Implement LWG-4218 Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG-2369 - #6324
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Stephan T. Lavavej (StephanTLavavej) merged 4 commits intoJul 11, 2026
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Pull request overview
This PR implements the LWG-4218 resolution to fix constraint recursion in basic_const_iterator's reversed relational operators (<, >, <=, >=) that occurs due to ADL combined with CWG-2369. The fix lives in the core <xutility> header that defines basic_const_iterator, and is accompanied by regression tests.
Changes:
- Add a deduced,
same_as<_Iter>-constrained template parameter_Jterto the four reversed relational friend operators and change theirbasic_const_iteratorparameter tobasic_const_iterator<_Jter>, breaking the constraint-satisfaction recursion. - Add compile-time
static_asserttests covering both a heterogeneous-comparison ADL scenario and areverse_iterator<basic_const_iterator<...>>scenario.
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| File | Description |
|---|---|
stl/inc/xutility |
Makes the four reversed relational friend operators' right-hand operand depend on a deduced same_as<_Iter> parameter to avoid the LWG-4218 ADL constraint recursion. |
tests/std/tests/P2278R4_basic_const_iterator/test.cpp |
Adds an lwg_4218 namespace with static_asserts that would fail to compile (constraint recursion) without the fix. |
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Stephan T. Lavavej (StephanTLavavej)
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Fixes #6316.
Remarks: Currently, MSVC doesn't give constraint recursion for the original example posted in LWG-4218, perhaps because MSVC hasn't completely implemented CWG-2369. In addition to the original example, this PR adds another example causing constraint recursion for both MSVC and Clang when LWG-4218 is not implemented.
Reported DevCom-11111696 for a bug encountered in CI (with workaround added).