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fix(folding): correct unique object rep checks [] - #326
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Fix __has_unique_object_representations for several cases: - A base class with no direct nonstatic data members that inherits some (and is thus not empty) was treated as an empty base. Its size was counted as tail padding, making the trait incorrectly FALSE for derived classes. Use is_empty_class_type instead of checking for an empty field list. - Treat unnamed bit-fields of nonzero width as padding in Clang 17+ and g++ 13.1+ (also g++ 10.5, 11.4, 12.3 backports). - Restrict treating volatile-qualified non-class types as lacking unique object representations to g++ modes before version 10, and no longer apply it in Clang modes. Add regression test has-unique-object-rep-inherited-base and update expectations for imported Clang type-traits and g++ has-unique-obj-representations3 tests.
nv-cmeerw
requested changes
Oct 20, 2026
| A base class that declares no nonstatic data members of its own but inherits | ||
| some (and is therefore not empty) was treated as an empty base class. Its | ||
| size was then counted as tail padding, so the trait was incorrectly FALSE for | ||
| classes derived from it. For example, with --g++: |
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The issue is not actually gcc specific, but also applies to default/standard mode, so should just have an example with --c++11.
| This is now fixed. In addition, the following version-dependent behaviors | ||
| of g++ and Clang are now emulated: | ||
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| - An unnamed bit-field of nonzero width is treated as padding (making the |
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What should the behavior for standard-mode be in this case?
| //remark:has_unique_object_representations with a base that has no direct fields | ||
| //type:fp | ||
| //options_all:--c++17 | ||
| //options:--g++ --gnu_version=150100:--clang --clang_version=190000 |
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The test case should also test standard mode (and ideally, also MSVC modes).
Also need to have test coverage for the other code changes (unnamed bit-field, volatile qualifiers). Should also test each version condition from the source code.
| field->source_corresp.assoc_info == | ||
| (char *)get_unnamed_field_symbol()) { | ||
| /* An unnamed bit-field of non-zero width is padding. */ | ||
| result = FALSE; |
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Does that need to be restricted to non-union class types?
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Discussion: #290
Fix __has_unique_object_representations for several cases:
A base class with no direct nonstatic data members that inherits some (and is thus not empty) was treated as an empty base. Its size was counted as tail padding, making the trait incorrectly FALSE for derived classes. Use is_empty_class_type instead of checking for an empty field list.
Treat unnamed bit-fields of nonzero width as padding in Clang 17+ and g++ 13.1+ (also g++ 10.5, 11.4, 12.3 backports).
Restrict treating volatile-qualified non-class types as lacking unique object representations to g++ modes before version 10, and no longer apply it in Clang modes.
Add regression test has-unique-object-rep-inherited-base and update expectations for imported Clang type-traits and g++ has-unique-obj-representations3 tests.