<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/97780>97780</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang-19 crash: bool clang::TreeTransform< <template-parameter-1-1> >::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl<clang::Expr*>&, bool*) [with Derived = {anonymous}::TemplateInstantiator]: Assertion `!Unexpanded.empty() && "Pack expansion without parameter packs?"' failed.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    ### Description:
The following code causes a crash in clang++.

Also ICE on trunk, compiler explorer:https://godbolt.org/z/a1YeKdsEW

### OS and Platform:
```
# uname -a
Linux ubuntu 4.15.0-213-generic #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

# clang++ -v
Ubuntu clang version 19.0.0 (++20240301064251+dd426fa5f931-1~exp1~20240301184412.1845)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Candidate multilib: .;@m64
Selected multilib: .;@m64
```

### Program:
```cpp
template <class... Ts>
void sink(Ts...);
template <class... Ts>
void f(Ts...) {
 sink([] { struct alignas:Ts) S {}; }...); }
}
int main() {
 f(0);
}
```

### Command Lines:
```
# clang++ z1.cpp -O1 -fsanitize=undefined -Wall -Wextra -pedantic -c -o z1.o
<source>:5:27: error: expected '('
    5 |   sink([] { struct alignas:Ts) S {}; }...); }
      |                           ^
      | (
<source>:5:30: error: expected '{' after base class list
    5 | sink([] { struct alignas:Ts) S {}; }...); }
      | ^
<source>:5:30: error: expected unqualified-id
<source>:6:1: error: extraneous closing brace ('}')
    6 | }
      | ^
clang++: /root/llvm-project/clang/lib/Sema/TreeTransform.h:4262: bool clang::TreeTransform< <template-parameter-1-1> >::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl<clang::Expr*>&, bool*) [with Derived = {anonymous}::TemplateInstantiator]: Assertion `!Unexpanded.empty() && "Pack expansion without parameter packs?"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: /opt/compiler-explorer/clang-assertions-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 -O1 -fsanitize=undefined -Wall -Wextra -pedantic <source>
1.      <eof> parser at end of file
2.      <source>:4:6: instantiating function definition 'f<int>'
 #0 0x0000000003a385c8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a385c8)
 #1 0x0000000003a362bc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a362bc)
 #2 0x0000000003980688 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007f2719e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007f2719e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007f2719e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007f2719e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007f2719e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007f2719e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x00000000070231fb clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl<clang::Expr*>&, bool*) SemaTemplateInstantiate.cpp:0:0
#10 0x000000000702d641 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#11 0x000000000702131d clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#12 0x000000000705e4bf clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformStmt(clang::Stmt*, clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::StmtDiscardKind) SemaTemplateInstantiate.cpp:0:0
#13 0x000000000705ec74 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#14 0x0000000007066aba clang::Sema::SubstStmt(clang::Stmt*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7066aba)
#15 0x00000000070bf088 clang::Sema::InstantiateFunctionDefinition(clang::SourceLocation, clang::FunctionDecl*, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x70bf088)
#16 0x00000000070bd39f clang::Sema::PerformPendingInstantiations(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x70bd39f)
#17 0x000000000655482f clang::Sema::ActOnEndOfTranslationUnitFragment(clang::Sema::TUFragmentKind) (.part.0) Sema.cpp:0:0
#18 0x000000000655502a clang::Sema::ActOnEndOfTranslationUnit() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x655502a)
#19 0x00000000063cb64a clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63cb64a)
#20 0x00000000063bed6a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63bed6a)
#21 0x0000000004326808 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4326808)
#22 0x00000000045ad759 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x45ad759)
#23 0x000000000453336e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x453336e)
#24 0x0000000004695bae clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4695bae)
#25 0x0000000000c710c5 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc710c5)
#26 0x0000000000c6a2ea ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#27 0x0000000004369f39 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
#28 0x0000000003980b34 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3980b34)
#29 0x000000000436a52f 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
#30 0x000000000432ffc5 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x432ffc5)
#31 0x0000000004330a2d clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4330a2d)
#32 0x0000000004338435 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4338435)
#33 0x0000000000c6e495 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6e495)
#34 0x0000000000b46364 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb46364)
#35 0x00007f2719e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#36 0x00007f2719e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#37 0x0000000000c69d9e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc69d9e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzsWl9z27gR_zTwC4YaEuDfBz_IspUmlzSes9ObPnlAEJTRUAAPAB35Hu6zdwBCf0DL7qljXTvTehJbIrG7v_1hd7EgSLTmK8HYJciuQHZ9QQbzKNUlJ2sipHhey0FTfVHL5vkSIDz-g9dMU8V7w6UAeA7iaxDP7x8ZbGXXyR9crCCVDYOUDJppSCBVRD9CLiDtiFgBdAXQ1WwUG3_POy3hx8UNlAIaNYjvAC0gleued0xBtuk7qZgCeP5oTK-tTbQEaLmSTS07M5NqBdDyN4CWJPk7-6nRN78cat8D_3oHiWjgbUdMK9V6Bx7ksf-3FYCDIGsGIzJe-czFsIFDPQgzwHSWZLM4QgmOVkwwxSkECCOURt_GAXdfbuEXKeCnQcCkggkGeI5jgOcJgt_uFxDFCMNNmT_k6eTPh79-A2jprE08OOQORk_jdW_P3YJPTGkurcVZPIshQOU4GsUojXGcxHmKsgSgq6ZJUd6SrK1wEiW_s02f_L4dlJRpmqBZUqYZQJWfWqJWzAA89yijnkadhRitxLCdfcVIA9eyYZ0d2EvNvQcfhTak61hzze0MQoCWg1YALTte29_d0zpKKoCWNRejxFIOooGUiIY3xDD4YbGAfNRCfMxNtawoBWjp4e2xoWVxBpWzbBa_v9pyVHnHOkYNa97D68UO2HroDLdCeA5nAF-BNF7n6cTiW4OmGTJJrFslV4ocSSja9-MVw9Z9Z6EAvKAd0Xo2m8F7DfDNeP9J8gZqblO_vLc3bfjhqxOE2wNJCAovutU51jd7HWqjBmog6fhKEFtN7rUVuXNCxTXAVxAU1zsI9ov3avuBCwPXhAuXY4fGLIb4EPle9m0CF3K9tqXpMxdMv1WXDsvAb8mM9j2MviYwajUR3PDfGMDXg2hYywVrYPQL6ToY_cI2RhEY9awhwnAKIwojaeWl14wXWg6KMkspnmcAz1Fh44ApJV3ask0_RglAhXPbxxiEEGYQFAsI35Nq6H5Gta_9gOxmOthaf80hV4Ffcai4AqiApDVMwZpoBl2UwY5rM3XzLE7uXDkB9yB-HUjHW86aiDdHxXO75kykjSKCyUFD2klt1-paEcqgn9Pi2s1vtQeYjwDfwHwQkr5IKSnNtrb3Sv6DUfvVj_PF646tCUDLe8XYvSJC2wV59gjwPEU5snpqKTsf7nhuWT0cCfDCFoNtYYh6osiaGaaiJEoAvoEjAU7Mi9xseqUBKg9U2ksAzSGVQhuA5rbtGITriBrIhbHfLQz71zozSt2tSdf9jVEj1cd13401aaLSmkf5Xn7u6kR29YObR3jNFH-ykYddAO06LRcsDrN3y62dwnBipAKZvQnnWjNl1wNoywJKvgm26YloWDNj6948b0uSNZ5DgNAtod-hG-O6AwtADgbuCIM9od81wEuAkE2ClvCONb49u_18M7-7gXqo19xAAuthBRXrpTLQSDjpxrh5HOoZlWs_9S8jgGs9MA3Q0rVhXNBuaBg0j8y3iDWh340NR0tcr1ivJGVaswb6qEYLJ0m0lpQTmwVqEHDsRj3kO2MdboZ1vyuj8QzElV-iIFGrYc2E0T5WZe9i0zeb0a7Z9PEakS3hOvKtqetUduE8NmSr5gdRbZTCaGUrq21F-95qH0w_mJmG0dpyAaNoU-YR0etIPwtDNgBfc2FYB6M7GEUrSiMjZUcf7eKCr9-EZwdrQXr9KA2MWio7qaKGk5WQ2nCqYdQKGTliw8unLxdBWXGUJpZSgBdMtjbbeqI0U5AYyEQDZQtb3rFxJPIjD-tS6mvT2OC4ALeVqB0EdaHt8PAxylHRArywyWhTarvqAIRjGG_i7Q8muMxoeZCl-lmPH24VF8ZFxf0YWuV-kCI_HqQ2ipG1T1eX9dVYDt8lNgC6ijce3r6wAoSTCf4c1fQY_kXHiBj6r-LOri8dQOWuRHXSWjkHWgsmQIsCtFUZ52UJFza4fmZUPjH1PML7CxFNZ42WnslgzEIKwzbGti0Az2P3f2cCexNFi4qkYinKkN_G-CXjSKfb8ZrOtJzlDrgTCWCnoc4qr1oKe-O2Kw_fededZsDJBwayKei0yOFKOy5OBZ8WeaA7D3WjsmgxJLWtvidpdoKB5uKF5qQ-WWdSBzrLUCe2bJ-m04kEOqvDqCtihJO2fqsnAKjcrabQ7uB171K-emNd_S9sFGxv9BIre5k2tozEE46aPE3OztGCdN3owSFN-6unu5FM3Ehw0vwpU310pk-Gj0L4GUvr9uzw78zahPDHKy4-z2TcWrjmmhLV_MRFczJReEoULdLzh6tc93IQzUvCwjvzfSKe6FYaupXnpCaHbrntzvhpqLX5ozP3ZegM_8yeWLdFMvf962euzbYe5WdoArwPu3JsncxCJ-s2LsvjTh7wtfRd3fWuqZs47hrDz5KOj7lC__fCtAumZ_r3DP479wL_84n_Da7a4_7fMmXj7paJhovVng1rGKDyfJgtpABzcYg5z7K0RK9gnlPzVdyI5mvr8mZ87PhNcLNUZGVDbjJvO8H7b9sR24oAUDnriTKzeJtIx7OmnIDLYvRK1rwKbrfhfVcmPZaAyaAZyTGt8zQAe-s2QQef72XvUncM3kPuvvbk14HdGhX2BHbgByWH_mfW7huDo3x8kc3QsY9ruxe_M8Sw81QB7-YhDygOeahZk7_kYX53fyRcDjuds2WuhxRgDrqLFKO8jIPKtZAN-8DEnG4PtWwnwOhgmL90lijzQAKkQSORZqQpsiqoiMruoURzBOqZQI4YApA4BIkxzhmcrKrW7Fj47Lb_GKOve3WWWPY4A0eCZTvNq6wmgSMe8t6fp906Ne0iAn_9A793dmDEFzgQLMkxLZKYZpDS5MGfT-x3I3OlyLMrLAv6SNR-L2MLzQKG19ACPknenMePEWXgRh66kRPECNySv0juXaUo39xavfApD3djVod__hD0TY3iT0wdXZ9QEZaNvGpx5Yg50Lt9avWgHLfu7piJuz6Ukq6rCf3-0Iqw4I-2fQwtEn8KNE3qo3OojR8n3UE8sRwc8GMUFyu_jIwTvBN4eKCbTZKMX2qiOX3QbrhnMRhsLzwYRbjR29sTdaTrbE5IdXDfDplPNrUj504EoKIj67oh28MkRxUqt8-zPsn6-HyU06dQNU4PpuLYs6bxzs-DuCMt654DOt-eujM8WBsRB5FfTUKMZGF_9j8XIpPm8bVYCJ__phi1ra18rxDn5o68WDG3bAbFfCLozmX3BWPxho3DofOg2Tnw7Z0bCOv2YUDhSauDY4KaU3n5JGv9OimfZD3Zfb791GsXBz3hyj_MP4FHHyx_BpuOrIBNNGGzTPGrUXZ98DloHI50DK_Nwp_I5hnaWUdPwB-eLOwsrTx_2w5li9-VlbkvCv962X7vnsQhC6AHvWFcpznOU_fax3vbHlUHtifHCqhqqhPPQpxIoHN6nFCxNIYPD1bsQRuizMOBc3_YCktDK8VkwqumYnDU__5zZpXvrE_eQ9i98zC-Htf6HQakPiPGg253IA7ZhpvxpcUEpxbmoBmMnqCRUDMG-UHb741tO36omBmUYDYnrfDRF3cumkvcVLgiF-wyKVCcpFlW4IvHS0TTtCBpUlNUZDHNiiytsrhFNG_jOK3qC37p3swr4ixGCcrwrM3jukYVS9oibUtagzRma8K7mc2YmVSrC3fIflkVRRlfdKRmnXYvdyLk-UEgu75Ql-5ovh5WGqRxx7XRew2Gm45dLtxkJNV4Nv__NzL-029kXAyqu_z3X7kYA-LpEv0zAAD__9bkO1w">