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<body><span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Non-const ref treated as const-ref in the compiler."
href="https://bugs.llvm.org/show_bug.cgi?id=43842">bug 43842</a>
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<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>What</th>
<th>Removed</th>
<th>Added</th>
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<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
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<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
</td>
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Non-const ref treated as const-ref in the compiler."
href="https://bugs.llvm.org/show_bug.cgi?id=43842#c1">Comment # 1</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Non-const ref treated as const-ref in the compiler."
href="https://bugs.llvm.org/show_bug.cgi?id=43842">bug 43842</a>
from <span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span></b>
<pre>In order to determine which 'fn' to call, the compiler needs to check
'is_invocable<F&&, T const&>'. As part of checking whether the lambda is
invokable, the compiler needs to check whether it has a valid return type.
Because the lambda has a deduced return type, the operator() must be
instantiated (with 'auto' = 'const int') to deduce that return type. That
instantiation fails as described in the given diagnostic.
So this is a bug in your code, not a bug in Clang (nor in GCC / ICC / MSVC,
which all reject this the same way).
You can fix your code by giving the lambda an explicit return type rather than
a deduced return type:
instance.fn([](auto& i) -> void {
i = 0;
});</pre>
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