The following code: int a[1]; void f() { decltype(({auto&& [x] = a; 0;})) b; (void)[]() -> decltype(({auto&& [x] = a; 0;})) { return 0; }; } when compiled with gcc 9.2 and --std=c++17 outputs: <source>: In function 'void f()': <source>:5:39: internal compiler error: in cp_parser_decomposition_declaration, at cp/parser.c:13786 5 | (void)[]() -> decltype(({auto&& [x] = a; 0;})) { return 0; }; | ^ Please submit a full bug report, with preprocessed source if appropriate.
Here's a motivating use case; https://godbolt.org/z/Tt-QmN This allows to effectively SFINAE on structured binding - a 'missing link'. Works in Clang since 5.0. GDB debugging gcc latest trunk build. The parse error occurs here gcc/cp/parser.c:13869 cp_parser_decomposition_declaration else if (decl != error_mark_node && DECL_CHAIN (decl2) != prev) { /* Ensure we've diagnosed redeclaration if we aren't creating a new VAR_DECL. */ gcc_assert (errorcount); /*** HERE ***/ decl = error_mark_node; } Haven't worked out exactly what is being checked there. So I tried commenting out that entire else-if block to see what happens. It then proceeds to mangling and crashes out in contains_struct_check. Backtrace looks like this: #0 contains_struct_check gcc/cp/mangle.c "write_closure_type_name" gcc/tree.h:3386 #1 write_closure_type_name gcc/cp/mangle.c:1641 in write_unqualified_name #3 write_prefix gcc/cp/mangle.c:1164 in mangle_decomp at gcc/cp/mangle.c:4050 #5 cp_maybe_mangle_decomp gcc/cp/decl.c:8024 #6 cp_parser_decomposition_declaration
Confirmed.
Even shorter: int a[1]; void f() { []() -> decltype(({auto&& [x] = a; 0;})) { return 0; }; } clang++ compiles it.
Not a regression though, so I'll leave it for GCC 11.