<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/155320>155320</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] UNREACHABLE executed at ...SemaDeclCXX.cpp: "should not generate implicit declarations for dependent cases"
        </td>
    </tr>

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

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

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

<pre>
    The [following invalid code](https://github.com/llvm/llvm-project/pull/151654#issuecomment-3207007466) crashes Clang:
```
template<class>
struct Trait;

template<class T>
constexpr bool TraitV = Trait<T>::value;

template<class T>
struct S {
    S() noexcept(TraitV<T>) = default;
};

template<class T>
struct S2 : S<T> {};

S2<char> s;
```

Godbolt link: https://gcc.godbolt.org/z/YEvdq5oha

Clang output:
```
<source>:5:25: error: implicit instantiation of undefined template 'Trait<char>'
    5 | constexpr bool TraitV = Trait<T>::value;
      | ^
<source>:9:18: note: in instantiation of variable template specialization 'TraitV<char>' requested here
    9 |     S() noexcept(TraitV<T>) = default;
      |                  ^
<source>:13:13: note: in instantiation of exception specification for 'S' requested here
   13 | struct S2 : S<T> {};
      |             ^
<source>:15:10: note: in evaluation of exception specification for 'S2<char>::S2' needed here
   15 | S2<char> s;
      | ^
<source>:2:8: note: template is declared here
    2 | struct Trait;
      |        ^
should not generate implicit declarations for dependent cases
UNREACHABLE executed at /root/llvm-project/llvm/tools/clang/lib/Sema/SemaDeclCXX.cpp:237!
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++ -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>:15:11: current parser token ';'
 #0 0x0000000004047698 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4047698)
 #1 0x0000000004044ac4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4044ac4)
 #2 0x0000000003f89208 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007303a1c42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007303a1c969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007303a1c42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007303a1c287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x0000000003f948da (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3f948da)
 #8 0x0000000006b1c384 clang::Sema::ImplicitExceptionSpecification::CalledDecl(clang::SourceLocation, clang::CXXMethodDecl const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b1c384)
 #9 0x0000000006b1c5c3 (anonymous namespace)::SpecialMemberExceptionSpecInfo::visitClassSubobject(clang::CXXRecordDecl*, llvm::PointerUnion<clang::CXXBaseSpecifier*, clang::FieldDecl*>, unsigned int) SemaDeclCXX.cpp:0:0
#10 0x0000000006b7597d (anonymous namespace)::SpecialMemberExceptionSpecInfo::visitBase(clang::CXXBaseSpecifier*) SemaDeclCXX.cpp:0:0
#11 0x0000000006b75d2a ComputeDefaultedSpecialMemberExceptionSpec(clang::Sema&, clang::SourceLocation, clang::CXXMethodDecl*, clang::CXXSpecialMemberKind, clang::Sema::InheritedConstructorInfo*) SemaDeclCXX.cpp:0:0
#12 0x0000000006b764dd clang::Sema::EvaluateImplicitExceptionSpec(clang::SourceLocation, clang::FunctionDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b764dd)
#13 0x0000000006ba940d clang::Sema::ResolveExceptionSpec(clang::SourceLocation, clang::FunctionProtoType const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ba940d)
#14 0x0000000006b81ec5 clang::Sema::DefineImplicitDefaultConstructor(clang::SourceLocation, clang::CXXConstructorDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b81ec5)
#15 0x0000000007fe3a61 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7fe3a61)
#16 0x0000000006d76d8a PerformConstructorInitialization(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>, clang::InitializationSequence::Step const&, bool&, bool, bool, clang::SourceLocation, clang::SourceLocation) SemaInit.cpp:0:0
#17 0x0000000006d86432 clang::InitializationSequence::Perform(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>, clang::QualType*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d86432)
#18 0x0000000006aaf35a clang::Sema::ActOnUninitializedDecl(clang::Decl*) (.part.0) SemaDecl.cpp:0:0
#19 0x000000000671696b clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x671696b)
#20 0x0000000006725d69 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+++0x6725d69)
#21 0x00000000066e30de clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66e30de)
#22 0x00000000066e387f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66e387f)
#23 0x00000000066eb6ba clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66eb6ba)
#24 0x00000000066ec655 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66ec655)
#25 0x00000000066de1da clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66de1da)
#26 0x00000000049bb8f8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49bb8f8)
#27 0x0000000004cb2225 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4cb2225)
#28 0x0000000004c2e51e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c2e51e)
#29 0x0000000004da41e1 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4da41e1)
#30 0x0000000000db3ebf cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb3ebf)
#31 0x0000000000daab6a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#32 0x0000000004a234b9 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 0x0000000003f896a4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3f896a4)
#34 0x0000000004a23acf 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 0x00000000049e5bad clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49e5bad)
#36 0x00000000049e6c3e 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+++0x49e6c3e)
#37 0x00000000049ee475 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49ee475)
#38 0x0000000000db0375 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb0375)
#39 0x0000000000c639e4 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc639e4)
#40 0x00007303a1c29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#41 0x00007303a1c29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#42 0x0000000000daa615 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdaa615)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

The test case was reduced by @frederick-vs-ja from a valid code that includes a libc++ version with a bug (which was fixed since then): https://github.com/llvm/llvm-project/pull/151654#issuecomment-3205410955


</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzkWl1z27iS_TXwC0oqEvzUgx8Y2Zo7s5mdbOSZzT65QLAp4YYCGABU5PvrtwBS5oek2J5rpWprXYklk0DjnNONRpMA1ZpvBMAtij6g6O6GNmYr1S2t4FBub3JZPN0-bAGj6EMpq0p-52KDudjTiheYyQJQdIdIujWm1ijIEFkhstpws23yOZM7RFZVtT9-zGol_wnMILKqm6pCZOVHfhyFiARc6waY3O1AmFlAvMTzkjCOEVlgpqjegsbLioqNHcPLUOx1_7zMwK6uqAEULFlFtUbBPfIybVTDDH5QlBsUfLB9Tpvih7Yxk0IbONQK51JWbae_MArujv2XrmGQoSDb06qBFy12w68xSmxLjDFeI5JaOkLCgUFtEEnbgY7mycINWUBJm-oIOrl79VgEoyDD686cG3nQe01svy1V9p7uLg9kRF72iyxyWRlccfHVmpo4lbH5pm0xl2qDyOpfiKz-535ffIvklrYmnI-wbEzdmFNXoWCpZaMYtGpGKMiI_YVBKansF76rK864wVxoQ4Xh1HApsCxxIwoouYACH3XAiCRH_3TEEEk6tSOMkiX-m47F7scaQNH9Ke4FCjI_tXCFtO7IMBengPdUcZpX0OPVNTBOK_6vts0R_l8j_FjBtwa0gQJvQUGHZuHQ_O0w6gmd_Jxl6AfHXz-k2CKwfzhqJWftrVIqy259iY4fOCyvCNzzwM9jtpHkexPMYN36asCDOdIGxZpYDgKgGBNow-vcnHohdggKslHoPEcH17gAVlE19jwZSjVIZyfKtMPprWyqwlrHGxCgnOXjpGrtO8racS6gBlGAMJhRDRp52Z__-fk-W_4j-_DxHsMBWGM9Rw1GZKWkNKeJvEvvRspKI7JiLkmTVcVzRFZr2NHu4w5YtfzyZc7q2soQJIj4yMs-fbzP1vdYN_mOG0xx3mywgloqg42cZqCXlxW3jFgcmIoCc8GqpgBsttCuIjin7KtRlAEiS1wrqJVkoDUUuHMSWbqeVGvJOLXkVSOwZorXZm7TqKHsKy6aXd2mN2-OvMUnJTeK7jBVm8YuYBayVUza6bliclfzCtQMDnUlFaijTDOqNSjnjZlRjfiKyCrnolfxAyIf8GyDZ9Jao3VtbbrcOtd4trPc8Wx2SOMZ1buZfhKGHlBwx4WBCs9KIWd7ULnUYO_j2RrPZhvGZtZXbEu5QMHdD1HaxlrQWm-lwbOSyUqqWcHpRkhtONPtGE7Z8WVtChTcsZYB8fBoEniZb0U7O319qxxrlLIxWVOlQWEjv4LLljbw2wSPSOBh7-Adf0IvTOJFil1QuImrn3T75ZPiwjivPbR-T_tGin5_lNoooDtEYut7LoxLoDbHvov7EPngHTp4iCw68P4EfEhZeA78sgIqmvoPseYbQStE0ka4cq3AlbRDXAOqBdNDJUOoQZkuiJfipXX5Z2ByD-qpxfYPKorKjph2Go7aLKUwcDDd7Pfc_9Z-0NlPAi-gPgtJZOPFkWpzyCGNH-NwVnHRHGYb0bQ32FzLedxCtl16wOHY4CJelAzXZquAFo9feVW9zbrr31uPpnDDJMYb7SR4K-wwiXvD8dgwSZMywDS3ifBNZl3H3mwydt8iTAv63jHTme0HTYeDxrnPgjTE7Fi-B5lbFty3X7ul6f64MK-H63I3CWhVQWHXD0TSoRWXPT7Kri1ZDodYfvnyO5itdP3aOhCR7AoTpqPXk19MyUcssINSIcXTTjYaC7oDXbtktOiotJXh77DLQY20-FWUsqtPueZmaQv_dZPLvF3w0jFlO91UJ1VmFelzyidplwX1p3C6Lsf9PlANnfJWg2wi5opD9WzVlphL_JyHutl-usQfJ7lNd95YkyRaJMW7aWLBnyhxyugljP4UY0EoXspd3Ri4a4tpKC6DmsSmK3ziiY6vj9hTHyy_fBkN_h9cFFPz_bQSW1DcQLG0gW9LR6mcaq8RgkyEiMOiOD_MfVtcw9lZ_IbJumoEs5efmV9hljoa7Sy1JIMxSboIvQskP4OW1R7-XW6flDTy4amG6yYjR6SnGY5ppj6w6DzNO_d0ffRkF_CD8Hlb7h10vKZTHZ-ebTRkm5QQ0NgfsbVl4P1hSxtt7R9LlrYWbMR_c7NdN2XJGYeuZly3KenH1PskW3bOflRQomC5l7xoSS-OT-bvq0DHsVcgHvm7SOIipfgTqFKq3SgXcNO_iXhN6nruAcW9MNw8HcP4YkNn2mapUcteq98bQ_MKMqXo02cn18DK_aFW_WJz0fwavjUg7CNE616oR4PlUlajr_3na2N5erNNnhbFmcyZjNVP4zAgrwXfOen_ki_-q6GVzWhXmt2tfn1sj6tKSssgoudzWcbMH-JPwXudTqvHcVqa11SZuTdcHM_4d1zaJX68iPMhgk_ugXXw_a5_4ZKVBtTxb6kyUWTGKJ43BvQpsrbRqUdPRigeuvdHdnm_FAfDXiupPlOxARsbV3JcK8yz48i4-ktIVMSLF2X7RcmmHgtj73GxsXfbRXjKs1eue948p8NI97fp-3JFN63afrbyTtte-XFNG0PgFfCi8n-ovh4rf7UPDe2rh7cL-WKLH3ozYwy0HhTx769Xq0ivF5nqlSblayf4SDaXeqYr61VUm0bcz1EtTcpetWCiWh7n9Ieq3R_aoBqo93OUuoYWlm2vRTjRgsVR9EMtHmT9EfZQna5Qf9T0WwOfjBovx8_J0S3U92dy0vM6-LssmsrW9FLZctaAa3wNESzNXoRoLEIBfnEaENn64WKxM67WrgDYQeoBj8rmcJHnaZmOnmhkAb-AyFj_Vuq-3RzpLrV7cu_9LrYF0sMc1Zchywkho-BaKbvsieIMzishbDH0CNMxQgKRD2Mh2zF_dVuJx9p3quVlSleJ3w5nz2JU54UFDX0YPUd2eHsy--cCIP0R2aukoA7fM_rxzohX5AHkJWbMf9xRa2Cw_zGs97dU9W8mulJ_dI0ssX2evQ6JFmXPwR9zoDSPKT7KvvQfXF4YMFnvaFX9Be7ZdldXZwlNnnmsja5MHNTNC1wovgd1Wv2Pd0JCSoIwXzhJXv0CwDVhtKpyyr4-lpP3sO3AXegs_aXc7agoprP4rPfcfpu9Jt0rKmoFGIhjFBebbrloXfvc4fGRHQ5u2w0FWU41Z4_aNT_ubA8b2wuPRlFu9PDMwqAFrSo7FaQabqTfd-HTZnMXPq3grgsiSUV3uU3HyeBdSXrc4PpN5mecEUy3pWI63EE7t_nUvc5rxJqWUD2NtPzZL246xH3Ah9PgomxUef6_C47JU_n5KIjG6zZEOS0uqua8Rk9WxqOUo9Q96WgbXHq4_kHTbFTODIi9b5XgaPehNClmIGYBvFWU32SuLyvym8w_cm0uvEw6zcXPEVBTbp3v9oxeL2IXJj9FSitWL2UykRLCJLoE-27wfVQgnKkMLrngJ0p5hWrVydOLl07qEC84inesRI7gXSrJTrYsL63Q7157WGQ97lH157E4WECILeL3Hrg1_Txw6E2OACyKxRuPQrguvUF_ahBCDz8-2j6P2lBlHge0Xj0EhIMhyLRQi_0It8bf3U_OeDv08N7w_Ki7jsvuoQGzLv5Lyiso8HduthgO3LiTy9gPQoux0YBne2wk1gCYD4p5O9KxiMcKTKME2Olne54en33YAjag27N0-DvVWEHRMChw_oRR6JUKClCcfZ3t9eyf1KLcYYr7o9TYbKk5HlrTmGKrfHcKbA9KcylaCu0xOUTS71vOtm6okh-gwJoL5o67iXYb_e0H6F46lx2FvreIopbxTXEbFItgQW_g1k-i2I89siA329s4CbyCBEkZR2GZBjHziijxUlqWhRfEi_yG3xKPRF5KIkL81PfnKSOhn6QLP8kXcZp7KPRgR3k1t_jmUm1uHJJbP4oC4t1UNIdKuzPrhHTBQFB0d6NuHaG82WgUehXXRvcmDDeVO-je9oju8KXjjvP5_HSTHCNC_r3zlYTcNKq6_fvHGjv6-1vyvwEAAP__ShO94w">