docs(test): explain posixtest's arg(1) in parenthesized sub-expressions - #672
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TestEval::posixtest() case 1 evaluates arg(1) rather than arg(pos), which
looks like an index bug when term() calls it recursively for a
parenthesized sub-expression (pos > 1). It is deliberate: bash 5.3's
posixtest evaluates ONE_ARG_TEST(argv[1]) in exactly the same situation,
so `test '(' '' ')' -a b' tests "(" -- not the empty string -- and
exits 0 in both shells. Verified against bash 5.3.15 across paren/-a/-o
combinations; bash's CHANGES entry for the forward-scan notes "Such
expressions remain ambiguous".
Document the quirk so it is not "fixed" into a divergence.
Closes #670
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Summary
Issue #670 reports that
TestEval::posixtest()case 1 evaluatesarg(1)instead ofarg(pos), producing a "wrong" result fortest '(' '' ')' -a b.Investigation shows this is deliberate bug-for-bug compatibility with bash 5.3, not an index bug:
test.cposixtest()does exactly the same thing:case 1: value = ONE_ARG_TEST(argv[1]);— even whenterm()calls it recursively for a parenthesized sub-expression withpos > 1(the forward-scan added in bash 5.3, CHANGES entry ee, which concedes "Such expressions remain ambiguous").test '(' '' ')' -a b— the issue's expected exit 1 comes from an older bash (≤ 5.2, whereshell_compatibility_level > 52routes the paren scan toexpr()instead).-a/-ocombinations, includingtest '(' '' ')' -a b,test x -a '(' '' ')',[ '(' '' ')' -a b ], and nested forms — all OK. Applying the proposedarg(pos)change makes 4 of the 11 diverge from bash.This PR adds a comment documenting the quirk so it is not "fixed" into a divergence later. No behavior change.
Closes #670
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