https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96251
Bug ID: 96251 Summary: co_yield incorrectly rejected in non-explicitly-constexpr generic lambda Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: yuri.kilochek at gmail dot com Target Milestone: --- Created attachment 48898 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48898&action=edit preprocessed test source Given `test.cpp`: #include <coroutine> struct coroutine { struct promise_type { auto get_return_object() { return coroutine(); } auto initial_suspend() { return std::suspend_always(); } auto yield_value(int) { return std::suspend_always(); } void return_void() {} auto final_suspend() { return std::suspend_always(); } void unhandled_exception() {} }; }; int main() { auto f = [](auto max) -> coroutine { for (int i = 0; i < max; ++i) { co_yield i; } }; f(10); } Compiled with: g++ -v -save-temps -std=c++20 -fcoroutines test.cpp The following diagnostic is produced: Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-libunwind-exceptions --disable-werror gdc_include_dir=/usr/include/dlang/gdc Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 10.1.0 (GCC) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++2a' '-fcoroutines' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/cc1plus -E -quiet -v -D_GNU_SOURCE test.cpp -mtune=generic -march=x86-64 -std=c++2a -fcoroutines -fpch-preprocess -o test.ii ignoring nonexistent directory "/usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../x86_64-pc-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0 /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/x86_64-pc-linux-gnu /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/../../../../include/c++/10.1.0/backward /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/include /usr/local/include /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++2a' '-fcoroutines' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/cc1plus -fpreprocessed test.ii -quiet -dumpbase test.cpp -mtune=generic -march=x86-64 -auxbase test -std=c++2a -version -fcoroutines -o test.s GNU C++17 (GCC) version 10.1.0 (x86_64-pc-linux-gnu) compiled by GNU C version 10.1.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.21-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C++17 (GCC) version 10.1.0 (x86_64-pc-linux-gnu) compiled by GNU C version 10.1.0, GMP version 6.2.0, MPFR version 4.0.2, MPC version 1.1.0, isl version isl-0.21-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 262bfe0a9d27898ea74cb944d94238f6 test.cpp: In instantiation of ‘main()::<lambda(auto:1)> [with auto:1 = int]’: test.cpp:21:10: required from here test.cpp:17:13: error: ‘co_yield’ cannot be used in a ‘constexpr’ function 17 | co_yield i; | ^~~~~~~~ Since coroutines cannot be `constexpr` (per [dcl.constexpr]/3.3), presence of `co_yield` should simply disqualify the lambda from being implicitly constexpr (as it would otherwise be due to [expr.prim.lambda.closure]/4). Interestingly, if we make the lambda non-generic (e.g. replace `auto max` with `int max`) it compiles and works fine.