<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/55743>55743</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [LTO][Templates]
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c++,
            c++14,
            clang:frontend
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          wolfy1961
      </td>
    </tr>
</table>

<pre>
    In the attached C++ example, when using LTO, clang instantiates a template function despite the fact that the C++14 standard says not to do so. 

> clang -O2 inst.cpp -o inst
/usr/bin/ld: /tmp/inst-05d30e.o:(.data+0x0): undefined reference to `auto foo<int>(int)'

> clang -flto -O2 inst.cpp -o inst
> echo $?
0

The relevant paragraphs in a recent C++ draft are as follows. They have been in the standard since C++14.

_An explicit instantiation declaration (13.9.2) does not cause the instantiation of an entity declared using a
placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its
type._ (section 9.2.8.5 in [this draft](https://isocpp.org/files/papers/N4860.pdf)).

and

_An entity that is the subject of
an explicit instantiation declaration and that is also used in a way that would otherwise cause an implicit
instantiation (13.9.1) in the translation unit shall be the subject of an explicit instantiation definition
somewhere in the program; otherwise the program is ill-formed, no diagnostic required._ (section 13.9.2 in the draft mentioned above).

[test.zip](https://github.com/llvm/llvm-project/files/8790228/test.zip)
</pre>
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