<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Explicit instantiation function template with auto deduced return type fails if soft instantiation occurred"
href="https://bugs.llvm.org/show_bug.cgi?id=49751">49751</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Explicit instantiation function template with auto deduced return type fails if soft instantiation occurred
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>11.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++14
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>phalpern@halpernwightsoftware.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Source code (4 lines, no preprocessor directives):
template <typename T> auto f(T v) { return v; }
extern template auto f(int);
int i = f(0);
template auto f(int);
Compiler command line: clang++ -std=c++14 -Wall -c autoreturnbug.cpp
Error produced:
autoreturnbug.cpp:4:15: error: explicit instantiation of 'f' does not refer to
a
function template, variable template, member function, member class, or
static data member
template auto f(int);
^
autoreturnbug.cpp:1:28: note: candidate template ignored: failed template
argument deduction
template <typename T> auto f(T v) { return v; }
^
1 error generated.
However, if line 3 (int i = f(0)) is removed, the code compiles successfully.
In the C++14 standard, section [dcl.spec.auto], paragraph 15, it says:
An explicit instantiation declaration (14.7.2) 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. [ Example:
template <typename T> auto f(T t) { return t; }
extern template auto f(int); // does not instantiate f<int>
int (*p)(int) = f; // instantiates f<int> to determine its return type, but an
explicit
// instantiation definition is still required somewhere in the program
— end example ]
I take that to mean that the last line is valid, even with the "soft"
instantiation of `f`.
This bug might be related to <a class="bz_bug_link
bz_status_NEW "
title="NEW - explicit instantiation confused for a function with a deduced return type if it's already been instantiated"
href="show_bug.cgi?id=19551">Bug #19551</a></pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>