refactor: disable default std::hash for CTransactionRef #35101

Sjors commented at 7:48 PM on April 17, 2026: member

While working on #33922 I initially forgot to add CTransactionRefComp to the std::unordered_map<CTransactionRef defined there. This PR turns that into a compiler error.

This change triggers a false positive IWYU error, and an inconsistent one at that: our CI wants <variant>, while a manual build on Ubuntu (version 0.26 with clang version 22.1.1) wants <string_view>.

Various workarounds were discussed in:

Addressed by back-porting:

https://github.com/include-what-you-use/include-what-you-use/pull/2013

DrahtBot added the label Refactoring on Apr 17, 2026

DrahtBot commented at 7:48 PM on April 17, 2026: contributor

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ACK	vasild
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Approach ACK	hebasto

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in src/primitives/transaction.h:426 in 276da6236a outdated

 421 | + *  comparing by pointer. Use CTransactionRefSaltedHash or provide a custom
 422 | + *  hasher. */
 423 | +template <>
 424 | +struct hash<CTransactionRef> {
 425 | +    size_t operator()(const CTransactionRef&) const = delete;
 426 | +};

hebasto commented at 5:54 PM on April 18, 2026:

e5d04fb46a68bd884d8eefae1014aaa3dd79fa6a:

This approach doesn't handle scenarios where a container is declared, but no member functions involving hashing are explicitly called. For example, this compiles.

I suggest a more defensive approach relying on the conditions that must be satisfied for the std::hash<CTransactionRef> specialization to be enabled:

template <>
struct hash<CTransactionRef> {
    hash() = delete;
};

Here are two examples on https://godbolt.org/:

A container with the default hasher: fails.
A container with the custom hasher: passes.

Sjors commented at 10:21 AM on April 19, 2026:

Thanks, marking draft while I study this suggestion.

Sjors commented at 8:35 AM on April 24, 2026:

I added hash() = delete;, but also kept the earlier variant for clarity.

in src/primitives/transaction.h:419 in 276da6236a

 414 | + * workaround.
 415 | + */
 416 | +#ifndef _LIBCPP_VERSION
 417 | +template <class T>
 418 | +struct hash; // IWYU pragma: keep
 419 | +#endif

hebasto commented at 6:00 PM on April 18, 2026:

276da6236af12bb7aee12f74f8138f2c9bb16176:

This changes follows this suggestion, excluding libc++, which is not used in the IWYU CI job.

Looks fine to me.

hebasto commented at 8:34 AM on April 20, 2026:

I can confirm that the bug has been fixed upstream in https://github.com/include-what-you-use/include-what-you-use/pull/2013.

Sjors marked this as a draft on Apr 19, 2026

Sjors commented at 10:55 AM on April 20, 2026: member

Since this isn't urgent, I think I'll wait for #35101 (review) to land in a release.

Sjors force-pushed on Apr 24, 2026

Sjors commented at 8:19 AM on April 24, 2026: member

I took the IWYU patch to see if it actually does the job (I still plan to wait for it to be backported to the clang_22 branch).

Sjors force-pushed on Apr 24, 2026

DrahtBot added the label CI failed on Apr 24, 2026

Sjors commented at 8:35 AM on April 24, 2026: member

The IWYU patch works. I also implemented @hebasto's suggestion: #35101 (review)

Sjors force-pushed on Apr 24, 2026

DrahtBot removed the label CI failed on Apr 24, 2026

Sjors commented at 5:02 PM on April 25, 2026: member

Apparently the patch won't be backported: https://github.com/include-what-you-use/include-what-you-use/pull/2013#issuecomment-4313034887

So I'll just keep it here.

Sjors force-pushed on Apr 25, 2026

Sjors marked this as ready for review on Apr 25, 2026

hebasto commented at 6:23 AM on April 27, 2026: member

Apparently the patch won't be backported: include-what-you-use/include-what-you-use#2013 (comment)

So I'll just keep it here.

FWIW, the IWYU maintainer wrote:

But if actually backported, it should be done along with the two preceding commits.

hebasto commented at 6:24 AM on April 27, 2026: member

Approach ACK 16a6b05441f0a195b3df89c8b570f57230ad0f1f.

Sjors commented at 8:28 AM on April 27, 2026: member

Previous two commits, I don't think we need them here:

https://github.com/include-what-you-use/include-what-you-use/commit/28d70e6a776c735fec8fe5ad630657533000591b: about false positive includes of <print>, which afaik we haven't encountered, and is unrelated to this change anyway.
https://github.com/include-what-you-use/include-what-you-use/commit/d1e42bf8bebae7269ae83e999b49f11144afeeda: related to <iosfwd>, so also seems unrelated to this PR

in ci/test/01_iwyu.patch:635 in 16a6b05441

 631 | @@ -632,3 +632,48 @@ See: https://github.com/include-what-you-use/include-what-you-use/blob/clang_21/
 632 |   };
 633 |   
 634 |   const char* stdlib_cpp_public_headers[] = {
 635 | +

vasild commented at 9:15 AM on April 27, 2026:

Offtopic, so we maintain patches for IWYU into a single file. Multiple patches appended into a single file. I think that is prone to conflicts when adding / removing stuff from that file and also a maintenance headache when two separate patches that are to be added in 01_iwyu.patch touch the same file.

In this case the newly patched std_symbol_map.inc was not present before in 01_iwyu.patch. It would be better to keep each patchset into a separate file. So, for example in this PR a new file like 52f85e1f4d99.diff is added.

Sjors commented at 12:26 PM on April 27, 2026:

I moved the new patch to its own file.

in ci/test/01_iwyu.patch:639 in 16a6b05441

 631 | @@ -632,3 +632,48 @@ See: https://github.com/include-what-you-use/include-what-you-use/blob/clang_21/
 632 |   };
 633 |   
 634 |   const char* stdlib_cpp_public_headers[] = {
 635 | +
 636 | +
 637 | +Map std::hash to its providing standard headers.
 638 | +Backport of https://github.com/include-what-you-use/include-what-you-use/pull/2013
 639 | +(commit 52f85e1f4d990f55fc6556d543eb051d79364a16) to the clang_22 release

vasild commented at 9:18 AM on April 27, 2026:

https://github.com/include-what-you-use/include-what-you-use/commit/52f85e1f4d990f55fc6556d543eb051d79364a16 contains changes also to mapgen/iwyu-mapgen-std-symbol.py which are omitted here. I think it is fine, just mentioning.

in src/primitives/transaction.h:408 in 16a6b05441

 402 | @@ -403,4 +403,16 @@ struct CMutableTransaction
 403 |  typedef std::shared_ptr<const CTransaction> CTransactionRef;
 404 |  template <typename Tx> static inline CTransactionRef MakeTransactionRef(Tx&& txIn) { return std::make_shared<const CTransaction>(std::forward<Tx>(txIn)); }
 405 |  
 406 | +namespace std {
 407 | +/** Disable default std::hash for CTransactionRef to prevent accidentally
 408 | + *  comparing by pointer. Use CTransactionRefSaltedHash or provide a custom

vasild commented at 9:20 AM on April 27, 2026:

There is no CTransactionRefSaltedHash.

Sjors commented at 10:38 AM on April 27, 2026:

Indeed, that's introduced in #33922.

Sjors commented at 12:27 PM on April 27, 2026:

(switched to CTransactionRefHash)

vasild approved

vasild commented at 9:25 AM on April 27, 2026: contributor

ACK 16a6b05441f0a195b3df89c8b570f57230ad0f1f (modulo the reference to the nonexistent CTransactionRefSaltedHash)

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DrahtBot requested review from hebasto on Apr 27, 2026

ci: backport iwyu PR 2013 std::hash mapping

Cherry-pick include-what-you-use commit 52f85e1f4d99 onto the clang_22
branch tracked by ci/test/00_setup_env_native_iwyu.sh, fixing the false
positive where std::hash was mapped to <string_view>/<variant> instead of
its real provider (<functional>, <memory>, etc).

47d68cd981

refactor: disable default std::hash for CTransactionRef

The default std::hash for shared_ptr compares by pointer.

CTransactionRefHash or a custom hasher should be used instead.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

a9301cfa07

Sjors force-pushed on Apr 27, 2026

Sjors commented at 12:31 PM on April 27, 2026: member

fixed comment to refer to CTransactionRefHash instead of CTransactionRefSaltedHash, see #35101 (review)
moved patch to its own file, see #35101 (review)
updated PR description to mention the patch

vasild approved

vasild commented at 1:47 PM on April 29, 2026: contributor

ACK a9301cfa0730d1d08c9e982f990dfc8aa0b3a27a

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w0xlt commented at 6:28 PM on April 30, 2026: contributor

Concept ACK