[llvm-bugs] [Bug 51910] New: Undefined symbol when using iostream in module interface unit

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51910

            Bug ID: 51910
           Summary: Undefined symbol when using iostream in module
                    interface unit
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: is7e9jnxv at protonmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Created attachment 25273
  --> https://bugs.llvm.org/attachment.cgi?id=25273&action=edit
test case

On x86-64 Ubuntu 21.04, when I use cout and endl together in a function defined
in a module interface unit, the linker (both ld and lld) reports that that
function is not defined, but only when there is also a corresponding module
implementation unit. Reference the following compile commands with the attached
file containing the text of the three source files clangbug.cpp, clangbug.cppm,
and main.cpp. Removing endl from the line causing the linker failure (line 2 of
somethingElse()) also resolves the issue. Omitting -flto does not affect the
outcome.

clang++ -fmodules -std=c++20 -stdlib=libc++ -Wall -Wextra -Wpedantic -O3 -flto
--precompile -o clangbug.pcm clangbug.cppm

clang++ -fmodules -std=c++20 -stdlib=libc++ -Wall -Wextra -Wpedantic -O3 -flto
clangbug.pcm clangbug.cpp -fmodule-file=clangbug.pcm -c

clang++ --ld-path=ld.lld -fmodules -std=c++20 -stdlib=libc++ -Wall -Wextra
-Wpedantic -O3 -flto -o clangbugTest clangbug.o main.cpp
-fmodule-file=clangbug.pcm

ld.lld: error: undefined symbol: _ZW8clangbugE13somethingElsev
>>> referenced by ld-temp.o
>>>               lto.tmp:(main)
clang-14: error: linker command failed with exit code 1 (use -v to see
invocation)

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