fix tertiary conditionals in ref context #23913
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GH #18669 In something like @{ expr } = ... the expression is expected to return an array ref. If the expression is something like $h{foo}, then the helem op needs to know both that: - it is in lvalue context, so should autovivify the foo element if not present; - it is in array ref context, so it should autovivify the value to an empty array ref, rather than just to undef. The function Perl_doref() is used to propagate this ref context at compile time, e.g. by setting the OPf_MOD and OPpDEREF_AV flags on the OP_HELEM op. My commit v5.31.1-87-ge9b0092a10 made this function non-recursive (so that deep expressions wouldn't SEGV during compilation), but introduced a bug when the expression included the ternary condition operator, '?:'. In particular, since '?:' is the only OP where doref() needs to recurse down *two* branches, I made the function just iterate down the tree, and then have special handling for OP_COND_EXPR. This involved, once having finished iterating down the tree, to work back up the tree looking for OP_COND_EXPR nodes, and if found, iterate back down the second branch. This had a fatal flaw: a 'type' variable indicated what context to apply. For example in @{$h{expr}} = ..., type would start off as OP_RV2AV, but as the tree was walked, would change to OP_HELEM and then to OP_RV2HV. When walking back up the tree, this value wasn't being restored. The specific bug in the ticket boiled down to something like @{ $cond ? $h{p} : $h{q} } = ...; where the correct OPpDEREF_AV flag was being set on the first helem op, but an incorrect OPpDEREF_HV on the second. Since I can't think of anything better, the fix in this commit restores some limited recursion to doref(). Namely, for an OP_COND_EXPR op, it now recurses down that op's first branch, then after it returns, iterates as normal down the second branch. Thus extremely deeply nested ternary code like: @{ $c1 ? $c2 ? $c3 ? .... } ... could start to SEGV during compilation again.
This compile-time function propagates lvalue ref context down a chain of ops. It does the same thing (setting OPf_MOD and OPpDEREF_XV flags) in three places. Consolidate this code into a single place. Should be no functional changes. Technically the code is slightly different in that OP_[AH]ELEM now checks for kids before following them, but since they always have kids, this makes no difference (except being infinitesimally slower during compilation).
Having just messed with this function, I understand it better, so can comment it better.
Contributor
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Tests look good, but C code should be reviewed by someone other than me. |
tonycoz
approved these changes
Nov 11, 2025
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Fix GH #18669 and tidy up the function where the bug was found, Perl_doref()