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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Concepts with parameter pack not at end silently ignores errors when used"
href="https://bugs.llvm.org/show_bug.cgi?id=48838">48838</a>
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<th>Summary</th>
<td>Concepts with parameter pack not at end silently ignores errors when used
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>trunk
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++2a
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mital.vaja@googlemail.com
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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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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=24407" name="attach_24407" title="Patch to reject concepts with parameter packs not at the end">attachment 24407</a> <a href="attachment.cgi?id=24407&action=edit" title="Patch to reject concepts with parameter packs not at the end">[details]</a></span>
Patch to reject concepts with parameter packs not at the end
There are no errors or warnings compiling the following with `clang++
-std=c++20 -fsyntax-only`:
template<typename, typename..., typename = int>
concept invalid = true;
template<typename> requires invalid<int>
no errors are printed
;
static_assert(invalid<int> also here ;
int main() {
bool b;
b = invalid<int> not just in declarations;
return b;
}
This happens because this assumption is violated with concepts:
<a href="https://github.com/llvm/llvm-project/blob/39239f9b5666bebb059fa562badeffb9f1c3afab/clang/lib/Sema/SemaTemplate.cpp#L5756-L5760">https://github.com/llvm/llvm-project/blob/39239f9b5666bebb059fa562badeffb9f1c3afab/clang/lib/Sema/SemaTemplate.cpp#L5756-L5760</a>
and no diagnosis is printed beforehand
I have attached a patch that makes concepts like `invalid` which have a
parameter pack not at the end an error.
There is currently no wording in the standard that makes these concepts
ill-formed (The current C++20 [temp.param]p14 only mentions "primary class
template, primary variable template, or alias template"), but I think this is
an oversight.</pre>
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