<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/109095>109095</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Clang][C23] Crash when trying to use a `constexpr` variable that could not be initialized
        </td>
    </tr>

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

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

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

<pre>
    I don't think I chose the best title, but I don't know how else to describe it, but the code is pretty straight forward: https://godbolt.org/z/1Tj8ba5bq

```C
constexpr char c[] = { 'a' };
constexpr int i = c[1];
static_assert(i == c[0], "whatever");
```

When compiled with `-std=c23` with clang 19.1.0 this happens:

```
<source>:2:19: error: constexpr variable 'i' must be initialized by a constant expression
    2 | constexpr int i = c[1];
      |                   ^~~~
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-19.1.0/bin/clang -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-13.2.0 -fcolor-diagnostics -fno-crash-diagnostics -std=c23 <source>
1.      <eof> parser at end of file
 #0 0x00000000036fdb08 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x36fdb08)
 #1 0x00000000036fbc74 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x36fbc74)
 #2 0x0000000003647be8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x0000795267642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007952677aedcd (/lib/x86_64-linux-gnu/libc.so.6+0x1aedcd)
 #5 0x0000000007257afd clang::Expr::getIntegerConstantExpr(clang::ASTContext const&, clang::SourceLocation*) const (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x7257afd)
 #6 0x0000000006181049 AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) (.constprop.0) SemaChecking.cpp:0:0
 #7 0x00000000061833fa clang::Sema::CheckCompletedExpr(clang::Expr*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x61833fa)
 #8 0x00000000066204e4 clang::Sema::ActOnFinishFullExpr(clang::Expr*, clang::SourceLocation, bool, bool, bool) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x66204e4)
 #9 0x0000000006385d7d clang::Sema::BuildStaticAssertDeclaration(clang::SourceLocation, clang::Expr*, clang::Expr*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x6385d7d)
#10 0x0000000005f81e36 clang::Parser::ParseStaticAssertDeclaration(clang::SourceLocation&) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x5f81e36)
#11 0x0000000005f71028 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x5f71028)
#12 0x0000000005f27cfa clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x5f27cfa)
#13 0x0000000005f29ac8 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x5f29ac8)
#14 0x0000000005f1877a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x5f1877a)
#15 0x0000000003fce825 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x3fce825)
#16 0x0000000004274201 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x4274201)
#17 0x00000000041f699b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x41f699b)
#18 0x000000000435a843 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x435a843)
#19 0x0000000000c874dc cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0xc874dc)
#20 0x0000000000c8146d ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#21 0x0000000004017309 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
#22 0x0000000003648013 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x3648013)
#23 0x0000000004017529 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
#24 0x0000000003fdd947 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x3fdd947)
#25 0x0000000003fde2fd clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x3fde2fd)
#26 0x0000000003fe7d4c clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0x3fe7d4c)
#27 0x0000000000c847f1 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-19.1.0/bin/clang+0xc847f1)
#28 0x0000000000b5c2d4 main (/opt/compiler-explorer/clang-19.1.0/bin/clang+0xb5c2d4)
#29 0x0000795267629d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#30 0x0000795267629e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#31 0x0000000000c80f0e _start (/opt/compiler-explorer/clang-19.1.0/bin/clang+0xc80f0e)
clang: error: clang frontend command failed with exit code 139 (use -v to see invocation)
Compiler returned: 139
```

Weirdly enough, it looks like adding some other flags avoids the crash, for example with `-std=c23 -O3`: https://godbolt.org/z/41bGxqdh8 (it still crashes with `-O3 -std=c23`).
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzkWU1z2zjS_jXwpYsqEqRE8uCDIkfzZipvJTVO7R5VIABKGEMABwBteQ7z27cAUh-QZScTa-eyqsSiyAb66aebjQaaWCvWivNbNP2Apnc3pHcbbW4FUYJuhOPiptHs-fYTMK0QLh24jVAP8AnoRlsObsOh4daBE05yhBfQ9A6O0g9KP8FGPwGXXloD45Ya0XAQbi_t56CacRAWOsOdewbrDBHrjYNWmydiGMrnsHGusyifI7xEeLnWrNHSTbRZI7z8E-Fl9u33qiHT5g-U3qF0Pv6dpcO_xfCbamUd33UG6IYYoIPVgPI7QOUHQLgkCJeAyjuUfzgfIpQDEWT9uAxNj0LWESfoiljLjUO4CmJ7ydRL4gUgjJ82xPFHbhDGCNeH4QeYp9j_veEKqN52QnIGT8JtAM3SxDqG8juKczRLh7tUErWGrJ5kk9T7x8KGdB1Xga1LZIw_84XVvaEc5R9RPscon2e1Z5obo42_OJr-SIwgjeSeIeEZ2vbWgXejEk4QKf7kDJpnIMMYohz4cdxaodWgDgAAAyoX8EOMQvh48ZcfNP34119_DXJfP3-c338E2zdb4YBA06_B8E4b58PtLGiE2_TNhOotwkspH_dfSWf075w6hJfC2p5bhJdAFAOhqOzZEObUELuBhtAHZwgNsd4Z3hlNubWcwcglXoSRxFpNBXGcgekV-KDv3GSAfO8IfQDWb7uDg9IJSuuvRq8N2QIx637LlfPAAeGl7jyyMRJMwned1MbH0DJ4Phk8j_CyEWp_E5I1eyKmTQpI1pBoPw_pOj9b77reTSwkW287JMmumiXEbhP7rBzZofxOKMclJK3SySM3jbbcP4fkHpJkTWnitJZ0Q4TyQf4WPi-c5RM8SSFpqZbaJEyQtdLWCWoHDYHX-PYhxCGK0cBU5plC-YLrFuUfoSPGcgPEAVcMdAutkHyMIITzFNJduv_ks5Y1aQXB8_kc5XP7bIeLr0YoF_zybXBudRQy5GmlrTOcbBGeeQcL5RCuAeHqJ72D8Id0N8LxeeCANzvD29CyuIR3ITlRffdF3Yu1IhLhqlchkTOQ2s9_DXReeYQOx-iKsuEVLLz_fuNUP3LzPMD5P6KY9EqqkalIZqGV4zs3oZ2P_zT8P6jIRxVlPcWzclbgKU5HU6RoEF7uqtlqViRSqH6XrFU_PKATqyezADwMiWAX0Zwl4YyyvzdnFsZEk05PuSjxtCQtGzLx4KGPu84MV2vuPinH19wsxtwYnuHqRHp-_22kZciOY6CdSNyH1-CzpsT5lIrnntcg-15Pj-Aj62an1s2yKkuLGuaKyOc_-adtJwUVbqHVIzc-wdvYmHu-JS8NGIyef8esBTRayzF-J8G-zujOA6_BT7zYcPog1Ppy-JRnuPO8JXAObXiD_DwLve0kd5y99MhPwH2PE0askROqyJgZTgteXDZmTt0XtRRK2M2yl_K91px_v9-6AXxkXR1Zl1dTVrLL1n3ohWT3ocKahwLrjlNJzAi6etOg73LwT_t5MPTAhM_50SI1bauM57NTMF_DGndy_VNczK6AfgQXoc9i9GWW4upN9K8i3j_QZsyF33HKeYYJs7O5c0Y0vfNV3N-XuJhm381aICViDces4ZLGieoFax93jhtF5KvsXcN6LyHU2uu47zi9lvnBusj8_Mz8mtC3g-ab7j7zRx7Mj-3-0pE_ev7VGZQvzoLpF6P77jfuC8ULnj7kl__XrJd-WdPG14COX-tlCWZFdhex3VlVli_dPr__9uqCeuXEPEKIMEaVTd5SXuHpKcaFZvwXruY0xOCYRDntHR9vhVB_d_05KI6QRVVJgcsCp9kpsqXxeUOxC9CuA2rUGYGKSo4ia2d13cR0DWo-hdrP72QuMfa6FVeJxRFXBDwqL4p8Sqoij1fGAPGI__GQFau37LtK0hjxRICjiiGlVVkwCpRmq63fip5u2ebGkOfw4i-Gc56hpp6HRLCA-B5ewKMW7Dq4B1SnsHF6BjsrZgz25C6yb-FNPgF_vyVS_otTp40vtS_aEHLBcYif4-X-oQZmxCM3L0tlDytat4s0K_O0DkSczNv2KgThygQuw9PhTRrOjFA-p0TKhtCHVaviBDzoHmNkkS30dksUO38pL_os7P_9Pd157cRzcMKPM0Ktx7Q-OPQwYLWiu12WDT8aYgVd2SA-shgJ-xsrZ4hwdv_4bDoipY95bU6ee5H5MQUf92HDEIRLSbYNIwiXR6pwtd-W_6qby_4431xXaZafuOLSFnp48luv7knL5XNE59uuu8b5QEAYRXp-HlJTz83_cEiETWxHjBv2r6_6voiXXMbqonyVuOAr8mKF27MZJeezgV7gtQOGN0TnUfFxpWOH0cwogM5KD8ZxfKjyIzz8qhv7Ogm_6uazsPEpy1up9-D3jgjv8nCg9eO8jcHxX2HPkxOxN4vZ4yUr6GtI706uo4X-wgr_Guv_IHtXKCcDHRFf5dnCXJTtWE7uK4o93pAm5uNL_v1l9901hEcSQY1qtbSZUswK8CDfq2uYKtJVx0ewuGb13zyCDUNO5zycwx_m5EUKq5UftrKOGLc6MeaHtfAi1pKdOTRtUw7D_O_3iZ_soG0fwSe9stB1acfaHegYyy05du_4Trih05nlIUh6yyF5BKfBcg7ipMAe1exrazDc9Ubx0AjN8vqtriEXhsln4Er3603oVjiQWj9YkOKBA2FMqDVYveWg3YYbaCVZWyC-QrDHTpcf2WoDfEe2neQX-o-QfMm98h_ozRZZ88vuD7apvNXCgXVCykEPt8epv-QQtTcRric37DZndV6TG36blXg2K6ZZiW82t9O6TeuqJU1as5rnLSunBU4Zr0lZTkld3ohbnOIirbMqLfM6xxOalbOS0mZakVnbMIyKlG-JkBP_Rnu0N6H7d5uldVpPbyRpuLShMY7xGAkYTe9uzG1oGjb92qIilcI6e5wiNML9oEUYMb3zlzhH07uhCwJPG67AmWfvBafBxwDx1h-6omiWHvutbkN8zPSSgdLnHdeb3sjbn29xjnY-3uL_BAAA___s1p4o">