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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - SFINAE does not work properly with type aliases"
href="https://bugs.llvm.org/show_bug.cgi?id=40242">40242</a>
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<th>Summary</th>
<td>SFINAE does not work properly with type aliases
</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
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mingxwa@microsoft.com
</td>
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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>Created <span class=""><a href="attachment.cgi?id=21299" name="attach_21299" title="The code that triggers the bug">attachment 21299</a> <a href="attachment.cgi?id=21299&action=edit" title="The code that triggers the bug">[details]</a></span>
The code that triggers the bug
This bug was found when I was writing traits for some types with `std::void_t`.
Although `std::void_t` is a component in C++17, I have tested some previous
versions of LLVM in C++11 mode with similar code, and the bug also exists.
To trigger the bug, we could simply define a class template which taskes a
default parameter of type `std::void_t<Expressions...>`, for example:
template <class T, class = std::void_t<decltype(std::declval<T>().func())>>
struct my_traits {};
In the code above, `my_traits<T>` shall be an ill-formed expression if
`std::declval<T>().func()` is not well-formed. However, LLVM will treat
`my_traits<T>` as a well-formed expression regardless whether
`std::declval<T>().func()` is well-formed.
To verify this conclusion, we could write another type traits for `my_triats`:
template <class T, class SFINAE = void>
struct has_my_traits : std::false_type {};
template <class T>
struct has_my_traits<T, std::void_t<my_traits<T>>> : std::true_type {};
The following assertion will fire in LLVM, but works fine with GCC or MSVC:
static_assert(!has_my_traits<int>::value, "Oops!");</pre>
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