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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [concepts] substitution into constraint expression resulted in a non-constant expression"
href="https://bugs.llvm.org/show_bug.cgi?id=51503">51503</a>
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<th>Summary</th>
<td>[concepts] substitution into constraint expression resulted in a non-constant expression
</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>Linux
</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>llvm@marehr.dialup.fu-berlin.de
</td>
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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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<pre>Hi clang-team,
the following code does not compile with clang trunk:
```c++
template <typename urng_t>
struct contiguous_range
{
template <bool const_range>
requires (const_range == false)
using basic_iterator = int;
auto begin()
{
return basic_iterator<false>{};
}
};
```
<a href="https://godbolt.org/z/jobYveoe3">https://godbolt.org/z/jobYveoe3</a>
with the error
```
<source>:5:19: error: substitution into constraint expression resulted in a
non-constant expression
requires (const_range == false)
^~~~~~~~~~~~~~~~~~~~
<source>:10:16: note: while checking constraint satisfaction for template
'basic_iterator<false>' required here
return basic_iterator<false>{};
^~~~~~~~~~~~~~~~~~~~~
<source>:5:19: note: subexpression not valid in a constant expression
requires (const_range == false)
^
```
MSVC 16.11, and gcc 11.2 are able to compile this code. A curious side note: If
you remove `template <typename urng_t>` clang does compile this code snippet.</pre>
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