<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/119084>119084</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [llvm] Input iterators should not const-overload `operator*` with varying return types
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          StephanTLavavej
      </td>
    </tr>
</table>

<pre>
    The C++ Standard requires that input (and stronger) iterators return the same type when dereferenced, regardless of the top-level constness of the iterator. This is [N4993](https://isocpp.org/files/papers/N4993.pdf) \[iterator.cpp17.general\]/1:

> In the following sections, `a` and `b` denote values of type `X` or `const X`, `difference_type` and `reference` refer to the types `iterator_traits<X>::difference_type` and `iterator_traits<X>::reference`, respectively

followed by \[tab:inputiterator\]:

> Expression: `*a`
> Return type: `reference`, convertible to `T`

In MSVC's STL, we recently merged https://github.com/microsoft/STL/pull/5135, which innocently added top-level constness to an iterator before dereferencing it, and that broke LLVM's build when this iterator was passed to `std::uninitialized_copy`:

https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/IR/DebugProgramInstruction.h#L373-L379

LLVM's iterator should not be overloading `const Value *operator*() const` and `Value *operator*()` with a varying return type like this. I searched this directory, and found a very similar occurrence:

https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/IR/IntrinsicInst.h#L212-L218

And more potentially affected occurrences:

https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/ADT/DenseSet.h#L120-L121
https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/ADT/StringExtras.h#L588-L590
https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/DebugInfo/PDB/Native/DbiModuleList.h#L46-L47
https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/Support/Error.h#L628-L634
https://github.com/llvm/llvm-project/blob/342c8db381129e908116f1059e97d235b62bcaf2/llvm/include/llvm/Support/ManagedStatic.h#L86-L96

I'm probably going to have to revert this specific change of adding top-level constness within MSVC's STL, but I wanted to let you know about these non-conforming LLVM iterators.
</pre>
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