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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Cast from past-the-end pointer from Derived to Base invalid inside constant expression"
href="https://bugs.llvm.org/show_bug.cgi?id=43856">43856</a>
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<th>Summary</th>
<td>Cast from past-the-end pointer from Derived to Base invalid inside 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>All
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<th>Status</th>
<td>NEW
</td>
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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>Frontend
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>ldionne@apple.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>Hi,
The following code produces an error with Clang trunk:
$ cat <<EOF | clang++ -xc++ -std=c++17 -
struct Base { };
struct Derived : Base { };
constexpr bool f() {
Derived array[5] = {};
Derived* ptr = array + 5;
Base* base = ptr; // <<<<< error HERE
return true;
}
static_assert(f());
EOF
The error is:
<stdin>:4:16: error: constexpr function never produces a constant
expression [-Winvalid-constexpr]
constexpr bool f() {
^
<stdin>:7:18: note: cannot access base class of pointer past the end of
object
Base* base = ptr;
^
<stdin>:11:15: error: static_assert expression is not an integral constant
expression
static_assert(f());
^~~
<stdin>:7:18: note: cannot access base class of pointer past the end of
object
Base* base = ptr;
^
<stdin>:11:15: note: in call to 'f()'
static_assert(f());
^
I believe this should still be a constant expression because we are not
actually accessing `ptr` (which is indeed past-the-end), we're just casting it
from a Derived class to a Base class.</pre>
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