<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/113598>113598</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] Assertion `(!Unexpanded.empty() || LambdaReferencingOuterPacks) && "Unable to find unexpanded parameter packs"' failed.
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend,
            crash-on-invalid,
            clang:frontend:fuzzer
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          yijan4845
      </td>
    </tr>
</table>

<pre>
    **This testcase is generated by a fuzzer.**

Compiler Explorer: [https://godbolt.org/z/KGTG7GWKz](https://godbolt.org/z/KGTG7GWKz)

This invalid code will crash on Clang Assertion Trunk.
```cpp
template <typename... T> void g(T&... t) {
  auto M = [](T&... tt = t) { };
}
```

It seems that this goes back to clang-10.

Stack dump:
```
clang++: /root/llvm-project/clang/lib/Sema/SemaTemplateVariadic.cpp:473: bool clang::Sema::DiagnoseUnexpandedParameterPack(clang::Expr*, clang::Sema::UnexpandedParameterPackContext): Assertion `(!Unexpanded.empty() || LambdaReferencingOuterPacks) && "Unable to find unexpanded 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 -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics <source>
1.      <source>:2:32: current parser token ';'
2.      <source>:1:42: parsing function body 'g'
3.      <source>:1:42: in compound statement ('{}')
 #0 0x0000000003bcced8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bcced8)
 #1 0x0000000003bcab9c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bcab9c)
 #2 0x0000000003b18248 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x0000716d0b442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x0000716d0b4969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x0000716d0b442476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x0000716d0b4287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x0000716d0b42871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x0000716d0b439e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x00000000074e081c (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x74e081c)
#10 0x0000000006a50682 clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a50682)
#11 0x00000000066cee8a clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66cee8a)
#12 0x00000000066dde69 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66dde69)
#13 0x00000000066e6787 clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66e6787)
#14 0x00000000066e6c49 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66e6c49)
#15 0x00000000067932b2 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67932b2)
#16 0x0000000006793fe0 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6793fe0)
#17 0x0000000006794eb5 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6794eb5)
#18 0x000000000679660a clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x679660a)
#19 0x00000000066aa193 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66aa193)
#20 0x00000000066de84d clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66de84d)
#21 0x00000000067a8006 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67a8006)
#22 0x00000000067b58cc clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#23 0x00000000067b5bea clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67b5bea)
#24 0x00000000066e6df3 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66e6df3)
#25 0x00000000066a5e77 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66a5e77)
#26 0x00000000066a6d1d clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66a6d1d)
#27 0x00000000066a71c0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66a71c0)
#28 0x0000000006699202 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6699202)
#29 0x0000000004542c48 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4542c48)
#30 0x00000000047fe239 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47fe239)
#31 0x000000000477cbde clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x477cbde)
#32 0x00000000048e3a0e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x48e3a0e)
#33 0x0000000000ce1c8f cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xce1c8f)
#34 0x0000000000cd998a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#35 0x0000000004586969 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
#36 0x0000000003b186f4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b186f4)
#37 0x0000000004586f5f 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
#38 0x000000000454c85d clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x454c85d)
#39 0x000000000454d94d 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+++0x454d94d)
#40 0x0000000004555275 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4555275)
#41 0x0000000000cdeb4f clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcdeb4f)
#42 0x0000000000badae4 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbadae4)
#43 0x0000716d0b429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#44 0x0000716d0b429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#45 0x0000000000cd943e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcd943e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
ASM generation compiler returned: 134
```
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzsW19z27by_TT0C0YaEvz_4AdFsvJz6048sds-akBwKaGhAP4A0JH96e8ApCyBkmU7Y7WZe9uxXYkEFmfPLhYHJEKUYksOcOnFn7x4dkFavRLy8pH9RXiURfFFIcrHSw9PPDy5XzGFNChNiQLEFFoCB0k0lKh4RARV7dMTyHHX2PNnnt__nYp1w2qQ6GrT1EKC9MIJ8uJPK60b5YUTD889PF-KshC1Hgu59PD8ycPzXz_ff04___nrkxfPPJy9vTnO94e3uBl_IDUrERUloO-srhGVRK2Q4GhaE75EE6VAaiY4upct_zbuLSR-90ObpruiYd3URAPywql-bICTNYzHY3TvhVfoQbASLT2c3Xs4MVe1h3PkpZ-6vgiRVgv0G_LCGeoY32-r7fVtF-SlMy_se5rPLqB9D681UgBrhfSKaKSNv0sBChWEfkNaIGo8HAX-eL_TnTZ3y3bdGE6PGbfdPPzJ_JiI4bkUQnt4XtcP61EjxV9Azde-3bxmhYfnd7Am_f_ue67-IJKRktGxYTGcRGlo7BVC1B00AyCc2I7204yRJRcKfuewaQgvobwlkqxBg7wl9JuHs71uV5tG2pybHjf2gpGp4Bo2hmyDZRd94z3OPBzs-o1h3ehHe9VEZuqlU3RD1kVJvkIFEjhlfPml7Q0r2wonHk6Qh_HvnBQ1mChUjJeofbaKmi0c1HTdsIdTVBFWQ9lH6vbmanJ3hVRbrJlGBBXtEklohNTG4GBCML1qizEV6z5Ch4FiSrWgPDxHhJeIcVq3JSC9gn4ymHzRklAwZDYSGikoKAUlUqKV3WXTkyglKLMzX7YcKSpZo8cvppXJu_xWiqUka0Tksl0D16pPKdHYFOpLxAi2JaJPqxHZRkaNtJmYHp4XjO-yzmQnGi3L70RWowiNlmgkjF3SNMZ6q5tWjxUarQ0XaDTaZMmIqPVIPXJNNl44Y1xDjUYVF6MHkIVQYO6j0R0ajZaUjrQQNV0Rxr1wdhKvaaw4adRKaDSqqKiFHJVdKmtGVTeGZdq57IXTnt7wqiMsMIQ5l8MJ9sJJaP4g2koJXJv8USCRFt-AIw-npljgtLOAj1gIzNSzFkxPxpeoajm1SW-qvDGxfDYQnjTAODIMiJaXSGmiwUQU2RmSmmqXzsyHbRlGHg595G_87X9hQSmUGbJZaiepelTdh1vJuLY5dN8lYrZrJMn3hVBaAlnb-TVFjHfV0gz8IZnk4U_-pofn4A8G-EmR02P4pzUQ3jZf-B1bclJ7OGu5XWJLVAszyjnQGjAOWuyiDTIcZWhqMu8rUPEA8rGD93-El7UZNOuZdNr0JbKv2r79fR4i7IdIg6T0iyjCMfZ717p1YJMliyQa1Yy3m9GSt90NOlZinFjgtosDO3Jt5kleUdTolQRSLr6ZRftdA9j-zgDxEHSUJmipLBfvBR-liWM7cW3jLK1CRApTq99l2XZ0LKcHloPi3TaDwrGZuTbDHPLkfTZtF8dmvp91aQR-FtCPzvbe7E7i4TBwaktCYj_J8HEtMCnLa840IzV7AnkvZkBrV010V4ZqYk9jGNVyhlncw3b8cmpOklCAjOyjurULwN5nA55IYsabVBrk9ruQE15OtJasaLURAEOPu0Z9UT01QrmVdNe8EogKrvTpXnMhvxK-BEO7ZfDjmeuIcZjDLnNlCUn-KnOfpWgbl5vbbqE0d-8aoIeu7sh71pOHVDjUv4_iw_Z3dkG-EdSG-TBV_27yLbcO-aFLPiRplp70-I6tm3o_eV9K0OMcDxk5yvDrMThs0U31t7P7WpzOwL4l12E_GrJPo9dT_-fi_R9gkUZuDscOi2ke4gKfzuGtCv4ih1XYKbw7yXi3JnX9B1BTeMPpvs96vc2nELdGer-haJhObwvT0YLx_jB9fBg6lp0wJMMwVOD_QBh-Ft7PwlkFvsNZOuAsgiI-6cO038k9c_dJlI8ezs6ldDpIDuZsgDlJ_NNKZ97vXIeYX1VyhxlDRQNny2jjiONp7pZnQoI8fJOnM6iYka7DKr0nUc6n4m6Ihl0VuGHqbHLCErLPGPYHWg6yqPxXy51HyxluHfLdLUhKMt9P3rUF2RLwPmnxXiL3Yvt1cn39pegeeL7G9_v1yIRSUMrkD6uYIfkMRcOS7ETB3c6kRZxRepKuLVt70fgiLWqz8_1HtN5x5sYNkXpsFjDkAD985mR4CIc8FPDmDfGdJlIzvvyT6dWPJeUPcXCG_LBuO_lxoPnL6vSi8q_m70hyWIwHS3MM6el969VGg-SkfpHNj2DjYNk607prvHXoSAZ0JGVwet29F80NPEB9-FDtS0P-v4VbPRDczwv1V6iOyu3dg7vfRNnWcL1uhNRW851pcnVuOjykAx7SgJ7eg8yZVPq_gwzjq0OGK9STPMf-4b54cnfvuty9EnaeqZxrc9FBcjA7kjuKI0yjbB_zVJTwGfjEyuzt816grYb-Uv_i92OR9kD2kbovyaK0Ahw6z27m0hRlXh6BeiaQHQYHZOCCTGlRgktnN-w1V5pwCscZfdmrs-Ryj9NxxFFVUQYh8cF96G8h7_x5eF6SslP-nqVC9_gcBxw55FMIaFYhSoPFmjD3mcdESvJoC8uUrojcbvkmttBMkXsNT-05lvP40aF03IhcN8o8zwjakj8N7m2lOP4A53rd1Ed9srVm18XY6AXN3nY3R6VkDyCPis0wdstGluRJ3h3w2dndvkBfSMutvdvNxO6NuRdOKKnrgtBvi4q7Bb8bu8-haTAV6zXh5XBSH42h0n070ZjRieFgjx8tGV_2y0gX4OcOiwXdbIKg-1IQxehC2eY9i05jc2GhJWFabW8PzJG6NnOie5LW3zdN9t6U2STqOLddPJzW9uyMPSGwpQpn27fjv4jieDyS4TvtpIr2QnHszXV352vL70gF9aND5-nQneM1vUXsZH46TLEqrtD_cooMtoQv5kI2WNJpFpcvEmdjRw5WzC2bTjEfdDQNXno-dqLpxBE7e759sIAwbjsJNZQ6ZR69m5dfRKFeJuUXUdwwpR1STpXm5zxoCDMpYM-ZvJ3HPln-FjYNWftsRq4ci-MYp_FLyGd7nx3hcEQxvBSFv5HNM8hZS4_DXzBY2KGI-vK2VShb_LasTPqi8Pqy_dGaxCJzoDva0C9ISSBCBvRHj92ZdsYenKzCeZm_82SV7eLYjIY2IfLRYmG6LZQmUi_2nHvzKBC5o8RDJReFgDr7Hx8zY_x59P3b4QSBlMKeMrfXUdXvMBDtZ0R3yBZ9Z3qFYMN0dyw8CCMDs1WARg9IC6QAENuT_f1gk7vftmffmehOQtrj7RJ0KzmYKWpsDc5UX5SXYZmHObmAyyDFeZrjKEguVpc4ijClRV4VSQpAKopDP6Y-LquKZEWUXbBL7OMoML9pHIbxmKaBTwoKEECc-H7gRT6sCavHZuqMhVxe2JO-l0EQxnl2UZMCamWP-GO8rRVbTuyx46m5YQ-mCj7qj8rvbgx7GPH09GSihr14diEv7THjol0qL_JrprTaAdFM1_ZfF3RW4tnPd9T6opX15Y-fpe5JfrjE_wkAAP__yEgqnQ">