It's time to default-allow C++20

[Peter Kasting]

To whomever is driving C++ style these days:

C++20 was initially supported in Nov 2023, which was more than two years ago. Per the Modern C++ Features policy, that means it's time to make final decisions on any remaining TBD items, biasing towards allowing, and then default-allow that language version (removing all the text from that page explicitly allowing parts of C++20).

I won't be driving that discussion, but I wanted to mention this here in hopes someone else would.

And while I'm at it, kudos to everyone who worked to get C++23 supported. I'm surprised and pleased it happened as soon as it did. Good work!

PK

[Kyle Charbonneau]

Thanks for bringing this up Peter. There are three features marked TBD that seem straightforward (IMO) to move to the banned list pending future discussion.

• <coroutine>
• It isn't clear how to integrate coroutines with the existing post-task system. There was discussion about it in this thread.
• Clang doesn't currently support coroutines for win32 which is problematic.
• <format> 
• This would require a large migration effort for existing base::StringPrintf() usage due to different format specifiers. 
• base::StringPrintf was changed to use absl::StrFormat internally already (thanks Peter!). A decision on absl::StrFormat vs <format> would have to be made.
• <source_location> 
• std::source_location would be used in place of base::Location. Leaving <source_location> banned until there is a plan for how that migration would work seems reasonable.

That would leave the following features as TBD. I don't have any initial thoughts on these but others might.

• Aggregate Initialization Using Paranthesis
• std::u8string

Kyle

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[Nico Weber]

On Wed, Jan 28, 2026 at 4:41 PM Kyle Charbonneau <kyle...@chromium.org> wrote:

Thanks for bringing this up Peter. There are three features marked TBD that seem straightforward (IMO) to move to the banned list pending future discussion.

This sounds right to me.

• <coroutine>
• It isn't clear how to integrate coroutines with the existing post-task system. There was discussion about it in this thread.
• Clang doesn't currently support coroutines for win32 which is problematic.
• <format> 
• This would require a large migration effort for existing base::StringPrintf() usage due to different format specifiers. 
• base::StringPrintf was changed to use absl::StrFormat internally already (thanks Peter!). A decision on absl::StrFormat vs <format> would have to be made.
• <source_location> 
• std::source_location would be used in place of base::Location. Leaving <source_location> banned until there is a plan for how that migration would work seems reasonable.

That would leave the following features as TBD. I don't have any initial thoughts on these but others might.

• Aggregate Initialization Using Paranthesis

I think I'd prefer to ban this, as it seems more bug-prone than brace-init and the difference is very subtle. I don't think we have great tools to enforce the ban though.

• std::u8string

Given we ban char8_t, we should probably ban this too.

 

Kyle

On Sat, Jan 24, 2026 at 2:28 PM Peter Kasting <pkas...@chromium.org> wrote:

To whomever is driving C++ style these days:

C++20 was initially supported in Nov 2023, which was more than two years ago. Per the Modern C++ Features policy, that means it's time to make final decisions on any remaining TBD items, biasing towards allowing, and then default-allow that language version (removing all the text from that page explicitly allowing parts of C++20).

I won't be driving that discussion, but I wanted to mention this here in hopes someone else would.

And while I'm at it, kudos to everyone who worked to get C++23 supported. I'm surprised and pleased it happened as soon as it did. Good work!

PK

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