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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang rejects private member access used in default template argument of hidden friend"
href="https://bugs.llvm.org/show_bug.cgi?id=45464">45464</a>
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<th>Summary</th>
<td>Clang rejects private member access used in default template argument of hidden friend
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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>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>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>lewissbaker@gmail.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>The following code is rejected by clang as it complains that referencing a
private data-member in a default template argument decltype() expression is not
allowed.
---
#include <type_traits>
#include <concepts>
template<typename T, typename B>
concept same_base_as = std::same_as<std::remove_cvref_t<T>,
std::remove_cvref_t<B>>;
template<typename T>
struct Y {
Y(T&&) {}
};
class X {
int value;
template<
same_base_as<X> S,
typename V = decltype((std::declval<S>().value))>
friend Y<V> foo(S&& x) {
return Y<V>{((S&&)x).value};
}
};
void test() {
X x;
auto a = foo(x);
}
---
With the following compile-error:
<source>:42:14: error: no matching function for call to 'foo'
auto a = foo(x);
^~~
<source>:34:17: note: candidate template ignored: substitution failure [with S
= X &, V = int &]: 'value' is a private member of 'X'
friend Y<V> foo(S&& x) {
Note that if I change the decltype() expression to use `.*&X::value` instead of
`.value` then clang accepts it.
MSVC and GCC both accept the above code, so I suspect it's a bug in Clang.
See <a href="https://godbolt.org/z/HcqBYn">https://godbolt.org/z/HcqBYn</a> for an example.</pre>
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