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Spurious error for pack expansions in nested generic lambdas [GH #491] - #492
Previously, valid pack expansions, sizeof... operands, and fold expressions in
nested generic lambdas (for example, in default arguments or exception
specifications) could be rejected during instantiation with spurious missing
parameter pack errors. For example, with --c++14 -tused:
int f() {
auto g = [](auto... x) {
return [](auto, int n = sizeof...(x)) { return n; }(0);
};
return g(1, 2); // Previously a spurious error, now okay.
}
Summary by CodeRabbit
Bug Fixes
Fixed spurious parameter-pack errors in nested generic lambdas, including pack expansions, sizeof..., default arguments, and fold expressions.
Valid pack-related code is no longer incorrectly rejected during instantiation.
Tests
Added regression coverage across C++ language versions and updated expected compiler diagnostics.
No actionable comments were generated in the recent review.
Walkthrough
The compiler now tracks pack expansions recorded by earlier scans in nested generic-lambda contexts. This state affects pack-reference checks and diagnostic handling. New regression tests cover default arguments, pack expansions, exception specifications, and fold expressions. Existing expected diagnostic outputs are updated.
Changes: Nested generic-lambda pack handling
Layer / File(s)
Summary
Record pack expansion state src/scope_stk.h, src/scope_stk.c, src/pending_changes/gh-491.txt
The pack-expansion stack entry gains a recorded_pack_expansion flag. Applicable scans set it, and the changelog describes the nested-lambda cases.
Apply recorded expansion state src/scope_stk.c, src/expr.c
Current-context checks recognize previously recorded expansions. The compiler skips related no-packs and non-pack identifier diagnostics when an earlier scan recorded an expansion.
New tests cover nested-lambda default arguments, pack expansions, exception specifications, and fold expressions. Test configurations select multiple language modes and compiler settings.
This change fixes spurious pack-expansion errors in nested generic lambdas. The new suppression logic could in rare cases hide a real diagnostic, or yield a wrong sizeof... value on a rescan. Neither case is confirmed. The change is mergeable, but the owner should consider adding a test that checks the returned sizeof... value.
Pre-merge checks: 3 · 1
Failed checks: 1 inconclusive
Check name
Status
Explanation
Resolution
Linked Issues check
[#491] The pack-expansion stack records earlier expansions and suppresses spurious no-pack diagnostics. Regression tests cover nested-lambda default arguments, exception specifications, and fold expre…
Provide reviewable test results or code evidence for the second issue reproducer with --c++20 -tused in linux-gcc-debug.
Passed checks: 3 passed
Check name
Status
Explanation
Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly describes the fix for spurious pack-expansion errors in nested generic lambdas.
Out of Scope Changes check
[#491] The compiler changes address recorded pack expansions. The regression tests, diagnostic baselines, and changelog support that fix. No unrelated change is evident in the reviewable summary.
Full details: Linked Issues check
Explanation
[#491] The pack-expansion stack records earlier expansions and suppresses spurious no-pack diagnostics. Regression tests cover nested-lambda default arguments, exception specifications, and fold expressions. The supplied evidence does not establish whether the issue's g(decltype(x){}...) default-argument example compiles with --c++20 -tused in linux-gcc-debug. The C++20 regression fixture uses Microsoft mode.
Resolution
Provide reviewable test results or code evidence for the second issue reproducer with --c++20 -tused in linux-gcc-debug.
Finishing Touches
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daveedvdv-nvidia
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Fixes #491
Previously, valid pack expansions, sizeof... operands, and fold expressions in
nested generic lambdas (for example, in default arguments or exception
specifications) could be rejected during instantiation with spurious missing
parameter pack errors. For example, with
--c++14 -tused:Summary by CodeRabbit
Bug Fixes
sizeof..., default arguments, and fold expressions.Tests