1768116 - (C++20) [meta] Compile as C++20

Status: NEW

(Firefox Build System :: General, task, P3)

Description

[Chris Peterson [:cpeterson]]

Who knows when we'll be able to compiled with -std=c++20, but in the meantime we can use this meta bug to gather the work needed to compile as C++20 and the new improvements that will become available with C++20.

When compiling as C++20, __cplusplus == 202002.

Summary of C++20 language changes: https://en.cppreference.com/w/cpp/20

Clang's C++20 support: https://clang.llvm.org/cxx_status.html#cxx20

GCC's C++20 support: https://gcc.gnu.org/projects/cxx-status.html#cxx20

Comment 1

[Chris Peterson [:cpeterson]]

This slide deck from a Google engineer details some of the problems (and fixes) found when updating Chrome from C++17 to C++20:

"C++20, How Hard Could It Be"
https://docs.google.com/presentation/d/1HwLNSyHxy203eptO9cbTmr7CH23sBGtTrfOmJf9n0ug/preview?resourcekey=0-GH5F3wdP7D4dmxvLdBaMvw

Chrome's C++20 meta bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=1284275

Comment 2

[Kelsey Gilbert [:jgilbert]]

Thanks for working on this!

Comment 3

[Mike Hommey [:glandium]]

FYI, I don't expect we'd be able to actually switch to C++20 for a few years.

Comment 4

[Kelsey Gilbert [:jgilbert]]

What are our unlisted blockers? Our good friends hazard-builds and android toolchains?

FWIW, we went c++17 on 2019-12-03, so it's we're neither really ahead nor behind our previous pace.

Comment 5

[Chris Peterson [:cpeterson]]

(In reply to Kelsey Gilbert [:jgilbert] from comment #4)

What are our unlisted blockers? Our good friends hazard-builds and android toolchains?

IIUC, we still need to support clang 5.0 for some older Linux distros, but I don't know which distros are blocking us today. There is some 2019 discussion in bug 1579189 comment 6 about clang support on Debian 8 and 9, though Sylvestre replied that clang versions up to 8 were available on Debian 8 at that time.

Comment 6

[Corey Bryant]

Chris, Does Firefox have a time frame for switching to C++20? I ask because we have a library in Thunderbird that's gone EOL and the new version requires C++20.

Comment 7

[Chris Peterson [:cpeterson]]

(In reply to Corey Bryant from comment #6)

Does Firefox have a time frame for switching to C++20? I ask because we have a library in Thunderbird that's gone EOL and the new version requires C++20.

We don't have a definite ETA, but the priority has increased because Gecko depends on Google's Skia graphics and the next version will require C++20. The main blocker is updating our minimum clang, gcc, and libstdc++ versions in bug 1880779.

Just curious for future reference, what Thunderbird library will require C++20?

Comment 8

[Corey Bryant]

(In reply to Chris Peterson [:cpeterson] from comment #7)

(In reply to Corey Bryant from comment #6)

Does Firefox have a time frame for switching to C++20? I ask because we have a library in Thunderbird that's gone EOL and the new version requires C++20.

We don't have a definite ETA, but the priority has increased because Gecko depends on Google's Skia graphics and the next version will require C++20. The main blocker is updating our minimum clang, gcc, and libstdc++ versions in bug 1880779.

Just curious for future reference, what Thunderbird library will require C++20?

Chatted with Chris offline about this.

Comment 9

[Corey Bryant]

(In reply to Corey Bryant from comment #6)

Chris, Does Firefox have a time frame for switching to C++20? I ask because we have a library in Thunderbird that's gone EOL and the new version requires C++20.

Chatted with glandium and he said updating the minimum versions would come somewhere in the 140 or 141 cycle, probably.

Comment 10

[Chris Peterson [:cpeterson]]

Attachment: WIP: Bug 1768116 - Compile as C++20.