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NilPy: -> ret is MANDATORY on every def; Python makes it optional

Found while writing test/test_nilpy_kwargs.npy (feature-nilpy-keyword-args, commit 66685acc) — every def in that file had to grow a -> None that CPython does not need. It cost three build-edit cycles to diagnose, because the cascade is misleading: a class whose __init__ header failed to parse simply is not registered, so the next statement fails with Nil Python: annotate the type / too dynamic on an unrelated line.

Repro

def f():
    print(1)
f()



pascal26:1: error: unexpected token
  near:  f   >>>

Same for def f(a: int):. Only def f(a: int) -> None: parses. CPython accepts all three.

Shape

The def-header parser (compiler/pyparser.inc, the Expected: - in -> site) requires the arrow unconditionally. It should treat a missing -> ret as -> None — the return type is already inferred elsewhere for nested defs, so the information is available; what is missing is accepting : where the arrow is expected.

Note the diagnostic quality issue too: Expected: - in -> names a character, not the construct. A def whose header does not parse should say so, and the class-registration cascade above should not surface as "too dynamic" on a later line.

Gate

test-nilpy green with a .npy case (unannotated def, unannotated method, unannotated __init__) diffed against CPython, plus --tier quick and self-host byte-identical.

Log