src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types by omghante · Pull Request #62507 · nodejs/node

omghante · 2026-03-30T13:45:22Z

On newer libc++ (shipped with macOS Xcode 16+), std::char_traits<T> for T not
equal to char, wchar_t, char8_t, char16_t, or char32_t is deprecated and
will be removed in a future release.

When the MaybeStackBuffer template is instantiated with unsigned char or uint8_t
(e.g., in test/cctest/test_util.cc), the compiler instantiates the declarations
for all member functions. The ToString() and ToStringView() methods historically returned
std::basic_string<T> and std::basic_string_view<T>, which triggered this
deprecation warning (-Wdeprecated-declarations) during class instantiation because
the return type references std::char_traits<unsigned char>.

Changes

This PR resolves the warning by:

Extracting the list of standard character types into a reusable C++20 concept
standard_char_type.
Converting ToString() and ToStringView() from normal member functions into
member function templates with a constrained default template parameter:
template <standard_char_type U = T>.

By making these member function templates, the C++ compiler defers instantiating
the return type (std::basic_string<U>) until the function is actually called. Since
these methods are never actually called for unsigned char or uint8_t variants
in the Node.js codebase, the deprecated std::char_traits<unsigned char> is never
evaluated or instantiated, completely bypassing the warning.

Refs: #62506

targos · 2026-03-30T14:33:41Z

Have you tried your changes? I applied the patch and still see a deprecation warning.

omghante · 2026-03-30T15:29:01Z

Have you tried your changes? I applied the patch and still see a deprecation warning.

Yes, I have checked my updated changes and the deprecation warning is now gone.

The issue with the first patch was that requires only stops the function from being callable, but the return type std::basic_string is still evaluated during class instantiation, which triggers the warning. By changing ToString() and ToStringView() to member function templates (template <standard_char_type U = T>), the compiler defers instantiating the return type (std::basic_string<U>) until the method is actually called. Since these methods are never used for MaybeStackBuffer<unsigned char>, the deprecation warning is completely bypassed.

targos · 2026-03-30T15:51:37Z

Thanks. I confirm there is no warning anymore with the latest push. I defer to @nodejs/cpp-reviewers for the code review.

codecov · 2026-04-02T07:40:10Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.81%. Comparing base (de5644c) to head (2951d47).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62507      +/-   ##
==========================================
- Coverage   89.81%   89.81%   -0.01%     
==========================================
  Files         699      699              
  Lines      216235   216466     +231     
  Branches    41336    41389      +53     
==========================================
+ Hits       194216   194412     +196     
- Misses      14129    14143      +14     
- Partials     7890     7911      +21

Files with missing lines	Coverage Δ
src/util.h	`91.26% <100.00%> (+0.06%)`	⬆️

... and 52 files with indirect coverage changes

🚀 New features to boost your workflow:

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On newer libc++ (shipped with macOS Xcode 16+), std::char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t, or char32_t is deprecated and will be removed in a future release. When MaybeStackBuffer is instantiated with unsigned char or uint8_t (e.g. in test/cctest/test_util.cc), the ToString() and ToStringView() methods trigger this deprecation warning because their return types reference std::basic_string<unsigned char> and std::basic_string_view<unsigned char>, even though these methods are never actually called for those types. Convert ToString() and ToStringView() into member function templates with a constrained default template parameter, so the return type is only instantiated when the function is actually called. Extract the type list into a reusable standard_char_type concept.

nodejs-github-bot · 2026-04-16T07:10:53Z

CI: https://ci.nodejs.org/job/node-test-pull-request/72713/

nodejs-github-bot · 2026-04-16T12:45:57Z

CI: https://ci.nodejs.org/job/node-test-pull-request/72722/

nodejs-github-bot · 2026-04-20T15:51:25Z

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- Loading data for nodejs/node/pull/62507
✔  Done loading data for nodejs/node/pull/62507
----------------------------------- PR info ------------------------------------
Title      src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types (#62507)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     omghante:fix/constrain-maybstackbuffer-tostring-char-traits -> nodejs:main
Labels     c++, author ready, needs-ci
Commits    1
 - src: fix MaybeStackBuffer char_traits deprecation warning
Committers 1
 - om-ghante <mr.omghante1@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/62507
Refs: https://github.com/nodejs/node/issues/62506
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/62507
Refs: https://github.com/nodejs/node/issues/62506
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Mon, 30 Mar 2026 13:45:22 GMT
   ✔  Approvals: 3
   ✔  - Yagiz Nizipli (@anonrig) (TSC): https://github.com/nodejs/node/pull/62507#pullrequestreview-4051985736
   ✔  - Tobias Nießen (@tniessen) (TSC): https://github.com/nodejs/node/pull/62507#pullrequestreview-4114804604
   ✔  - Anna Henningsen (@addaleax): https://github.com/nodejs/node/pull/62507#pullrequestreview-4141239569
   ✘  1 GitHub CI job(s) failed:
   ✘    - x86_64-darwin: with shared libraries: FAILURE (https://github.com/nodejs/node/actions/runs/23905606841/job/69714214012)
   ℹ  Last Full PR CI on 2026-04-16T12:45:57Z: https://ci.nodejs.org/job/node-test-pull-request/72722/
- Querying data for job/node-test-pull-request/72722/
✔  Build data downloaded
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/24676220710

On newer libc++ (shipped with macOS Xcode 16+), std::char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t, or char32_t is deprecated and will be removed in a future release. When MaybeStackBuffer is instantiated with unsigned char or uint8_t (e.g. in test/cctest/test_util.cc), the ToString() and ToStringView() methods trigger this deprecation warning because their return types reference std::basic_string<unsigned char> and std::basic_string_view<unsigned char>, even though these methods are never actually called for those types. Convert ToString() and ToStringView() into member function templates with a constrained default template parameter, so the return type is only instantiated when the function is actually called. Extract the type list into a reusable standard_char_type concept. PR-URL: #62507 Refs: #62506 Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com> Reviewed-By: Tobias Nießen <tniessen@tnie.de> Reviewed-By: Anna Henningsen <anna@addaleax.net>

addaleax · 2026-04-20T16:23:15Z

Landed in e02087c

nodejs-github-bot added c++ Issues and PRs that require attention from people who are familiar with C++. needs-ci PRs that need a full CI run. labels Mar 30, 2026

omghante force-pushed the fix/constrain-maybstackbuffer-tostring-char-traits branch from ecb3efb to 7c9594c Compare March 30, 2026 13:48

jasnell reviewed Mar 30, 2026

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Comment thread src/util.h

omghante force-pushed the fix/constrain-maybstackbuffer-tostring-char-traits branch 2 times, most recently from b42db3d to 8eb1cad Compare March 30, 2026 15:27

omghante force-pushed the fix/constrain-maybstackbuffer-tostring-char-traits branch from 8eb1cad to 9f3892d Compare April 2, 2026 14:30

targos mentioned this pull request Apr 2, 2026

src: restrict MaybeStackBuffer string helpers to text types #62564

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anonrig approved these changes Apr 2, 2026

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omghante force-pushed the fix/constrain-maybstackbuffer-tostring-char-traits branch 3 times, most recently from 2951d47 to 9f3892d Compare April 15, 2026 15:16

tniessen approved these changes Apr 15, 2026

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src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types#62507

src: constrain MaybeStackBuffer::ToString and ToStringView to standard char types#62507
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