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From: "alexander.lelyakin at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99227] [meta] [modules] Bugs relating to header-units of
 STL header files
Date: Sat, 06 Mar 2021 10:08:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99227

Alexander Lelyakin <alexander.lelyakin at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alexander.lelyakin@googlema
                   |                            |il.com

--- Comment #1 from Alexander Lelyakin <alexander.lelyakin at googlemail do=
t com> ---
Just want to express my current understanding of situation.

Here:

https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Modules.html#C_002b_002b-Mod=
ules

is stated:
"The Standard Library is not provided as importable header units. If you wa=
nt
to import such units, you must explicitly build them first. If you do not do
this with care, you may have multiple declarations, which the module machin=
ery
must merge=E2=80=94compiler resource usage can be affected by how you parti=
tion header
files into header units."


Any header file in "/usr[/local]/include/c++/11[.0.1]" can be precompiled a=
s a
header module like:
----
g++ -std=3Dc++20 -fmodules-ts -x c++-system-header iostream
----
And can be successfully used as:
----
import <iostream>;

int main()
{
  std::cout<<"Hello\n";
  return 0;
}
----
g++ -std=3Dc++20 -fmodules-ts test.c++
----

However if you will compile more than one header into the same 'gcm.cache'
directory, you will receive various compiler errors depending of order of
compilation of headers.

Currently there is no way to compile all headers. Longest sequence i have s=
een
so far was 35 headers, shortest - just 2.

This meta bug will accumulate bug reports discovered during attempts to
precompile system headers.=