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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang cannot deduce return type for the implicit instantiation of template specialization"
href="https://bugs.llvm.org/show_bug.cgi?id=52025">52025</a>
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<th>Summary</th>
<td>clang cannot deduce return type for the implicit instantiation of template specialization
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>12.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++14
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>harashimahashi@gmail.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>The following code fails to compile, however must to.
template <typename T>
struct S
{
template <typename U>
decltype(auto) operator[](U u);
};
template <>
template <typename U>
decltype(auto) S<int>::operator[](U u)
{
return u;
}
int main()
{
S<int> s;
std::cout << s[1] <<std::endl;
return 0;
}
<a href="https://godbolt.org/z/MqqY5cTjx">https://godbolt.org/z/MqqY5cTjx</a>
Impicit instantiation of S<int> definition implies implicit instantiation of
the operator[] template declaration at the line
13(<a href="https://eel.is/c++draft/temp#inst-3.1">https://eel.is/c++draft/temp#inst-3.1</a>). After that a[1] shall be resolved in
favor of declaration instantiated from template at the line
13(<a href="https://eel.is/c++draft/temp#inst-10">https://eel.is/c++draft/temp#inst-10</a>), which implies implicit instantiation
of the operator[] definition(<a href="https://eel.is/c++draft/temp#inst-4">https://eel.is/c++draft/temp#inst-4</a>), which allows
compiler to deduce return
type(<a href="https://eel.is/c++draft/dcl.spec.auto#general-12">https://eel.is/c++draft/dcl.spec.auto#general-12</a>)</pre>
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