<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/129620>129620</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang] Crash on CTAD for alias template declared with respect to another alias template
        </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... Ts>
struct A {
    constexpr A(Ts...) {}
};

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

template <class T>
using Bar = Foo<T, T>;

Bar a{0, 0};
```

See https://compiler-explorer.com/z/rzhxc7nPv.

Assertion:
```console
clang++: /root/llvm-project/clang/lib/Sema/SemaTemplateInstantiate.cpp:1474:
static clang::TemplateArgument {anonymous}::TemplateInstantiator::getTemplateArgumentPackPatternForRewrite(const clang::TemplateArgument&):
Assertion `TA.pack_size() == 1 && "unexpected pack arguments in template rewrite"' failed.
```

<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>:12:12: current parser token ';'
 #0 0x0000000003e53898 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e53898)
 #1 0x0000000003e51554 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e51554)
 #2 0x0000000003d9de88 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007b90cb442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007b90cb4969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007b90cb442476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007b90cb4287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007b90cb42871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007b90cb439e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x0000000006f462e3 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f462e3)
#10 0x0000000006f4fa50 clang::declvisitor::Base<std::add_pointer, clang::TemplateDeclInstantiator, clang::Decl*>::Visit(clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f4fa50)
#11 0x0000000006f7f52d clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeLocBuilder&, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#12 0x0000000006f83baa clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeSourceInfo*) SemaTemplateInstantiate.cpp:0:0
#13 0x0000000006f83ccc clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::QualType) SemaTemplateInstantiate.cpp:0:0
#14 0x0000000006f84840 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformSubstTemplateTypeParmType(clang::TypeLocBuilder&, clang::SubstTemplateTypeParmTypeLoc) SemaTemplateInstantiate.cpp:0:0
#15 0x0000000006f7f42d clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeLocBuilder&, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#16 0x0000000006f83baa clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeSourceInfo*) SemaTemplateInstantiate.cpp:0:0
#17 0x0000000006fa0bbd (anonymous namespace)::TemplateInstantiator::TransformTemplateArgument(clang::TemplateArgumentLoc const&, clang::TemplateArgumentLoc&, bool) SemaTemplateInstantiate.cpp:0:0
#18 0x0000000006fa294f clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformTemplateSpecializationType(clang::TypeLocBuilder&, clang::TemplateSpecializationTypeLoc, clang::TemplateName) SemaTemplateInstantiate.cpp:0:0
#19 0x0000000006f7ff36 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformType(clang::TypeLocBuilder&, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#20 0x0000000006f80781 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformTypeTraitExpr(clang::TypeTraitExpr*) SemaTemplateInstantiate.cpp:0:0
#21 0x0000000006f880ff clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#22 0x0000000006fb23d2 clang::Sema::SubstExpr(clang::Expr*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fb23d2)
#23 0x0000000006efde0c (anonymous namespace)::buildAssociatedConstraints(clang::Sema&, clang::FunctionTemplateDecl*, clang::TypeAliasTemplateDecl*, llvm::ArrayRef<clang::DeducedTemplateArgument>, unsigned int, clang::Expr*) SemaTemplateDeductionGuide.cpp:0:0
#24 0x0000000006f018d8 (anonymous namespace)::BuildDeductionGuideForTypeAlias(clang::Sema&, clang::TypeAliasTemplateDecl*, clang::FunctionTemplateDecl*, clang::SourceLocation) SemaTemplateDeductionGuide.cpp:0:0
#25 0x0000000006f43cc5 clang::Sema::DeclareImplicitDeductionGuides(clang::TemplateDecl*, clang::SourceLocation) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f43cc5)
#26 0x0000000006b2fd17 LookupDirect(clang::Sema&, clang::LookupResult&, clang::DeclContext const*) SemaLookup.cpp:0:0
#27 0x0000000006b1b012 clang::Sema::LookupQualifiedName(clang::LookupResult&, clang::DeclContext*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b1b012)
#28 0x0000000006af9c2e clang::Sema::DeduceTemplateSpecializationFromInitializer(clang::TypeSourceInfo*, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6af9c2e)
#29 0x000000000677e43d clang::Sema::deduceVarTypeFromInitializer(clang::VarDecl*, clang::DeclarationName, clang::QualType, clang::TypeSourceInfo*, clang::SourceRange, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x677e43d)
#30 0x0000000006783207 clang::Sema::DeduceVariableDeclarationType(clang::VarDecl*, bool, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6783207)
#31 0x00000000067a19a2 clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x67a19a2)
#32 0x0000000006430345 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6430345)
#33 0x000000000643f8c9 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+++0x643f8c9)
#34 0x00000000063ff73e clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ff73e)
#35 0x00000000063ffef9 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ffef9)
#36 0x00000000064076d3 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64076d3)
#37 0x00000000064085ad clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64085ad)
#38 0x00000000063faa3a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63faa3a)
#39 0x0000000004812598 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4812598)
#40 0x0000000004ada245 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ada245)
#41 0x0000000004a5d92e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4a5d92e)
#42 0x0000000004bc809e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4bc809e)
#43 0x0000000000d4ab2f cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd4ab2f)
#44 0x0000000000d4244a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#45 0x000000000485a4c9 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
#46 0x0000000003d9e334 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3d9e334)
#47 0x000000000485aabf 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
#48 0x000000000481d6dd clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x481d6dd)
#49 0x000000000481e75e 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+++0x481e75e)
#50 0x00000000048266f5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x48266f5)
#51 0x0000000000d47923 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd47923)
#52 0x0000000000c11464 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc11464)
#53 0x00007b90cb429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#54 0x00007b90cb429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#55 0x0000000000d41ef5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd41ef5)
```

</details>
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzkW19z26q2_zTKC2OPhGT9eciD6sT7dt_27tzG01cPAuRwIgsdQKnTT38GkC0jy07dibNn9umkiS0Wi7V-LNb6gSQkJVvXlN56s0_e7O4GteqJi9uvaE1rhZaC0ifeSnpTcPJ668W-_cEe_KR__FzRTVMhRYEXznGFpJxOp2ApvfDe83OpRIsVyIGXaFkAAMC8lopuGwFyD6ZLLe7BzAgkd56f69-hFn5LdytZvQYLzoEX3oHcC-dWWXh_uj9YHvb9hITpu-Bc9_bg3Lbvuut25CWffN3i94btQLBSj5SCJ6Ua6YW5BxceXGC-aVhFxYRum4oLKqaYbzy4-OnBhfj5tMVJ_fAytb1zKalQjNe694FuDROvqOfnuEL1uoM7zIHWwbny4KKqXjaTRvB_Uay_dnKLihUeXDzSDer-LDsUPtdSoVoxpOgUN40X5kGURHZcqZBiGFgdYe6F-a5XLtbthtZKzxCqef264a00WBxK9bq5sC1rqoYqHhB-fkBKUVEvuPhGfwimqAdTExLnxvZg7MHMWroHDHixv8ynDcLPK8l-akUmkMI7PacBMJ1i4EHY1nTbUKwoAVoaoE6tBKwG-xgRO3ugBxNQIlZRMj2ebi-cE6oQq7owtJdku9kg8eqF949KD0HaTeOFcw8u-pZO-HiG_annZw-CrwXa9MZ1k80bNRZSuwmfoB0ecqJEWz97cFGwuo8HHTdgsiY_kCgnEZiswYRrvahptPZWNa2aSjDZ6HACk8k2jSdIbibytVZo64V3rFa0ApOy5pMXKgouqW4Hk0cwmawxnijOK_yEWK2RP2evFpY1auQTV2BSYl5xMSEMrWsuFcPSjoEFkk_uZamIF951KQf6ejVL3gpMLaiBhs-5FuYB3P0CuBVCx2-DhKQCKP5Ma-DBRC9mmOis5MHQB_7W3_0L6SxMsxRoRGxAyldpPzwIViszxUuBsI65XkigHysulaBoY2JvDpgO3QyYyHyXifTgJ3_bmadXhDU-GBgfzGbRmPHziqK6bf6qH9m6RpUH07Y2yZ-AiushrmGqNqY3FTqmkozQNAVzPeXfKOYvVLxa2_4H1aTSI6Ydho7MnNeKblWXxXzz3-oPO_1Jkfm4iCI40_FinLJ5cZvGqziaVKxut5N13doGPJV8GhuTTZfe4MhVmMVZiUGjngRFZPXMquoy7aZ_r302NDdKYrCWBoJLzY6SuFccu4phmpQhQAUX6jK1pmOvNjlSGxQXKwyKXmHqKgwzmsWXKTRdeoXZYYDFZRRDGr53VHdq7aB6_fmDQUs08w_rGaG4emGS7YvjJySpzlg6senviJBVw3WmFTpvjJTCO4orp8i6YrrZg7lNfl6Yf9ej6dJ6LPL-q7zzuMcjcPFIyhkkjlOC0qVAtSy52Jgyme6pBajRhsrGJNfsHMm43-nq9CxfG-p6rK984fhTyyqi_YqH2Np2jch5nrTLMNo36PqWhgVCf5Nvj6bgfa5L3s3rr3sRDr3AGP8NXvx_iyp79RLjo4HxURr5Vzf-sS3kntFqmx_Q78TcSTUXB-JsuMiif84ii_8RiyxxvUB-URDwW6a6dh7tjtIze6cvHNs998jUHIt2QgXn1WW-pgNfYRaV15-xTuqxoZihiv1Eulz9Roye1GMgGZX9P7S5MG1lwwVbhvE_ZMHCAQNK_SQNPsS3pUBM3W8bcezkQdNlCxcO-Eua-uX1g_nYid-yfcBPChgS6BQgukEHpejMqE5UfG0rxb7QF1odZQ0m1UGGuQK7ND7s2SV0uQstCfXxW2m10NGeS8mxho3MtbkCsVpJ13l7bDZcEou2xiYjHJDxY4h0zOUVQ3JErN-N50Kg12-0tEeSPTknLabkKLWH97r3fptuNsPOoCdixOjTJv_RMjIWJgMO5QcpSd8C0aQMV_OCi73bv4LkOYwuBtyW5i8cm3R9MQQDAhWFGM_GV4oeHgn6edNUDDPlapbj9fdXTX7_vZj2o18tLpUqYEmCBHzh_Llt7pgwR8dvT5uV_0ZlW43wCO1qdySzSwT7gLQ9R9B3yVERFH5wIk9ZFXqrwEpGiS286e9Y183Hjt28N_TWiR56lxOhMsOQngowvfzHachC8M3nmilzhY7UuQFDdZzvO5L7WjH1eooL7gXNmP_LauJI9vnra6tQUdHxNLbLRjptvT--FsEeX5dQJQmNQjKOLzH4fkcmWZ0F9DsS4yvX5gCDjo1Ap7XfyB6lu3PTY9u-oXpN-8A8ld_fG04L2B5O9ww8TtIQ-sm5cP2OBNOhcIDMMdt08fxgB40LvYMurUtQkKETKScn5CBClty6MHqaNu7M9ZKMNbt3yuV7UeiHkVPFHsxtj4PPB9OVl4qK3Xcu8prkSglWtGpY1Xqh49RxNALpmWrJTyWcw14LLswa0JhfKRYsMD1s4QC2MsXZm7D9IXjbuMDoNlavdatO2-PlxyK3L0LHODi4X4bvyBHTgGMMg_TDkdfY9si7zDMsyySkbyL_l9hRwjtafq4VFfYG1uVAvilxdjZzjKmUpkCXTMPx_nhZRHq8ZkO8aPl2pHYL3IGN6YymA-LqqA0j7mNQo-VBlLm0N_KTmIRnUbvf2qA6QO9jkLrCijPe9lgkAyzSGSJnsVjyxmz1j8veXw36d0sflBhuX7vkaBjh_UhO2pfWr5y0ld5KcaEeFVL0SqcG1s0ehHSwjBAK0REI-ePy5HZox12uVditSb3BDruN0gDOsvTQ4Dkn9A9a53j39JCmHxS3inaXuudh3tfMzpC9mZHDGiNEEHT5x0LosleTETuvZKG1obcwcC2ckczdhs27Me25GqbjWJ526Srx29nZe-HwvKjAqZ9Rl3oae3tnXvYEID3n7FVSUGdfb71Dt3wSoQKWAONgtUFaQTp-PvaERH-gYI_C3GtwDl44I9dxwlrZ-xANfIBRhMAO9nmwNHnhwJPHDaqq7xQrLj5vmmrUocHmWusYnKSY4CKCvVBxfIoSzdwkMUMRzgwkB0rLjgKshEHVtNqltz-ExqiqCoSfV2XtJnY7cBc682DONxtUk-EqHp29_cMNvNGjIw3AAThKsHrdlQs7tfsOqxXeboOgO7lFkuGVNOIdhI6wvrBSAjEld80Ddaiq9FLg4qBdixxs00z4WMBNFw8mFdoUBHkw6aGC6e4xqT95MTIZ8eDhJqrJbu_y2CNMtuVbWz-iklavDpbn5-0KD2tZi_uAT4bBhQrnNsh_XXBoxKcNEmrqn46CdFC3SUzISdTMrKGjyriD0kndg45a4NTm-oxoPrjFu3fsnVmCdrsPpSGZocmMXgrKn7yQpxH5kxeDO1GHifU4F-8joEFMT_7xrZWzIHZh8iFQarD2UM5cwpXCOC5np8y-O_jsEIQRZnBqCj4QyiuwVQNPD14wqOFJBrtt4Y6J7Iw3qSQ_un13qkK_O_fQlvV2O-zPx0EQxRHQFr_3wFZ1P_DggVqYkezCB2pNl15hNFRIIx-sVrrPSiok1OrArV8egkYHQ8wGkxzQcgas8vefJ628G_roRQEPLvp3BW7IbUiyMEM39DZIosCPkiwLbp5ukygo0zgqi5imaYHCOCmSNChTCklC0wjfsFvow5kf-pHvwzj0p3RWxqiESeqjLEp86EU-3SBWTXWcTrlY3zApW3obwCyG_k2FClpJ84IPhJ0H0Jvd3Yhb8w5J0a6lF_kVk0r2KhRTlXkraG56zO7sM9iA12C-zO9AyQVAFUOyf4WC2PukBPxg6gkIKhuKFVAcoJqrJzqUv2lFdeu-OrNm6qktuvdlzKo7es_FuCY9uOi8e7mF_wkAAP__GPBjaA">