[libcxx-commits] [libcxx] [libc++] Port #200996 to ostream (PR #226372)

[Nico Weber via libcxx-commits]

https://github.com/nico created https://github.com/llvm/llvm-project/pull/226372

Reduces the time to compile a TU with just `#include <ostream>` from 254.7 ms ± 3.0 ms to  227.7 ms ± 2.1 ms on my system (-std=c++23, -O2, arm64 macOS).

See #195466 for details. Without this, the call is evaluated when the template is parsed, which evaluates format_string's consteval constructor for "\n" in every translation unit that includes <ostream>, even if nothing calls println().

No behavior change.

>From 57b2c0c5520b00f431028e809ee80898f65a1deb Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis at chromium.org>
Date: Sun, 20 Sep 2026 13:23:25 -0400
Subject: [PATCH] [libc++] Port #200996 to ostream
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Reduces the time to compile a TU with just `#include <ostream>`
from 254.7 ms ± 3.0 ms to  227.7 ms ± 2.1 ms on my system
(-std=c++23, -O2, arm64 macOS).

See #195466 for details. Without this, the call is evaluated
when the template is parsed, which evaluates format_string's consteval
constructor for "\n" in every translation unit that includes <ostream>,
even if nothing calls println().

No behavior change.
---
 libcxx/include/__ostream/print.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/__ostream/print.h b/libcxx/include/__ostream/print.h
index 4a519c177b990d..f9810a4d7d26a0 100644
--- a/libcxx/include/__ostream/print.h
+++ b/libcxx/include/__ostream/print.h
@@ -159,9 +159,9 @@ _LIBCPP_HIDE_FROM_ABI void println(ostream& __os, format_string<_Args...> __fmt,
 #    endif // _LIBCPP_HAS_UNICODE
 }
 
-template <class = void> // TODO PRINT template or availability markup fires too eagerly (http://llvm.org/PR61563).
+template <class _Void = void> // TODO PRINT template or availability markup fires too eagerly (http://llvm.org/PR61563).
 _LIBCPP_HIDE_FROM_ABI inline void println(ostream& __os) {
-  std::print(__os, "\n");
+  std::print(__os, (_Void(), "\n"));
 }
 
 #  endif // _LIBCPP_STD_VER >= 23