This commit implements P2819R2 for C++26, making std::complex destructurable and tuple-like (see [complex.tuple]). std::get needs to get forward declared in stl_pair.h (following the existing precedent for the implementation of P2165R4, cf. r14-8710-g65b4cba9d6a9ff), and implemented in <complex>. Also, std::get(complex<T>) needs to return *references* to the real and imaginary parts of a std::complex object, honoring the value category and constness of the argument. In principle a straightforward task, it gets a bit convoluted by the fact that: 1) std::complex does not have existing getters that one can use for this (real() and imag() return values, not references); 2) there are specializations for language/extended floating-point types, which requires some duplication -- need to amend the primary and all the specializations; 3) these specializations use a `__complex__ T`, but the primary template uses two non-static data members, making generic code harder to write. The implementation choice used here is to add the overloads of std::get for complex as declared in [complex.tuple]. In turn they dispatch to a newly added getter that extracts references to the real/imaginary parts of a complex<T>. This getter is private API, and the implementation depends on whether it's the primary (bind the data member) or a specialization (use the GCC language extensions for __complex__). To avoid duplication and minimize template instantiations, the getter uses C++23's deducing this (this avoids const overloads). The value category is dealt with by the std::get overloads. Add a test that covers the aspects of the tuple protocol, as well as the tuple-like interface. While at it, add a test for the existing tuple-like feature-testing macro. PR libstdc++/113310 libstdc++-v3/ChangeLog: * include/bits/stl_pair.h (get): Forward-declare std::get for std::complex. * include/bits/version.def (tuple_like): Bump the value of the feature-testing macro in C++26. * include/bits/version.h: Regenerate. * include/std/complex: Implement the tuple protocol for std::complex. (tuple_size): Specialize for std::complex. (tuple_element): Ditto. (__is_tuple_like_v): Ditto. (complex): Add a private getter to obtain references to the real and the imaginary part, on the primary class template and on its specializations. (get): Add overloads of std::get for std::complex. * testsuite/20_util/tuple/tuple_like_ftm.cc: New test. * testsuite/26_numerics/complex/tuple_like.cc: New test.