[cfe-dev] libc++: Unexpected pointer-to-member-function behavior

[Adam C. Emerson via cfe-dev]

Hello. I'm not sure if this is a bug or if I'm using undefined
behavior or otherwise doing something screwy.

My Minimal Working Reproducer is:

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#include <functional>
#include <iostream>
#include <string>

int main(void) {
    auto s = ::std::string("Meow.");
    auto l = ::std::mem_fn(&::std::string::size);

    ::std::cout << l(s) << ::std::endl;

    return 0;
}

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If I compile this under g++ or with clang++ using libstdc++, this
compiles and works as expected. However, if I use clang++ (v4.0.0 on
Arch Linux x86_64) with libc++ (4.0.0), I get a link error:

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% clang++ -v -std=c++14 -stdlib=libc++ foo.cc
clang version 4.0.0 (tags/RELEASE_400/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-pc-linux-gnu/6.3.1
Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.1
Found candidate GCC installation: /usr/lib64/gcc/x86_64-pc-linux-gnu/6.3.1
Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
 "/usr/bin/clang-4.0" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name foo.cc -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -v -dwarf-column-info -debugger-tuning=gdb -resource-dir /usr/bin/../lib/clang/4.0.0 -internal-isystem /usr/bin/../include/c++/v1 -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/clang/4.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c++14 -fdeprecated-macro -fdebug-compilation-dir /home/azure -ferror-limit 19 -fmessage-length 119 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/foo-1fe78b.o -x c++ foo.cc
clang -cc1 version 4.0.0 based upon LLVM 4.0.0 default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/bin/../include/c++/v1
 /usr/local/include
 /usr/bin/../lib/clang/4.0.0/include
 /usr/include
End of search list.
 "/usr/bin/ld" --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64/crt1.o /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64/crti.o /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/crtbegin.o -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1 -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64 -L/usr/bin/../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../.. -L/usr/bin/../lib -L/lib -L/usr/lib /tmp/foo-1fe78b.o -lc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/crtend.o /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64/crtn.o
/tmp/foo-1fe78b.o: In function `main':
foo.cc:(.text+0x44d): undefined reference to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::size() const'
/usr/bin/ld: a.out: hidden symbol `_ZNKSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE4sizeEv' isn't defined
/usr/bin/ld: final link failed: Bad value
clang-4.0: error: linker command failed with exit code 1 (use -v to see invocation)

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Thank you very much.