<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/127539>127539</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Crash on CTAD for alias template involving variadics
        </td>
    </tr>

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

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

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

<pre>
    ```c++
template <class...>
struct A {
    template <class... ArgTs>
    A(ArgTs...) {}
};

template <class... ArgTs>
A(ArgTs...) -> A<typename ArgTs::value_type...>;

template <class... Ts>
using AA = A<Ts...>;

AA a{};
```

See https://compiler-explorer.com/z/3c8bYM5To.

Unreachable:
```console
Can't get a NULL template argument here
UNREACHABLE executed at /root/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:2894!
```

<details>

<summary>Stack dump</summary>

```console
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 -std=c++20 <source>
1.      <source>:13:7: current parser token ';'
 #0 0x0000000003e9bfd8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e9bfd8)
 #1 0x0000000003e99c94 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e99c94)
 #2 0x0000000003de6098 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x000072f907a42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x000072f907a969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x000072f907a42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x000072f907a287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x0000000003df19fa (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3df19fa)
 #8 0x0000000006f33f4c clang::Sema::getTrivialTemplateArgumentLoc(clang::TemplateArgument const&, clang::QualType, clang::SourceLocation, clang::NamedDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f33f4c)
 #9 0x0000000007046a57 (anonymous namespace)::TemplateInstantiator::TransformTemplateArgument(clang::TemplateArgumentLoc const&, clang::TemplateArgumentLoc&, bool) SemaTemplateInstantiate.cpp:0:0
#10 0x000000000704851e clang::Sema::SubstTemplateArgument(clang::TemplateArgumentLoc const&, clang::MultiLevelTemplateArgumentList const&, clang::TemplateArgumentLoc&, clang::SourceLocation, clang::DeclarationName const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x704851e)
#11 0x0000000006fa3827 (anonymous namespace)::BuildDeductionGuideForTypeAlias(clang::Sema&, clang::TypeAliasTemplateDecl*, clang::FunctionTemplateDecl*, clang::SourceLocation) SemaTemplateDeductionGuide.cpp:0:0
#12 0x0000000006fe8b75 clang::Sema::DeclareImplicitDeductionGuides(clang::TemplateDecl*, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fe8b75)
#13 0x0000000006bd4517 LookupDirect(clang::Sema&, clang::LookupResult&, clang::DeclContext const*) SemaLookup.cpp:0:0
#14 0x0000000006bbf842 clang::Sema::LookupQualifiedName(clang::LookupResult&, clang::DeclContext*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6bbf842)
#15 0x0000000006b9e2fe clang::Sema::DeduceTemplateSpecializationFromInitializer(clang::TypeSourceInfo*, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b9e2fe)
#16 0x0000000006825c1d clang::Sema::deduceVarTypeFromInitializer(clang::VarDecl*, clang::DeclarationName, clang::QualType, clang::TypeSourceInfo*, clang::SourceRange, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6825c1d)
#17 0x000000000682a9e7 clang::Sema::DeduceVariableDeclarationType(clang::VarDecl*, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x682a9e7)
#18 0x0000000006849282 clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6849282)
#19 0x00000000064d89f5 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64d89f5)
#20 0x00000000064e7f19 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+++0x64e7f19)
#21 0x00000000064a791e clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64a791e)
#22 0x00000000064a80d9 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64a80d9)
#23 0x00000000064af883 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64af883)
#24 0x00000000064b077d clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64b077d)
#25 0x00000000064a2c1a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64a2c1a)
#26 0x000000000485dbb8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x485dbb8)
#27 0x0000000004b24a55 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b24a55)
#28 0x0000000004aa835e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4aa835e)
#29 0x0000000004c1324e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c1324e)
#30 0x0000000000d65b8f cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd65b8f)
#31 0x0000000000d5d60a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#32 0x00000000048a41c9 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
#33 0x0000000003de6544 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3de6544)
#34 0x00000000048a47bf 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
#35 0x000000000486793d clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x486793d)
#36 0x00000000048689be 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+++0x48689be)
#37 0x0000000004870975 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4870975)
#38 0x0000000000d62983 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd62983)
#39 0x0000000000c2b674 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc2b674)
#40 0x000072f907a29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#41 0x000072f907a29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#42 0x0000000000d5d0b5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd5d0b5)
```

</details>
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzkmt9zm7i3wP8a5UVjDwgw8JAHase73Zvu9jbZztwnjxDC0S1IXEl4nf3r70hgg_CPNDNxX76dtI3R0dE5Hx0dHSxhpdiWU3oPok8gWt3hVr8Ief8FbynX-FlS-iJaRe9yUbzeg4XX_RCAPpkfL9O0biqsKQTBklRYqfl8DoIH4GVKy5ZomEEQG0EIITwnDDO5fVZdFyOTAZTYR0YRSm3veAW8zPwbGE0XRh0rmiqZgeABZiBY6teGclzTg3AGgmyHq5ZuTEtv-9VRDkO0ivEtzDIIgpVV_awm3bMM4t767tGBXtf8RCl80bqxVqA1QGsi6oZVVM7ovqmEpHJORA3Q-l-A1gFJ8v_5Ej2Ledf7by4pJi84r6jpP9JOBFeiosDLlpgDFGu4pRpi-Offj4_DDGC5bWvKNXyh0sj-_ee3h2z5e_bp8QHSPSWtpgXEGgK0lkJogNZVtatnjRT_S4n5SCrMt-YxywFaP9Ea9_8990OsaNESzQSfk6YBQYaSNATIPwUBgmVBNWZVT7Z7pNq6xvIVBA9PGpMfsGjrBgRLgNZDSy986rk3B176VYqtxPXRVQPa-CMafQ72wacZVopKY7iaadnyHwCtc8YHl03kw9m2-AfLchbC2RbOhNGLm8Zob3XT6rmCs9oQg7PZPlnMsKpn6pVrvAfBinFNKzgruZjtqMyFoqYdzp7gbLYlZKaFqMgLZhwEq6v2GmHFcaNehIazkohKyFnB8JYLpRlR3RhEYvXiPla6AMGqX8XIMwGuRCsJ7aD6Bp_zLMj8AARZbBCSVkoTOQ2WikqoxQ_KIUCxiXIUm2UMUOBBb-8d_gQ0zcsigQZIt-bUa7_4vkrGtZ3hZ4kJBSgZhCT-ZyOUlhTXAC0AWkLGtc0JKPmgeQTok7fvzQMo7Y33J8anJA3PGb-sKOZt8xd_YluOK4CSltt0WsBKmCFuYaoxZjAVOaYWdOGlCVyaGf9GidhR-drZ9jvmRWVGTHqGjsxScE33ul-pnv3b6Q96_TEqUy_GIYpMuFinupW_TxabRTirGG_3sy1vuwYyV2K-sCbbLoPBoaswXaQlgY1-kRQXmx-sqt6n3fYftEdTc8N4AbfKIniv2WG8GBQvXMUoicsA4lxI_T61tuOgNnanr_TTEn94zHRqh0GT8aCLMgjKkMCulw1rm8ztb1uqnyXbMVwdsnrWJ9NHQQBKRp2mAtBkY92v25Hcf7e4en5t6OTxk001j4Jg49Kk8U9c02JFSQVQdoNF1SMYAKVjQLEXLnAUm0ExF_y1Fq2CpoZQjU1Yqev_Z6405pphLWTfIjFXpZD1FNF1gI-CXGJ4djKsUC5EZQiNN-LBIjpZ4CbVeRNfk8in54Phqc2V_jAfvrSVZo90R09Di6mL0XPZ85-NJRNGWNoWE1ajgT48rnqaXVwZ1r678HCQoDfj6lPLquJYS_3WsoKuhTQrKKsYVi7_rgw7gXYQHkqzfik5YuuW2zGuS03hurHmGnom3JCLgCZ5HJ0Pt26i6Oe6qRhh2tWszsfdz5r84QnE-jFMdOB4mRdh5MfwUYgfbbNi0hbQb09bJ_-NqrY6sxKMq_22fYjh7DAbXc8z9EPXrrxMQnSefqfCJGtWMlqYpeLa_NPW9fNxyEwfjb5zYkAfuS6mFJUX8pmNKHoInaeGEoYr9q8NkrUU9WfOtH1iy6bJeupi6jMvxWm8DR2LB66Zfr2UzY6Cdsz_YrxwJIeK80urzZteJiV-_UbL7n30oOVh30hjRPBwE76W4MB34fBNUET84jzfwvL9jm2yugr0O5bnV-4kWf9cKfHW9HRt3zDf0iEwHYkD0Rvg7IANOOMJTpzS-Fq4fseSmVAYkekQXOb5qx00LgwOusVmEqYouZBysqIYRciz6FxIJuFwOp9HZ26XZDqzB6ecAnERFklaOrvYV_tqPPp9NF1Zqak8fBYy40WmtWR5q6e72iB0mjpORiiGcq8UlxLOuNdaSLsGDPMbxUIH5ogNeS42Gpd--ia236RoGxeMaWN8a1pN2j6__XTkjpvQKQeH-_v4vl1zToP0V5O3bAfybuUZ4jh1q_yz5P-Sh5JwRcvPXFPZfcnxfpBvSlydzYwQqpTdoEtmcNyAlyUy8EITXolXvB2p_QJ3sDGT0UxA3JzaNOJ-CTXDZaAWTKiVSRJcpfaw74JqRO_XkLoFC-PtwMIttcPci-PiKotn0djX4dNt768G_19Lv2rp1n3H5GgrwoczOem4tX4RRVuZVykh9ZPGmt7mhbd3c4AQTQICER-fQMieni--Dh1ql1tt7J1Jg8FOdRsmUZHnydjgpSjob5Rndnkfyg97WtI_MvZ9uJm9IYOZTtUY5ijEkVN_rKXZ9nhxxs4bWdjZMFjolH0hxkkQURdkN2b35RSh51ledukm8dvbOXjh1Hkh8QMUUrf0tPYOzuyOBUByzdmbpKDevqP17hmMVyyiPCkhIf6mxkbB6KRl_GL5guXwhYJJK0voPkNLuBOsuI0TnZWDD77rQ1QsPAwP2Jf-s80LI0-ealxV3ynRQn6um-qsQ5OXa6Nj8k2KDa5Csh2Vp9-iuGcuYYJDn6QWyUhp2ZcAG2mp2tZu6XXHaSDICK6qHJMfm5K7ib0buA-dpb8UdY15MV3FZ2fPHuyZZ6Ixo2MDYARHS8a3_XbRTe2xw2ZD9nvf7z7kWDGyUVa8R-gImwcbLTHT6tA8UYeryiwFIUftRmT0mmbDpwNuuwAUV7jOCwxQPKBCyeEo7Q-Rn5mMYHoAFoXjs7pzx1xdy7eWP-GSVq8Oy-vzdoMDvc7iIeDDaXDFeQn_k4PDEJ83WOq5dzkKIpfaIk6D4iI1O2v4ZGc8oHRS96SjEbj0cn1FNJsczxwd-9gqwbo9hNKkmFkkaU7fC-UPkavLRP4Q-clpzbVcfIyABjMz-fYs-uch9mHyS1AaWANKt-BKYi91jy3GZq9GvzsFwpnK4NIU_EKUN6hWLZ4BXjKpQ1B6eC08VCIH420qyfpE8PYO_eG1h7FssNup_jyC8kUcQmPxRw_cqT4OHHqTywZpkb7z0oXtMij0pwpp6MHNxvTZKI2l3ozc-ukhaDgaAk0LNS-PYKf8w-fJKu-HPrlLBtB6uE52V9wHRRqk-I7e-3GQJshP4_Du5b6MqO_5QRQWxF8Qb-F7IYkCvyhzPw6ixLtj98hDkYf82E8jFIXzNI4QjcMgSQqULPwQhB6tMavmJk7nQm7vmFItvfdRHAXpXYVzWil7rRKh3gMEotWdvLc36fJ2q0DoVUxpNajQTFf2LubS9ohW3T0dKDhcPmcrWAoJccWwGm7yMb4T1Y7xLdxhyXDBiLprZXXv3i3cMv3S5v2FQruyTm70WfMVQOveg909-v8AAAD__9kn9cU">