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    <head>
      <base href="https://bugs.llvm.org/">
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    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <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>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Cast from past-the-end pointer from Derived to Base invalid inside constant expression
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>trunk
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>All
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>Frontend
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>ldionne@apple.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
          </td>
        </tr></table>
      <p>
        <div>
        <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>
        </div>
      </p>


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