<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/86279>86279</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [C++26] Crash when indexing a pack with a pack of indices
        </td>
    </tr>

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

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

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

<pre>
    The following code crashes on the most recent clang trunk when compiling with `-std=c++2c`. 
The intent is to transform a pack of types into two sub-packs using the new pack indexing feature in C++26.

```
#include <utility>

template<typename... T>
struct TypeList{};

template<typename... T, std::size_t... left, std::size_t... right>
auto SplitImpl(TypeList<T...>, std::index_sequence<left...>, std::index_sequence<right...>)
{
 return TypeList<TypeList<T...[left]...>, TypeList<T...[sizeof...(left) + right]...>>{};
}

template<std::size_t N, typename... T>
auto Split(TypeList<T...> tl)
{
    return SplitImpl(tl, std::make_index_sequence<N>{}, std::make_index_sequence<sizeof...(T) - N>{});
}

auto tl1 = TypeList<char, bool, float, int, double>{};
auto tl2 = Split<2>(tl1);
```

https://godbolt.org/z/Px5c5971o

```
Stack dump:
0.      Program arguments: /opt/compiler-explorer/clang-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++2c <source>
1.      <eof> parser at end of file
2.      <source>:19:6: LLVM IR generation of declaration 'tl2'
3.      <source>:19:6: Generating code for declaration 'tl2'
 #0 0x0000000003695e68 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3695e68)
 #1 0x0000000003693fb4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3693fb4)
 #2 0x00000000035dd148 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007f3b03e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00000000030aa562 llvm::Type::isIntegerTy(unsigned int) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x30aa562)
 #5 0x0000000003aaf7a5 clang::CodeGen::CodeGenTypes::ConvertTypeForMem(clang::QualType, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3aaf7a5)
 #6 0x000000000397a5dd clang::CodeGen::CodeGenModule::EmitNullConstant(clang::QualType) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x397a5dd)
 #7 0x0000000003a3f3fa clang::CodeGen::CodeGenModule::EmitGlobalVarDefinition(clang::VarDecl const*, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3a3f3fa)
 #8 0x0000000003a62f45 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3a62f45)
 #9 0x0000000003a635f3 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3a635f3)
#10 0x0000000003a6bdd7 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#11 0x0000000003ef1e4e (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#12 0x0000000003ee2fb9 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3ee2fb9)
#13 0x0000000005da18e4 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x5da18e4)
#14 0x0000000003eef935 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3eef935)
#15 0x0000000004186c01 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4186c01)
#16 0x000000000410c99b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x410c99b)
#17 0x0000000004269503 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x4269503)
#18 0x0000000000c33a4c cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc33a4c)
#19 0x0000000000c2d9fd ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#20 0x0000000003f38799 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#21 0x00000000035dd573 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x35dd573)
#22 0x0000000003f389b9 clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#23 0x0000000003efef77 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3efef77)
#24 0x0000000003eff92d clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3eff92d)
#25 0x0000000003f0974c clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3f0974c)
#26 0x0000000000c30eb1 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc30eb1)
#27 0x0000000000b28884 main (/opt/compiler-explorer/clang-trunk/bin/clang+++0xb28884)
#28 0x00007f3b03e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#29 0x00007f3b03e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#30 0x0000000000c2d55e _start (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc2d55e)
clang++: error: clang frontend command failed with exit code 139 (use -v to see invocation)

```

</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzkWU9z27oR_zTwBUMNCUr8c_BBluI0b5I0jT25akBwKaEGARYAbfl9-g5ASiZk2Ule1V6aiW0JXOz-9ofF7hKgxvCtBLhGixu0WF_R3u6Uvn6gvG005VeVqp-v73eAGyWEeuJyi5mqATNNzQ4MVhLbHeBWGYs1MJAWM0HlFlvdywf8tAOJmWo7LtzUJ253GGVxZGyN0jVD5AaRG8JQFs8witcoXjpbXFqniBtsFbaaStMo3WKKO8oesGqwfe7AODGF7ZPCpq8i98jg3jgzDpGEp0Gcyxr2brQBanvttOPVaDibDUbH31k8_h--kpRLJvoaMEpXveWC22eUfphOsdB2glpA6cphkrSF2WyG749ixuqeWXz_3MFnbizKb1C-RunNLykhK-yJWqJ0afifsLFuXEBj33ik-XZnj7ZpbxW-6wS3n9pOIFIcUaSr-9ls5gSnejxTGwP_6kEyh8ZZ-hU5b_YgWI6u5aOPWIPttcRT4yc4Fjfep8X6xdhrEeelatxnUgwUlBiRm9Hn49z0wynH-foNsk8IxF-d4fPL-ELlWRqxFWccx_jg-3QRnOiEzJY-wOYVo19f_Pi58JSZe0dLhKfzy7eo8E5ZkWCUrqeEsx3VzmyllMfaCEV9xHHp_9SqrwS8ZnrUR7y-gax0RfyCFlYkAZKTreZ_76ztjHOU3CJyu1V1pYSdKb1F5PZPRG6_7RdsUeaJemfT3lm35-u-7ZwiPxTPUFx-02qraYup3vYtSOvsYERuVWcRuR0yFOgI9p1QGrQbc2ks8mkMkduKy8PYkDtwtK2fqG6iOY62OFJOGe06p7K3XW9nBketEI8tjqJ9kUXUtJF5lpbuUbp2CU7g6A5H0ZaxyCol2I5yidL1u5icsJG0MztlcdQwJZSOak63UhnLmcFRI1Xkc3M4fJJvXUIzqtcMjhGeOJZQugLVuIDuqDagMbUYZO1SbsMFDJJklHxRkC6TEqXLzHH6-fOPL_jTd7wFCZparqSbXQMTdPyKSG4FQSQf1KXvqvs4qjnUnUbp95RhRNIYx_v48C_NygVkBXYrMW72ZzN8-Ka5tD5e7jVl4NLKUUjTp40yVgNtEcmOse8yTvGfRA0iN_F-xHTMGA50cgI6bar5OdArAVT23d_lHd9K6vJJL30Fr7FQzsrFIDoEAUQSQFzUdTIv8MrF2ndg6hH084Dpb1TWwlkqRs4CmZWSFvZ2xjq3Q2P_czSRjibyJq3iFOZkQeLRH8ErRG73RbbJ5pHgst9HW9kPD9jMqFnmgfspAex5ADumdJGRCbMu741lzXySFrag75-ntI5OMCWNvQy3A4YA5CIASWmT08XQSI3Lrmr4CDL44pAfokLJR9DWjdwq_QVaRIrJ7H_0VHg_j0n9QlEyAA08yQJPypwu6vpnnnxRdS_GVfjQcvu1F2Ll-KaO-_OeXMiDAWDgQR6uRdqkDf1dDz4KVVHxg-o1NFxyl6pCR_wjJoawQmR5-aXxuAPHitCxjDTznwbZecfe8urwlPm-4WWTHegQPXhfL-WjdyHwsTzxMV006V_z8W3PLobegXvpG0maxCfoq7rOfxf9veo-wyOMWKc-DCMH-mcd1XYW-wQ91fQ6MztkYX2CJoE5OC1UKvncqt5g1zSbzhfSMgDo6rfSvvv1w0N9eB_mR6367js0Dt0A66bnogZ9Hl1YmgBIU5VT3m4oewBZu5zSt6D_Go6LLPqALVj0dAp-UdOkgPkU_DfXii3v7kN0d9DSsTU59OmXzB8jjgDo_ITlpkzP5Y8l84lhCMg9sN7COORwXYxHZz2AF9TQeVJkLE6m8G61az1kfQbfBZGNhgNkWYgsZmVZhcQNtj75ksfgPHdvu-LC4FLoPbgAfVAP5yQrF3GQUkecL048KkZfY37l5OUKwQgqQB0Uu5ilKZ0zzFiyad171rTXX2pNn90eH958X0rycBIRjpEVflS8viD4AVqAvQyxk7psanygeZXc-30-8eCupUL8AHZMs2ccycKK7HSMrfhBzgdRrfnjG1mWhNWpSYu8LD0bE71NL31MbrQn1D8ddtfwZofSJaNCVJQ9bBrpkL5EyGB7jJZVslJtS2V9ulHPLtzxhER1zjp1HEz4sZrLrZf8cHqYtdmw_T5Jhi8VNZxtjBefHIQchd3AxmrKrTk8PlFHhXDRr_TkuROZNHjL4-vEMAWRXNC2qql7iz1SRYrD-9wfqjq_HsnpC9kiTydLce61a3jyvZd3tAHxHND5_tJdKnEPMKfhTshpXJVh9f6_i4uT9uzNAEhPmjJo8vxN4vyC0Vel78BmkKtPJjqBSZZYvWNjKroM-pOLvkIPvgZRdNKdNE1J6t8l4w9VmbeZ-ENVn7mxARPvJeHj4neUu3Ufj09_lbwxQv57FDqGAgrDU4gmLnNXM8_DXU8-Bx3AmdL_FvX_QwovlL0GTgLSspNGI4ZqbDsPrcYBtM8ay3HP_7wUX6a5cHACvEE7F1ekKIo5dkgvYnDQFxgswkM9Utblbx7q-SmBzvJUJ8xjvNm4aRtjqbabiUe_bAXmgZXwJNn1YYsF4EH_ZVbHazyanD5Olxi0duViOV6oNmPPj9kY5Q3lAurhThX23A4H5EnqI6c3gKNHbBU2AJhPevLynduTq_o6rcu0pFdwneRJkiQky-Or3XUTZ9mczIElLCOE0JgWrM5JkhYVTRJgV_yaxGQep4QkaTwnxaxhLGOLOmV5VeSULdA8hpZyMXOBP1N6e8WN6eG6yEheXglagTD-EpoQCU_YP0SEoMX6Sl-7OVHVbw2ax4Iba160WG6Fv70-3umixXo4ch5un4_3v-P1sefr5SqZy5ozMFe9FtcnF1Dc7vpqxlTrQsVt1-FP1Gn1T2Bu4T1Kg8it9-LfAQAA___h8mN_">