See https://godbolt.org/z/o944oWafx. ```C++ void f(auto i) { ([]<class... T>(T...) { return (..., [] { if constexpr (T{0}) ; }()); }(i)); } void g() { f(0); } ``` ``` <source>: In instantiation of 'f<int>(int)::<lambda(T ...)> [with T = {int}]': <source>:7:4: required from 'void f(auto:1) [with auto:1 = int]' <source>:9:13: required from here <source>:6:8: internal compiler error: in tsubst_pack_expansion, at cp/pt.c:13102 6 | }()); | ^ 0x1dadf09 internal_error(char const*, ...) ???:0 0x6e5511 fancy_abort(char const*, int, char const*) ???:0 0x962157 tsubst_pack_expansion(tree_node*, tree_node*, int, tree_node*) ???:0 0x94de47 instantiate_decl(tree_node*, bool, bool) ???:0 0x7f5dbe maybe_instantiate_decl(tree_node*) ???:0 0x7f7520 mark_used(tree_node*, int) ???:0 0x710ad5 build_op_call(tree_node*, vec<tree_node*, va_gc, vl_embed>**, int) ???:0 0x9b4935 finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool, bool, int) ???:0 0x94de47 instantiate_decl(tree_node*, bool, bool) ???:0 0x99020b instantiate_pending_templates(int) ???:0 0x7fb2f9 c_parse_final_cleanups() ???:0 Please submit a full bug report, with preprocessed source if appropriate. Please include the complete backtrace with any bug report. See <https://gcc.gnu.org/bugs/> for instructions. Compiler returned: 1 ```
Confirmed.
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:c8b2b89358481d36755dbc99e585a251780453b0 commit r12-3488-gc8b2b89358481d36755dbc99e585a251780453b0 Author: Patrick Palka <ppalka@redhat.com> Date: Mon Sep 13 10:29:32 2021 -0400 c++: parameter pack inside constexpr if [PR101764] Here when partially instantiating the first pack expansion, substitution into the condition of the constexpr if yields a still-dependent tree, so tsubst_expr returns an IF_STMT with an unsubstituted IF_COND and with IF_STMT_EXTRA_ARGS added to. Hence after partial instantiation the pack expansion pattern still refers to the unlowered parameter pack 'ts' of level 2, and it's thusly recorded in the new PACK_EXPANSION_PARAMETER_PACKS. During the subsequent final instantiation of the regenerated lambda we crash in tsubst_pack_expansion because it can't find an argument pack for this unlowered 'ts', due to the level mismatch. (Likewise when the constexpr if is replaced by a requires-expr, which also uses the extra args mechanism for avoiding partial instantiation.) So essentially, a pack expansion pattern that contains an "extra args" tree doesn't play well with partial instantiation. This patch fixes this by forcing such pack expansions to use the extra args mechanism as well. PR c++/101764 gcc/cp/ChangeLog: * cp-tree.h (PACK_EXPANSION_FORCE_EXTRA_ARGS_P): New accessor macro. * pt.c (has_extra_args_mechanism_p): New function. (find_parameter_pack_data::found_extra_args_tree_p): New data member. (find_parameter_packs_r): Set ppd->found_extra_args_tree_p appropriately. (make_pack_expansion): Set PACK_EXPANSION_FORCE_EXTRA_ARGS_P if ppd.found_extra_args_tree_p. (use_pack_expansion_extra_args_p): Return true if there were unsubstituted packs and PACK_EXPANSION_FORCE_EXTRA_ARGS_P. (tsubst_pack_expansion): Pass the pack expansion to use_pack_expansion_extra_args_p. gcc/testsuite/ChangeLog: * g++.dg/cpp1z/constexpr-if35.C: New test.
Fixed for GCC 12.
*** Bug 101595 has been marked as a duplicate of this bug. ***
*** Bug 96815 has been marked as a duplicate of this bug. ***
The master branch has been updated by Joseph Myers <jsm28@gcc.gnu.org>: https://gcc.gnu.org/g:f5f1d92fe2e1d75c3fae34497929a1965af704ae commit r13-3366-gf5f1d92fe2e1d75c3fae34497929a1965af704ae Author: Joseph Myers <joseph@codesourcery.com> Date: Tue Oct 18 23:25:47 2022 +0000 c: Diagnose "enum tag;" after definition [PR107164] As noted in bug 101764, a declaration "enum tag;" is invalid in standard C after a definition, as well as when no definition is visible; we had a pedwarn-if-pedantic for the forward declaration case, but were missing one for the other case. Add that missing diagnostic (if pedantic only). (These diagnostics will need to be appropriately conditioned when support is added for C2x enums with fixed underlying type, since "enum tag : type;" is OK both before and after a definition.) Bootstrapped with no regressions for x86_64-pc-linux-gnu. PR c/107164 gcc/c/ * c-decl.cc (shadow_tag_warned): If pedantic, diagnose "enum tag;" with previous declaration visible. gcc/testsuite/ * gcc.dg/c99-tag-4.c, gcc.dg/c99-tag-5.c, gcc.dg/c99-tag-6.c: New tests.