1927545 - `...fmt/format.h(3466,8): error: variable of non-literal type 'DTSC' (aka 'double_conversion::DoubleToStringConverter') cannot be defined in a constexpr function before C++23` with C++20 enabled

Status: RESOLVED FIXED

(Firefox Build System :: General, defect)

Description

[Nick Alexander :nalexander [he/him]]

Folks, some code that used to compile is now failing with some constexpr stuff:

In file included from C:/Users/nalexander/Mozilla/gecko.git/toolkit/mozapps/pushproxy/pushproxy.cpp:5:
In file included from C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\mozilla/CmdLineAndEnvUtils.h:20:
In file included from C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\mozilla/Vector.h:25:
In file included from C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\mozilla/Span.h:36:
In file included from C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\mozilla/Casting.h:15:
C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\fmt/format.h(3466,8): error: variable of non-literal type 'DTSC' (aka 'double_conversion::DoubleToStringConverter') cannot be defined in a constexpr function before C++23
 3466 |   DTSC converter(DTSC::NO_TRAILING_ZERO |
      |        ^
C:/Users/nalexander/Mozilla/objdirs/objdir-browser-compile/dist/include\double-conversion/double-to-string.h(36,7): note: 'DoubleToStringConverter' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors
   36 | class DoubleToStringConverter {
      |       ^
1 error generated.

This is in a standalone binary with some unusual flags, including

        "-Xclang",
        "-fcxx-exceptions",
        "-Xclang",
        "-std=c++20",

It would be nice to accommodate C++20 for when we do, in fact, switch over.

Comment 1

[Nick Alexander :nalexander [he/him]]

Attachment: Bug 1927545 - Allow fmt with C++20. r?nika

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/07c23bd64100
Allow fmt with C++20. r=nika

Comment 3

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/07c23bd64100

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

The patch landed in nightly and beta is affected.
:nalexander, is this bug important enough to require an uplift?

• If yes, please nominate the patch for beta approval.
• If no, please set status-firefox133 to wontfix.

For more information, please visit BugBot documentation.

Comment 5

[Nick Alexander :nalexander [he/him]]

This doesn't need an unmotivated uplift. If we somehow end up with C++20 in Beta, we can uplift then.