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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Rejects valid subscript expression on array of unknown bound in constant expression"
href="https://bugs.llvm.org/show_bug.cgi?id=48402">48402</a>
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<th>Summary</th>
<td>Rejects valid subscript expression on array of unknown bound in constant expression
</td>
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<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++11
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>leni536@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>clang version 12.0.0 (<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
2518433f861fcb877d0a7bdd9aec1aec1f77505a)
command line options: -std=c++11 -pedantic-errors
Also reproducible with c++14, c++17 and c++20
```
extern const int arr[];
constexpr const int (&arrref)[] = arr;
constexpr int arr[2] = {1,2};
constexpr int x = arrref[0];
```
error: constexpr variable 'x' must be initialized by a constant expression
constexpr int x = arrref[0];
^ ~~~~~~~~~
note: read of element of array without known bound is not allowed in a constant
expression
constexpr int x = arrref[0];
Accessing elements through arrays of unknown bound is not explicitly forbidden
in constant expressions, so I think it is valid.
The followings are accepted:
```
extern const int arr[];
constexpr const int * ptr = arr;
constexpr int arr[2] = {1,2};
constexpr int x = ptr[0];
```
```
extern const int arr[];
constexpr const int (&arrref)[] = arr;
constexpr int arr[2] = {1,2};
constexpr const int* decay() {
return arrref;
}
constexpr int x = decay()[0];
```
P0388 is accepted in C++20 and will enable other ways of creating subscript
constant expressions to arrays of unknown bound. Related gcc bug:
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97645">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97645</a></pre>
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