This fixes a number of std::tuple bugs by no longer making use of the
empty base-class optimization. By using the C++20 [[no_unique_address]]
attribute we can always store the element as a data member, while still
compressing the layout of tuples containing empty types.
Since we no longer use inheritance we could also apply the compression
optimization for final types and for tuples of tuples, but doing so
would be an ABI break.
Using [[no_unique_address]] more liberally for the unstable std::__8
configuration is left for a later date. There may be reasons not to
apply the attribute unconditionally, e.g. see the discussion about
guaranteed elision in PR 94062.
libstdc++-v3/ChangeLog:
PR libstdc++/55713
PR libstdc++/71096
PR libstdc++/93147
* include/std/tuple [__has_cpp_attribute(no_unique_address)]
(_Head_base<Idx, Head, true>): New definition of the partial
specialization, using [[no_unique_address]] instead of
inheritance.
* testsuite/libstdc++-prettyprinters/48362.cc: Adjust expected
output.
* testsuite/20_util/tuple/comparison_operators/93147.cc: New test.
* testsuite/20_util/tuple/creation_functions/55713.cc: New test.
* testsuite/20_util/tuple/element_access/71096.cc: New test.
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