clang-tidy 22 reports cppcoreguidelines-pro-bounds-avoid-unchecked-container-access on locale_format.hpp, which the repo's own config makes an error

[Yaraslaut]

Found while implementing #591. Filed rather than folded in: it is a
repository-wide tooling question, not part of that change.

What

clang-tidy 22.1.8 reports a finding on include/morph/render/locale_format.hpp
that this repository's own .clang-tidy asks for, and that nothing in the tree
suppresses. The header was otherwise untouched by any in-flight work when I
measured it.

Verification status

Reproduced, on master @ 0e3b8823, by running clang-tidy directly over a
one-line translation unit that includes the header. Not inferred from reading.

$ cat /tmp/tu.cpp
#include <morph/render/locale_format.hpp>

$ clang-tidy --version | head -3
LLVM (http://llvm.org/):
  LLVM version 22.1.8
  Optimized build.

$ clang-tidy --config-file=.clang-tidy /tmp/tu.cpp -- -std=c++23 -I include
include/morph/render/locale_format.hpp:457:29: error: possibly unsafe 'operator[]', consider bounds-safe alternatives [cppcoreguidelines-pro-bounds-avoid-unchecked-container-access,-warnings-as-errors]

Line 457 on that revision is the display loop in formatCanonicalNumber:

        grouped += wholePart[i];

cppcoreguidelines-* is in the Checks list and WarningsAsErrors is "*",
so this is an error by the repository's own configuration, not a check I
enabled.

One methodological note, because it changes how to reproduce this. The
finding only appears when the header's path matches HeaderFilterRegex
(include/morph/.*). Pointing clang-tidy at a copy of the header under some
other path reports nothing at all, silently — which is how I initially
convinced myself a different check was not firing when it was.

What I did not verify

• Which clang-tidy version CI runs, and whether it reports this. This is the
  load-bearing unknown. The check
  cppcoreguidelines-pro-bounds-avoid-unchecked-container-access is recent; an
  older clang-tidy does not have it, which would explain why a green CI has been
  sitting on top of this. I did not read the workflow to find the pinned version
  and I did not run CI's toolchain locally.
• Whether the rest of the tree has the same finding. I measured one header,
  because it is the one I was working in. A tree-wide sweep is exactly what
  would size this, and I am told a separate lane is already running one — this
  issue should be reconciled with that lane's result rather than duplicated by
  it.
• Whether this is a real safety problem. i is bounded by the loop condition
  three lines above, so the access is in fact in range. This is a suppression /
  policy question, not a defect report.

Why it matters

Not because the indexing is wrong — it is not. It matters because the
repository has decided it wants clang-tidy as strict as possible (#580), and a
check that the configuration enables and that fires on first-party code is
either a finding to fix, a check to disable deliberately, or a suppression to
write with a reason. Which of the three it is has not been recorded anywhere,
and the difference only shows up when someone upgrades the compiler on the CI
image.

The existing code already knows about this check: the same header carries

        // i is bounded by the loop condition.
        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-avoid-unchecked-container-access)

twice, in groupingIsWellPlaced and in normalizeLocaleNumber. So it fires in
CI at least sometimes, or did once — and formatCanonicalNumber's identical
pattern simply never got the same treatment.

Incidental note

PR #630 rewrites that loop for unrelated reasons (#591 needed the digits mapped
through a base rather than copied), and the rewrite happens to use a range-for
with a separate index counter, so the finding is gone on that branch. That
is a side effect of the rewrite, not a fix, and it says nothing about the rest
of the tree. This issue is about the policy and the sweep, not about that line.

What would change the verdict

• Close as fixed when a tree-wide clang-tidy run at the version CI pins
  reports no
  cppcoreguidelines-pro-bounds-avoid-unchecked-container-access outside
  suppressions that state a reason.
• Close as invalid if CI's pinned clang-tidy does not implement this check
  and the project decides not to move to a version that does — in which case the
  two existing NOLINTNEXTLINEs for it in this same header are dead weight and
  should go, which is a finding in itself.
• Re-open / widen if the tree-wide sweep finds this in more than a handful
  of places, which would make it a configuration decision rather than a few
  annotations.

[Yaraslaut]

Independently reproduced — and here is where it sits in the tree-wide picture

I ran the full-tree clang-tidy census for #580 in parallel with this being
filed, and it reproduces your finding at the same line and column, from a
completely different tool path: all 695 translation units in a
compile_commands.json configured with the CI clang-tidy job's exact option
set, on a020e69c (one commit behind the 0e3b8823 you measured; the diff
between them touches no include/morph/ file).

include/morph/render/locale_format.hpp:457:29: error: possibly unsafe 'operator[]', consider bounds-safe alternatives [cppcoreguidelines-pro-bounds-avoid-unchecked-container-access]

Two pieces of context that may change how this is resolved:

1. It is not one finding — it is one of 250.
cppcoreguidelines-pro-bounds-avoid-unchecked-container-access is the single
largest check in the tree by a wide margin: 250 of 596 distinct findings
under include/morph/, 42% of the whole census.

 250  cppcoreguidelines-pro-bounds-avoid-unchecked-container-access
          76 include/morph/forms/forms.hpp
          61 include/morph/session/session_auth.hpp
          29 include/morph/forms/views.hpp
          19 include/morph/net/detail/sha1.hpp
          15 include/morph/net/detail/base64.hpp
          12 include/morph/util/quantity.hpp
           ... and 38 more files

locale_format.hpp:457 is the one instance in that file. Fixing it alone
would leave 249, and would settle by accident the policy question those 249
raise. The
full report on #580
lays out the three exits (disable the check in .clang-tidy, suppress per site
with reasons, or rewrite) and recommends one.

2. This exact line was already reported, on #600. The second comment there
recorded locale_format.hpp:457 as a second instance of the changed-lines gate's
blind spot, in July, with the same reproduction. It also made the observation
that decides the right fix here: the same file already carries a
NOLINTNEXTLINE for this very check two functions earlier, at
groupingIsWellPlaced's char const chr = text[i];, with the reason "i is
bounded by the loop condition" — and i < wholePart.size() is that same bound.
So the file suppresses the check in one place and not in the other, for
identical reasons. That inconsistency, not the line itself, is the finding.

On your methodological note — it is right and worth keeping. I hit the
mirror-image version of the same trap: the same header reaches clang-tidy
through several ../ include chains, so a raw diagnostic count double-counts
badly (60265 raw lines → 3581 distinct findings by normalised path+line+column
over the full run, a 17x factor). Anyone measuring this check set needs both
halves of your note: filter by HeaderFilterRegex and normalise the path
before counting.

Suggested handling

Not a duplicate of #580 in the mergeable sense — #580 is the census and the
posture decision, this is a specific line. But its fix is item 4 of #580's
batch A (the 21-finding tail: version.hpp, attributes.hpp, journal/,
render/, qt/forms/, detail/), which I recommended dispatching first
precisely because locale_format.hpp is where the suppression precedent has
to be written — it is the one file that already contains both halves of the
argument. Folding this in there gets a consistent file and a precedent for the
other 249 in one commit; fixing it standalone gets a consistent file and no
precedent.

Not verified by me: whether .at(), a NOLINT matching the one at :181,
or disabling the check is the right answer. I reproduced the diagnostic; I did
not make that call, because it is the same call #580 needs from the owner and
250 sites ride on it.

No triage: label applied — per AGENTS.md that verdict belongs to the
triage-issue skill, not to a commenter.

[Yaraslaut]

Resolved by #630 — closing as fixed, not as invalid

This was real when filed. Re-running the issue's own reproduction verbatim on master (95aa5686):

$ printf '#include <morph/render/locale_format.hpp>\n' > /tmp/tu.cpp
$ clang-tidy --config-file=.clang-tidy /tmp/tu.cpp -- -std=c++23 -I include
(no output)
  exit=0

and the only change to that file since the revision you measured on:

$ git log --oneline 0e3b8823..HEAD -- include/morph/render/locale_format.hpp
513145a1 render/forms: escape the bidi controls in the QML i18n tests, and support
         native digit sets at both control edges (fixes #610, fixes #591) (#630)

So the finding at :457 in formatCanonicalNumber's display loop was removed by #630 — the PR implementing #591, which is the same work during which you found it. Incidental, but genuinely fixed rather than suppressed: the file carries exactly one NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-avoid-unchecked-container-access), at :181, and that one predates this issue (#581) and covers a different site.

My clean result is not silence. I mutation-checked the probe, because a clang-tidy run reporting nothing is exactly as quiet when it analysed nothing:

with the NOLINT at :181 removed  -> 1 unchecked-container-access finding
restored                          -> exit 0, clean

So the analysis runs, the check is live (cppcoreguidelines-* with WarningsAsErrors: "*"), and it has nothing to say about this header any more.

Not applying a triage: label. The verdicts available are about whether a premise holds, and this one held when written — calling it invalid would misrepresent the history for whoever reads it next. Closing as completed with the measurement instead.

What would re-open it

A finding of this check in locale_format.hpp from a TU that includes it, on a named revision. Note the two modes differ and both are worth trying if you suspect it: the header as its own main file (main-file diagnostics bypass HeaderFilterRegex), and via an including TU (governed by the regex, which does match include/morph/.*).

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