[Bug 229479] clang unused code triggers link failure

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229479

            Bug ID: 229479
           Summary: clang unused code triggers link failure
           Product: Base System
           Version: 11.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: adridg at freebsd.org

With 11.2-STABLE, using either base clang or clang50 from ports, this C++
program compiles, but fails to link:

```
  #include <string>
  static std::string foop() { return std::string(); }
  int main() {
    (void)(foop);
    return 0;
  }
```

The error message is

```
[adridg at beastie]$ c++ t.cpp 
/tmp/t-0b5183.o: In function `foop()':
t.cpp:(.text+0x20): undefined reference to `std::__1::basic_string<char,
std::__1::char_traits<char>, std::__1::allocator<char> >::basic_string()'
/usr/bin/ld: a.out: hidden symbol
`_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEC2Ev' isn't
defined
/usr/bin/ld: final link failed: Nonrepresentable section on output
```

The same problem occurs with clang50 on 11-STABLE, and with (base) clang60 on
12-CURRENT from mid-june. The program compiles and links without issue with
g++6 on 11-STABLE.

Removing the line `(void)(foop);` allows the program to link; it does generate
one warning about an unused function, though.


**On Linux**: The program compiles and links with clang5 and clang6 on Linux
using the default C++ library (e.g. `clang++ t.cpp`). I am told that it does
**not** link when using libc++ (e.g. `clang++ -stdlib=libc++ t.cpp`). I have
not been able to verify that, though

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