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.
