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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>
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<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED WONTFIX - Clang++ accepts invalid template code"
href="https://bugs.llvm.org/show_bug.cgi?id=44034">bug 44034</a>
<br>
<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>What</th>
<th>Removed</th>
<th>Added</th>
</tr>
<tr>
<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
</tr>
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<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>WONTFIX
</td>
</tr></table>
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED WONTFIX - Clang++ accepts invalid template code"
href="https://bugs.llvm.org/show_bug.cgi?id=44034#c2">Comment # 2</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED WONTFIX - Clang++ accepts invalid template code"
href="https://bugs.llvm.org/show_bug.cgi?id=44034">bug 44034</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>Well, the intent is that you are allowed to pass Args as explicit template
arguments:
algorithm_impl::do_n<Function, Args...>(
std::forward<Function>(func),
std::forward<Args>(args)...,
std::make_integer_sequence<size_t, count>{});
... and in the case where Args is empty, you can instead write that as:
algorithm_impl::do_n<Function>(
std::forward<Function>(func),
std::forward<Args>(args)...,
std::make_integer_sequence<size_t, count>{});
(This specifies Args as an empty pack.)
You can also omit the "Function" in this case, because it's deducible:
algorithm_impl::do_n<>(
std::forward<Function>(func),
std::forward<Args>(args)...,
std::make_integer_sequence<size_t, count>{});
(This still -- presumably! -- specifies Args as an empty pack.)
And finally, [temp.arg.explicit]p4 says you can omit the empty <> in this case
and it means the same thing as including an <>:
algorithm_impl::do_n(std::forward<Function>(func),
std::forward<Args>(args)...,
std::make_integer_sequence<size_t, count>{});
... so by that argument, the code is valid, and MSVC is wrong to reject it, and
Clang and GCC are correct to accept.
This whole area of the C++ standard is underspecified and imprecise, but the
behavior of Clang and GCC here is consistent, reasonable, and in line with
discussions on the C++ committee reflectors about how deduction should work.
MSVC rejects all of the above cases, so I think this is just an MSVC bug. (ICC
has even weirder behavior: it accepts the first two examples but rejects the
last two.)
We're unlikely to make any changes here unless the C++ standard text is
clarified to indicate Clang's behavior is incorrect, so I'm resolving WONTFIX
for now.</pre>
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