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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang rejects creation of struct with mutable member during constant evaluation"
href="https://bugs.llvm.org/show_bug.cgi?id=48606">48606</a>
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<th>Summary</th>
<td>Clang rejects creation of struct with mutable member during constant evaluation
</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>Windows NT
</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
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mschellenbergercosta@googlemail.com
</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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<p>
<div>
<pre>Consider the following code with C++20:
```
#include <memory>
struct WithMutable {
mutable int val_{0};
};
constexpr bool createOnStack() {
[[maybe_unused]] volatile WithMutable var;
return true;
}
constexpr bool createOnHeap() {
WithMutable* var = new WithMutable{};
delete var;
return true;
}
constexpr bool createWithAllocator() {
WithMutable* var = std::allocator<WithMutable>{}.allocate(1);
std::construct_at(var);
std::destroy_at(var);
std::allocator<WithMutable>{}.deallocate(var, 1);
return true;
}
int main() {
static_assert(createOnStack());
static_assert(createOnHeap());
static_assert(createWithAllocator());
}
```
Clang trunk rejects the last function `createWithAllocator` with the error
message:
"construction of mutable member 'val_' is not allowed in a constant
expression"
However, I cannot find any restriction on construction of mutable members
during a constant expression. Given that the other two functions are accepted
it is unclear why clang rejects the call to std::construct_at
(Godbolt link: <a href="https://godbolt.org/z/71zb7j">https://godbolt.org/z/71zb7j</a>)</pre>
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