NilPy: a ONE-character literal through a ctor str param becomes a char — silent, then segfaults

Found 2026-07-20 while landing [[bug-nilpy-method-returning-str-garbage]]. Independent of it: reproduces with no method involved, and a plain def with the same parameter type handles it correctly.

Repro

class W:
    def __init__(self, n: str) -> None:
        self.n = n

a = W("x")
print(a.n)          # CPython x  -> pxx SEGFAULT

b = W("xyz")
print(b.n)          # CPython xyz -> pxx xyz   (multi-char is fine)

A def is NOT affected, which is the tell:

def g(n: str) -> None:
    print(n)
g("x")              # correct

Cause

pxx types a one-character literal "x" as tyChar — Python has no character type, so to NilPy that is a str of length 1. The def parameter path already widens it; the CTOR parameter / field path does not, so the field ends up holding the character CODE (0x78) where a string handle is expected. Using it then dereferences 120 as a pointer:

mov (%rax),%rdi     ; rax = 0x78

Same tyChar-vs-str family as the int("0") case fixed in 62c4e457 (which returned the character code 48) and the "a" * 3 handling in a123c875. Worth fixing as the family rather than one site: audit every place a NilPy value crosses into a str context and confirm tyChar widens.

Gate

make test + self-host byte-identical, test-nilpy green, both repros above matching CPython, plus a sweep of one-character strings through: ctor param, def param, method param, field assignment, list/dict element, return value, concatenation and comparison.

Log

• 2026-07-20 — fixed at the ROOT the ticket asked for: a 1-character literal is no longer a char in NilPy at all (da66e43d), rather than widening it at each crossing. Filed separately as [[bug-a-nilpy-one-char-string-literal-is-a-char]] before this duplicate was spotted; both describe the same defect.
• The sweep this ticket asks for was run against CPython and passes: ctor param, def param, method param, field assignment and re-assignment, list/dict element, return value, concatenation, comparison, repeat, in, int(), str(), ord(), chr(), str methods, +=. Regression test: test/test_nilpy_one_char_string.npy in make test-nilpy.
• One unrelated gap surfaced by the sweep and filed on its own: [[bug-a-nilpy-print-of-a-list-prints-a-pointer]].
• 2026-07-20 — resolved, commit da66e43d.