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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 - -Wunused-lambda-capture too strict with double variadic usage"
href="https://bugs.llvm.org/show_bug.cgi?id=41212">41212</a>
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<th>Summary</th>
<td>-Wunused-lambda-capture too strict with double variadic usage
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>jvapen@gmail.com
</td>
</tr>
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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>This might be a variation of <a class="bz_bug_link
bz_status_NEW "
title="NEW - -Wunused-lambda-capture too strict with variadic usage"
href="show_bug.cgi?id=41211">BUG41211</a>
Reproduction on compiler explorer: <a href="https://godbolt.org/z/_Uanrt">https://godbolt.org/z/_Uanrt</a>
In the example below, I get the following warning: lambda capture 't' is not
used [-Wunused-lambda-capture]
When removing the suggested 't', I get: error: variable 't' cannot be
implicitly captured in a lambda with no capture-default specified
t.cpp
-----
#include <type_traits>
#include<algorithm>
#include <optional>
#include <tuple>
#include <mutex>
template <typename T, typename TMutex>
struct W
{
using mutex_type = TMutex;
bool acquire();
};
template<typename ...T>
int convert(T && ... t);
struct S
{
template <typename TValue, typename TMutex>
static TValue &v(W<TValue, TMutex> &);
template <typename TValue, typename TMutex>
static TMutex &m(const W<TValue, TMutex> &);
};
template <typename... T>
auto acquire(T &... t)
{
auto locks = std::make_tuple(std::unique_lock<std::mutex>{ S::m(t),
std::defer_lock }...);
auto dispatch = [&t...](auto &&... a)
{
static_assert(sizeof...(t) == sizeof...(a));
return std::make_tuple(convert(S::v(t), std::move(a))...);
};
return std::apply(std::move(dispatch), std::move(locks));
}
void f()
{
W<int, std::mutex> a;
acquire(a);
}
run.sh
------
clang++ -std=c++17 -O3 -Wunused-lambda-capture -Werror t.cpp</pre>
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