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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - missing auto in `concept auto arg` error diagnostic needs improvement"
href="https://bugs.llvm.org/show_bug.cgi?id=49848">49848</a>
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<th>Summary</th>
<td>missing auto in `concept auto arg` error diagnostic needs improvement
</td>
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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
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++2a
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>ldalessandro@gmail.com
</td>
</tr>
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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>If you forget the `auto` in a `concept auto arg` declaration specifically for a
member function template clang provides a confusing diagnostic
<a href="https://godbolt.org/z/M5859vqjd">https://godbolt.org/z/M5859vqjd</a>.
```
template <class T>
concept thing_t = true;
struct Foo {
void foo(thing_t const&);
};
```
clang produces
```
<source>:5:14: error: expected parameter declarator
void foo(thing_t const&);
^
<source>:5:22: error: expected ')'
void foo(thing_t const&);
^
<source>:5:13: note: to match this '('
void foo(thing_t const&);
^
2 errors generated.
Compiler returned: 1
```
where gcc produces
```
<source>:5:14: error: expected 'auto' or 'decltype(auto)' after 'thing_t'
5 | void foo(thing_t const&);
| ^~~~~~~
Compiler returned: 1
```</pre>
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