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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [P0692R1] Access checking not waived for declarations of explicit specializations of function and variable templates"
href="https://bugs.llvm.org/show_bug.cgi?id=51769">51769</a>
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<th>Summary</th>
<td>[P0692R1] Access checking not waived for declarations of explicit specializations of function and variable templates
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Keywords</th>
<td>invalid-bug
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++2a
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>davveston@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>[temp.spec]/6[1] was added to the C++20 spec by implementation of P0692R1[2]:
"The usual access checking rules do not apply to names in a declaration of an
explicit instantiation or explicit specialization, with the exception of names
appearing in a function body, default argument, base-clause,
member-specification, enumerator-list, or static data member or variable
template initializer."
Meaning that programs (A) through (C) below are (arguably) well-formed:
(A) (DEMO: <a href="https://wandbox.org/permlink/P2GpDGy6mxMydtii">https://wandbox.org/permlink/P2GpDGy6mxMydtii</a>)
class A { class B {}; };
template<typename T> struct S {};
template<> struct S<A::B> {};
int main() {}
(B) (DEMO: <a href="https://wandbox.org/permlink/CZFZlMwC3VMzSBky">https://wandbox.org/permlink/CZFZlMwC3VMzSBky</a>):
class A { class B {}; };
template<typename T> void foo() {};
template<> void foo<A::B>() {}
int main() {}
(C) (DEMO: <a href="https://wandbox.org/permlink/LX3LWSesx2tBCbMh">https://wandbox.org/permlink/LX3LWSesx2tBCbMh</a>)
class A { class B {}; };
template<typename T>
constexpr bool v = false;
template<>
constexpr bool v<A::B> = true;
int main() {}
Moreover, program (D) below:
(D) (DEMO: <a href="https://wandbox.org/permlink/hjEw0n4UN0wQ55Fv">https://wandbox.org/permlink/hjEw0n4UN0wQ55Fv</a>)
class A { class B {}; };
template<typename T, typename U> struct S {};
template<typename U> struct S<A::B, U> {};
template<typename T> struct S<T, A::B> {};
int main() {}
is also (arguably) well-formed as per [temp.class.spec]/10[3], which covers
waiving of access checking for template arguments in the simple-template-id of
partial specializations, a paragraph that was also added as part of P0692R1.
However whilst Clang accepts (A), it rejects (B) through (D).
As per [4], the implementation status of P0692R1 is "partial" (so this may
actually not be a bug report but an enhancement request).
---
[1] <a href="https://timsong-cpp.github.io/cppwp/n4861/temp.spec#6">https://timsong-cpp.github.io/cppwp/n4861/temp.spec#6</a>
[2] P0692R1 - Access Checking on Specializations
<a href="http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0692r1.html">http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0692r1.html</a>)
[3] <a href="https://timsong-cpp.github.io/cppwp/n4861/temp.class.spec#10">https://timsong-cpp.github.io/cppwp/n4861/temp.class.spec#10</a>
[4] <a href="https://clang.llvm.org/cxx_status.html">https://clang.llvm.org/cxx_status.html</a></pre>
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