https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117783
--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:dff57d76ad3c78cedaf2f8caa1686acb5059303d commit r16-3069-gdff57d76ad3c78cedaf2f8caa1686acb5059303d Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Aug 7 16:38:51 2025 +0200 c++: Implement C++26 P1061R10 - Structured Bindings can introduce a Pack [PR117783] The following patch implements the C++26 P1061R10 - Structured Bindings can introduce a Pack paper. One thing unresolved in the patch is mangling, I've raised https://github.com/itanium-cxx-abi/cxx-abi/issues/200 for that but no comments there yet. One question is if it is ok not to mention the fact that there is a structured binding pack in the mangling of the structured bindings but more important is in case of std::tuple* we might need to mangle individual structured binding pack elements separately (each might need an exported name for the var itself and perhaps its guard variable as well). The patch just uses the normal mangling for the whole structured bindings and emits sorry if we need to mangle the structured binding pack elements. The patch just marks the structured binding pack specially (considered e.g. using some bit on it, but in the end I'm identifying it using a made up type which causes DECL_PACK_P to be true; it is kind of self-referential solution, because the type on the pack mentions the DECL_DECOMPOSITION_P VAR_DECL on which the type is attached as its pack, so it needs to be handled carefully during instantiation to avoid infinite recursion, but it is the type that should be used if something else actually needs to use the same type as the structured binding pack, e.g. a capture proxy), and stores the pack elements when actually processed through cp_finish_decomp with non-dependent initializer into a TREE_VEC used as DECL_VALUE_EXPR of the pack; though because several spots use the DECL_VALUE_EXPR and assume it is ARRAY_REF from which they can find out the base variable and the index, it stores the base variable and index in the first 2 TREE_VEC elts and has the structured binding elements only after that. https://eel.is/c++draft/temp.dep.expr#3.6 says the packs are type dependent regardless of whether the initializer of the structured binding is type dependent or not, so I hope having a dependent type on the structured binding VAR_DECL is ok. The paper also has an exception for sizeof... which is then not value dependent when the structured bindings are initialized with non-dependent initializer: https://eel.is/c++draft/temp.dep.constexpr#4 The patch special cases that in 3 spots (I've been wondering if e.g. during parsing I couldn't just fold the sizeof... to the INTEGER_CST right away, but guess I'd need to repeat that also during partial instantiation). And one thing still unresolved is debug info, I've just added DECL_IGNORED_P on the structured binding pack VAR_DECL because there were ICEs with -g for now, hope it can be fixed incrementally but am not sure what exactly we should emit in the debug info for that. Speaking of which, I see DW_TAG_GNU_template_parameter_pack DW_TAG_GNU_formal_parameter_pack etc. DIEs emitted regardless of DWARF version, shouldn't we try to upstream those into DWARF 6 or check what other compilers emit for the packs? And bet we'd need DW_TAG_GNU_structured_binding_pack as well. 2025-08-07 Jakub Jelinek <ja...@redhat.com> PR c++/117783 gcc/c-family/ * c-cppbuiltin.cc (c_cpp_builtins): Change __cpp_structured_bindings predefined value for C++26 from 202403L to 202411L. gcc/cp/ * parser.cc: Implement C++26 P1061R10 - Structured Bindings can introduce a Pack. (cp_parser_range_for): Also handle TREE_VEC as DECL_VALUE_EXPR instead of ARRAY_REF. (cp_parser_decomposition_declaration): Use sb-identifier-list instead of identifier-list in comments. Parse structured bindings with structured binding pack. Don't emit pedwarn about structured binding attributes in structured bindings inside of a condition. (cp_convert_omp_range_for): Also handle TREE_VEC as DECL_VALUE_EXPR instead of ARRAY_REF. * decl.cc (get_tuple_element_type): Change i argument type from unsigned to unsigned HOST_WIDE_INT. (get_tuple_decomp_init): Likewise. (set_sb_pack_name): New function. (cp_finish_decomp): Handle structured binding packs. * pt.cc (tsubst_pack_expansion): Handle structured binding packs and capture proxies for them. Formatting fixes. (tsubst_decl): For structured binding packs don't tsubst TREE_TYPE first, instead recreate the type after r is created. (tsubst_omp_for_iterator): Also handle TREE_VEC as DECL_VALUE_EXPR instead of ARRAY_REF. (tsubst_expr): Handle sizeof... on non-dependent structure binding packs. (value_dependent_expression_p): Return false for sizeof... on non-dependent structure binding packs. (instantiation_dependent_r): Don't recurse on sizeof... on non-dependent structure binding packs. * constexpr.cc (potential_constant_expression_1): Also handle TREE_VEC on DECL_VALUE_EXPR of structure binding packs. gcc/testsuite/ * g++.dg/cpp26/decomp13.C: New test. * g++.dg/cpp26/decomp14.C: New test. * g++.dg/cpp26/decomp15.C: New test. * g++.dg/cpp26/decomp16.C: New test. * g++.dg/cpp26/decomp17.C: New test. * g++.dg/cpp26/decomp18.C: New test. * g++.dg/cpp26/decomp19.C: New test. * g++.dg/cpp26/decomp20.C: New test. * g++.dg/cpp26/decomp21.C: New test. * g++.dg/cpp26/feat-cxx26.C (__cpp_structured_bindings): Expect 202411 rather than 202403.
