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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - RVO fails in template function with 'auto' return type"
href="https://bugs.llvm.org/show_bug.cgi?id=39326">39326</a>
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<th>Summary</th>
<td>RVO fails in template function with 'auto' return type
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>7.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++14
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>wenzel.jakob@epfl.ch
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<pre>I've run into the following rather bizarre bug on Clang that appears to be a
standard violation: when a function template has an 'auto' return value, return
value optimization (RVO) no longer works. Everything is okay for non-template
functions and functions with an explicitly specified return value type.
Consider the following piece of code that reproduces this issue:
// ========================================================
#include <stdio.h>
struct A {
A() { printf("A::A()\n"); }
A(const A&) { printf("A::A(const A&)\n"); }
A(A&&) { printf("A::A(A&&)\n"); }
~A() { printf("A::~A()\n"); }
A& operator=(const A&) { printf("A::operator=(const A&)\n"); return *this;
}
A& operator=(A&&) { printf("A::operator=(A&&)\n"); return *this;}
};
A test1(int z) { A x; return x; }
auto test2(int z) { A x; return x; }
template <typename T> A test3(T z) { A x; return x; }
template <typename T> auto test4(T z) { A x; return x; } /// <- problem!
int main(int argc, char *argv[]) {
printf("\n-- Test 1:\n");
test1(1);
printf("\n-- Test 2:\n");
test2(1);
printf("\n-- Test 3:\n");
test3(1);
printf("\n-- Test 4:\n");
test4(1);
return 0;
}
// ========================================================
This produces the following output:
$ clang++ test.cpp -o test -std=c++14
$ ./test
-- Test 1:
A::A()
A::~A()
-- Test 2:
A::A()
A::~A()
-- Test 3:
A::A()
A::~A()
-- Test 4:
A::A()
A::A(A&&)
A::~A()
A::~A()
Comparison (GCC 8):
-- Test 1:
A::A()
A::~A()
-- Test 2:
A::A()
A::~A()
-- Test 3:
A::A()
A::~A()
-- Test 4:
A::A()
A::~A()</pre>
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