doctest: Undefined reference of `operator<<(ostream&, const string&)` when compiling with clang 10 and libc++ 10 on Ubuntu 16.04.6 LTS

Description

I tried to compile a minimal Doctest example of two files on Ubuntu 16.04.6 LTS with clang 10 and libc++ 10, which resulted in link errors.

I assume it’s because <iosfwd> is included without <iostream>, and some_ostream << some_string; compiles, while yielding wrong ABI,

Steps to reproduce

Create two files:

// a.cpp
#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
#include "doctest.h"

and

// b.cpp
#include <string>
#include "doctest.h"

void foo() {
    std::string a, b;
    CHECK(a == b);
}

and run clang++-10 -O2 -stdlib=libc++ -std=c++17 a.cpp b.cpp. I expected the example to compile, but it resulted in

/tmp/b-c494e1.o: In function `doctest::String doctest::detail::stringifyBinaryExpr<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)':
b.cpp:(.text._ZN7doctest6detail19stringifyBinaryExprINSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEES8_EENS_6StringERKT_PKcRKT0_[_ZN7doctest6detail19stringifyBinaryExprINSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEES8_EENS_6StringERKT_PKcRKT0_]+0x23): undefined reference to `std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<< <char, std::__1::char_traits<char>, std::__1::allocator<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)'
b.cpp:(.text._ZN7doctest6detail19stringifyBinaryExprINSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEES8_EENS_6StringERKT_PKcRKT0_[_ZN7doctest6detail19stringifyBinaryExprINSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEES8_EENS_6StringERKT_PKcRKT0_]+0x5e): undefined reference to `std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<< <char, std::__1::char_traits<char>, std::__1::allocator<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Extra information

If I add #include <iostream> on top of b.cpp, it compiles successfully.

Strictly speaking, <iosfwd> does not define any operations on ostream, but <string> does. So I’m not sure if this is an issue with doctest not including <iostream> or libc++ including <string> without all necessary dependencies.

• doctest version: v2.3.7
• Operating System: Ubuntu 16.04.6 LTS
• Compiler+version: clang version 10.0.0-++20200412072846+50d7e5d5e7d-1~exp1~20200412053445.132

I’ve used clang and libc++ from apt.llvm.org (sudo apt install clang-10 libc++-10-dev libc++abi-10-dev libc++1-10 libc++abi1-10), and doctest 2.3.7:

clang version 10.0.0-++20200412072846+50d7e5d5e7d-1~exp1~20200412053445.132 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin