and this is Python 3.3: it predates every single feature NilPy does implement.

That last point is worth stating because it inverts the natural assumption. The gap is not "NilPy is a bit behind modern Python" — NilPy has f-strings (3.6), dict | (3.9) and str.removeprefix (3.9). The implemented subset is not an interval; it is the imperative-scripting core plus modern conveniences, with generator delegation and coroutines missing from underneath it.

How it was found, and what that says about coverage

Not by a failing program — by a probe suite written to answer a different question: what version should sys.version_info claim ([[decide-nilpy-what-version-does-sys-version-info-claim]], owner 2026-08-30). Nine feature probes, and this was one of two that failed far below the level anyone expected.

A gap this old surviving in a suite with real coverage — SQLite CRUD, classes, variants, string methods — says the suite is broad in library surface and thin in language surface. Worth a look at whether the same is true elsewhere, independently of this fix.

Scope note

yield from is not sugar for a for loop that re-yields: it forwards send(), throw() and close(), and propagates the sub-generator's return value as the expression's value. How much of that NilPy's generator machinery can support is the actual question — a partial implementation that forwards values but drops the return value would be silently wrong, which is worse than the current refusal. Establish what the generator implementation can carry before choosing scope, and if the full semantics are out of reach, say so in the ticket and keep refusing rather than half-landing it.

Relationship to the version claim

[[feature-n-sys-version-info-implementation-and-the-probe-suite]] ships a test asserting this is refused — as a tripwire on the 3.9 claim, not as a wish that it stay broken. Fixing this should trip that test by design; whoever lands the fix updates it and re-checks whether 3.9 is still the right number.

Gate

make test-nilpy green + self-host byte-identical + cross. Plus the repro above, and a test that pins the two behaviours a re-yield loop would get wrong: the sub-generator's return value reaching the yield from expression, and send() reaching the inner generator.