NilPy: subscripting a string LITERAL is a parse error

Found 2026-07-20 while writing the regression test for [[bug-a-nilpy-one-char-string-literal-is-a-char]].

Repro

print("abc"[1])     # error: unexpected token (  — CPython prints b

A string in a variable subscripts fine (s = "abc"; print(s[1])), so this is the literal base only.

Cause (suspected)

PyMakeStrIndex is reached from the lvalue postfix path, which a literal never enters — the same asymmetry PyParseStrMethod needed its ParseFactor wrapper for ("a".upper() works because of that wrapper). The subscript suffix likely needs the same treatment at factor level.

Gate

make test + self-host byte-identical, test-nilpy green, the repro matching CPython — plus a negative index ("abc"[-1]) and a chained one ("abc"[1].upper()).

Log

• 2026-07-20 — DUPLICATE of [[bug-nilpy-subscript-on-literal]], filed 2026-07-19 with the same repro and the same root-cause reading. Rejected in favour of the older ticket; nothing here is lost.