<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/135801>135801</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang++ crash on nested template specialization instantiation
        </td>
    </tr>

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

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

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

<pre>
    This following piece of code makes clang++ crash:

```cpp
template <int Index>
struct Node {
    template <typename = void>
    struct State;
};

template <>
template <typename T>
struct Node<0>::State {};

int main() { Node<0>::State a; }
```

* **Clang++ version**: any version from 10.0.0 to the latest
* **Compilation options**: `-std=c++20`

Compiler explorer link: https://godbolt.org/z/4GaEjnref

I'm not sure if this is valid C++ code:
gcc compiles it successfully; msvc reports an error; clang crashes.

<details>
<summary>Clang++ output</summary>

```
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-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>:11:30: current parser token ';'
2.      <source>:11:12: parsing function body 'main'
3.      <source>:11:12: in compound statement ('{}')
 #0 0x0000000003bd6b78 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3bd6b78)
 #1 0x0000000003bd4cbc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3bd4cbc)
 #2 0x0000000003b255e8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007cfb96442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x0000000006d964e0 clang::Sema::hasAcceptableDefinition(clang::NamedDecl*, clang::NamedDecl**, clang::Sema::AcceptableKind, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6d964e0)
 #5 0x0000000006d9817a clang::Sema::RequireCompleteTypeImpl(clang::SourceLocation, clang::QualType, clang::Sema::CompleteTypeKind, clang::Sema::TypeDiagnoser*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6d9817a)
 #6 0x0000000006d98580 clang::Sema::RequireCompleteTypeImpl(clang::SourceLocation, clang::QualType, clang::Sema::CompleteTypeKind, clang::Sema::TypeDiagnoser*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6d98580)
 #7 0x000000000683c1a3 TryConstructorInitialization(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>, clang::QualType, clang::QualType, clang::InitializationSequence&, bool, bool) SemaInit.cpp:0:0
 #8 0x000000000684394e clang::InitializationSequence::InitializeFrom(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x684394e)
 #9 0x00000000064fe2f8 clang::Sema::ActOnUninitializedDecl(clang::Decl*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x64fe2f8)
#10 0x0000000006176bec clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6176bec)
#11 0x000000000618cbcc clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x618cbcc)
#12 0x000000000618f6ca clang::Parser::ParseSimpleDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, bool, clang::Parser::ForRangeInit*, clang::SourceLocation*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x618f6ca)
#13 0x000000000618fbe7 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x618fbe7)
#14 0x000000000624a3e6 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 24u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x624a3e6)
#15 0x000000000624ad0e clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 24u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x624ad0e)
#16 0x000000000625380d clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x625380d)
#17 0x000000000625424a clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x625424a)
#18 0x00000000061404d6 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x61404d6)
#19 0x000000000618d02f clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x618d02f)
#20 0x00000000061388c7 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x61388c7)
#21 0x00000000061396de clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x61396de)
#22 0x00000000061431ea clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x61431ea)
#23 0x0000000006144a98 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6144a98)
#24 0x0000000006132eda clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x6132eda)
#25 0x000000000450f27d clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x450f27d)
#26 0x000000000481d2ea clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-trunk/bin/clang+++0x481d2ea)
#27 0x000000000479986b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x479986b)
#28 0x000000000490d663 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x490d663)
#29 0x0000000000db27b1 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdb27b1)
#30 0x0000000000daaf0d ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#31 0x0000000004580919 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
#32 0x0000000003b25a03 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x3b25a03)
#33 0x0000000004580b39 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
#34 0x00000000045440ed clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-trunk/bin/clang+++0x45440ed)
#35 0x00000000045450b1 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+++0x45450b1)
#36 0x000000000454f3fc clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0x454f3fc)
#37 0x0000000000daf4e1 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdaf4e1)
#38 0x0000000000c33cf4 main (/opt/compiler-explorer/clang-trunk/bin/clang+++0xc33cf4)
#39 0x00007cfb96429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#40 0x00007cfb96429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#41 0x0000000000daa9a5 _start (/opt/compiler-explorer/clang-trunk/bin/clang+++0xdaa9a5)
clang++: error: clang frontend command failed with exit code 139 (use -v to see invocation)
Compiler returned: 139
```

</details>
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzsmlFzozjywD8NeVHZBQLb-CEPjBPvP_uf3Zkb5-41JUTjaAMSK4mMs5_-ShKOgNjOzBaztVV7U5kKAbXU_VOruxEiSrE9B7gOFh-Cxc0VafWjkNebqs0Z3Ai-v8pF8XJ9_8gUKkVVia-M71HDgAISJaKiAFSTJ1CIVoTvA_whwB8QlUQ9BnEWhPZnGbof2jRBmGmom4poQEG8YVyjO17AIYhvgzBTWrZUo19Nr8HqQxBmCCHUF9AvDXBSm-sb9CxY4QRNs054p4mGIDbCweqmuxiO6mROdnv_RpEg3oTmZpwFcWY7t6r1ujZG1ITxAKcBXpun5wRJEH9ARrQHpWOEMxRg82vT4_gMUjHB3YMgzhDhL8ebqJSiRlE4D-ch0gLpR0DGHKWH3Ym6YRXRRkI05pfy3QXLcKZ0EcQ31I2IvUZOECSCQ1MJCRJVjD8ZqUetG2XMwtsAb_eiyEWl50LuA7z9I8Db5Cdy-xuXULqO7gK8qhEXGqlWAmIl0sabmELPpGIF2hydxjIzEntKEXXDK8SMHKWgVNlW1YshWKtniiQ0QmqFCEcgpZDmgfVB532g5h3ZeFOAJqxSbmqDeKPauibyJYhv-7BFq5tWG_fAW99i5MJBmH3-eJvtbpFq85ppRFDe7jtlzDSM4DD92OZzKuoAb6vq-fhr1kjxG1Ad4C1TqgUV4C0ivECM06otwM6mtQPlhD5pSSgEeIMaCY0UBgYUSIlWuttGkiglKCMaCiRbjhSVrNGGwU4T-oSKtm4c3XAehOvPUuwlqRGR-7YGrpV1B7wVjdGpgy9nx7k39wyqmZYtfwrwNjfuvu2v-dm--EpkOUvQbI9mwnRGmsZ0abnOFZrVxnQ0mx3S5YyoeqZeuCaHIL5hXEOFZiUXs2eQuVBgnqPZDs1me0pnWoiKPpolFt9cVNI0Vpw06lFoNCupqIScFYzsuVCaUeXGsGCHt0erwMSEDq91gcgwG9yLsygK4iwODTjaSglco4ZIBRJp8QQcBXhlQgReBWGGz8lH2MgbORNXy5ZTu1JN0DUduLBieogv98C4XTGi5QVSJtaYSUU2Iq26cIVXAV6bWBngOEThITz-i_Nima9SZP3TRiv1otzFZ8m4tg5071ww9Y0k-foglJZA6gAvjRsyrm0ANKP-eU8K8Ifw0OnkNY5GGic0p6c03lRAeNt84ju256QKcNpym-IKVAkzxGT6GQ28fnioH14sIEUb42hfgIpnkC9Oof8jvKjMMGlHa9BmI7iGg56bTBlnof3v-o-7_le0zNfLJMEL46XWkorlAd4e0uXDMplVjLeH2Z637gGdKzFfWpWtiFc46Su8LNbLBMIuibuMBTVxV49EZZRCo0lewQ2UjDNts1Laa_4rqaG4AVrZ7LJBZx-Nn_px_CD_z3hhmuVCVFPNWGeiB7AYAUijFTmt2Bf4vWUSTEasQMP9SwN3dVMNAezswvwoKHFwBkb-qyWVETtre7_vo_UnG5oGNy5yGdOzCfEYAB7PcoxnkZ7xj38KnkXa857VAE8a04jE6N6uYFc8Cnln1gmp2B_k7XKxGruo2bv7KgHFLddMvyBqurvQ0HZtiAxa-rj4S2sXVCYlefkCZRBver3cHhrLKL79tvk4c3uozA5-b4GbXLH0S9gvZWO4ETgR4tIh0iReJ_AN44zIbaUptv7mrMdUJnFRR8y76HrAMykBl-m5yKs_8X9z5pG4cN2n-BrAJ9LW6eO0Nfl9UJIso9UyB9rX9rOtrXrXRiEi7aRkpQZ5_FvIjBeZ1pLlrTaV9dgK1-jtRL8ZobjvXhHveCnOuUdfaivkF8L3YFxmSlaOhmcVDVmlNKfvs_pJirYZ0vjs6k7zdNeYAcbGeVxdZXLK-AHs74P6tv04T4zLhb8EtwXqceMR7nJJyUUbd8ykq56DnnPC01THDE4yfZ_62xbHsPOtPN-bmcl4G6KedzzmncPqW0PB34T0X8Uth5XnNizpcUJiWF720-Ob4ic5jqaDAOpz3K4mVfUfMOXNML_tdH30GZy0Ns29HwqM0LdNzMkw8P0TMxF4h9aDX4zBFyH8CfB_F9LTUSpC8JSGbxR4EadhcVHrTbej8UrrgyheApxOWjU5PbyWq5GWCU4uB_ttt2kz1vJU7XR5lnZUNDCpn1rtvW3DCjtKwqS4HCCOtp176-8VED-uqvpINPjV_JGpaZO9peAZrUfJpwhx-b_aasKcZYC-4sajsj9OU_p-rv8ke455xzVIt9U3ceZ-dwoz-13CPGMlMwymgmQxeEijej9eL4vL-aWXVAaszi7iyVGNfevHoTIwPKpRrZ7EEVwO37cH5z5na8gfhWey-GVM9ABGxXOSkHV6EcC9aD7CM1RvX_k_NeT3Fj7rUQHyGvDsJseJ8sNvLPwiiraCu7oRUrsPsxNmN2ebtzwZrRIMxdupz3b3Z3eHfsiujNPDazmoFZNFWOLVoAraiAJ-Ap7ZJXvcOwLaauhudV-ZJ9CtG93rNqjQkjQq8HDxbKXJVLw4odyUarmBvVqDkixZrdfpMh8icwPdcaXJ65bgmNp5O6bzyU45r_qg4krWYbFcxmiwLWiV9BY8v2bn9JKF0wWQTimv8qAACoscr_IIURo9dGcc_CtKf6fzkchjLfe6oTy4hzfuvMZ0mjvVXhUfftQMC0LKsEBHwJvo3q7q029Y9nvFKStGW7ymj65a61Wua1RI9gxytKdtdIqGqz0N19Hacuh1evzq-yAtSvvULafX4yOUVFVO6NNDyYex2A3cOckm2oi6JrwYr8yTU2Y_eZt77lgIMQB6cLRkfN9FeDefrwIPD_RwsB-fgzjLiWL0QdnmHcJBY3PjQUvCtDo-HnVHqso4vXvP7Z6bJpmPxdZnHHArEuBVRercxNWVR4XT4yfen0V-YjLefKMlYdybh1NfYt2TLy3fkRKqlwHLy_M22Ydmq6b38njsUXm8Rv9kjzCY5w2Reh6en_pkSC1JQijOUvNHpsZB2qIcROaRoGlw7p32QtNsUIH0DJsgx1tbvf-M6o9kEZrw_n0kfha5Oo_hZ5Gbl_QzX8neRt3XaW8Ik-5Q4PeQ63zjx_EzhDy_5YhfGZf0nK43vetBpj-R4s9x_wv5TVVVWiae2GqUl8sEOo87lhRHjW2kOJ4ReT_rTlJEWHW8soOCLaRxTMvEnu6cZDTXnx9tPTzVg9fF-jtP9ViR1w6TcNwhJCF6eDAyD0oTqR96tnzzEJD0hojGZdaaLJDrfKIZMT268frP4ux40DPrDnqWXRmPaOfTJWEVFOgr048IDky788lRbF2lVYBmz0gLpAAQ61Xa6_5xVwm6lRzMkjKSJ47q2hOi_kzpVXEdF-t4Ta7gOlolySrGabq4erwmaZQUqyjBsCBJvsQ0T4sC55iGURyGK7hi1zjEizCJFtEyjPBinpOcprCENY4IREkUJCHUhFVzsxbmQu6v7GnR6yhepGF0VZEcKmWPbmPcocLB4uZKXttDpnm7V0ESVkxp5bvQTFdw3T_56g6aCo44KA2FP3CtGqD-6ANi9u1DM_vXVSur6z9_2rUz4fka_zcAAP__l0ZGIw">