<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/140130>140130</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang assertion failure in ASTStmtReader::VisitConstantExpr()
        </td>
    </tr>

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

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

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

<pre>
    Clang crashes when reading AST with constexpr structs containing unions. A test case reduced from real-world code:
```
$ split-file - q
//--- a.cppmap
module "a" {
 header "a.h"
}
//--- a.h
#include "b.h"
//--- b.h
#ifndef _B_H_
#define _B_H_
struct B {
  consteval B() {}
  union {
    int a;
  };
};
constexpr B b;
#endif
//--- c.cppmap
module "c" {
header "c.h"
}
//--- c.h
#include "b.h"
^D
$ cd q
$ $CLANG -iquote . -fmodule-name=c -Xclang=-fno-cxx-modules -Xclang=-fmodule-map-file-home-is-cwd -xc++ -Xclang=-emit-module -fmodules -fno-implicit-modules -fno-implicit-module-maps -std=c++20 -c c.cppmap -o c.pcm && $CLANG -iquote . -fmodule-name=a -fmodule-map-file=a.cppmap -Xclang=-fno-cxx-modules -Xclang=-fmodule-map-file-home-is-cwd -xc++ -Xclang=-emit-module -fmodules -fno-implicit-modules -fno-implicit-module-maps -Xclang=-fmodule-file=c.pcm -std=c++20 -c a.cppmap -o a.pcm
assert.h assertion failed at llvm/include/llvm/Support/Casting.h:109 in static bool llvm::isa_impl_cl<clang::FieldDecl, const clang::Decl *>::doit(const From *) [To = clang::FieldDecl, From = const clang::Decl *]: Val && "isa<> used on a null pointer"
    @     0x55c79770988a  __assert_fail
    @ 0x55c7878e6fb0  llvm::isa_impl_cl<>::doit()
    @     0x55c7878e6f38 llvm::isa_impl_wrap<>::doit()
    @     0x55c7878e6f15 llvm::isa_impl_wrap<>::doit()
    @     0x55c7878e6e88 llvm::CastIsPossible<>::isPossible()
    @     0x55c7878e6e55 llvm::CastInfo<>::isPossible()
    @     0x55c7878e6db8  llvm::isa<>()
 @     0x55c7878e7230  llvm::cast<>()
    @     0x55c78a88b77c clang::serialization::BasicReaderBase<>::readAPValue()
    @ 0x55c78a891260  clang::serialization::ReadDispatcher<>::read<>()
    @ 0x55c78a88f2bd clang::serialization::DataStreamBasicReader<>::readArray<>()
    @ 0x55c78a88b399  clang::serialization::BasicReaderBase<>::readAPValue()
    @ 0x55c78a9eaaed  clang::ASTStmtReader::VisitConstantExpr()
    @ 0x55c78aa186f4  clang::StmtVisitorBase<>::Visit()
    @     0x55c78aa0a095 clang::ASTReader::ReadStmtFromStream()
    @     0x55c78a7e4a2f clang::ASTReader::GetExternalDeclStmt()
    @     0x55c78a7e4ac1 clang::ASTReader::GetExternalDeclStmt()
    @     0x55c78d7583bf clang::LazyOffsetPtr<>::get()
    @     0x55c78d742662 clang::VarDecl::getInit()
    @     0x55c78a982e92 clang::ASTReader::PassInterestingDeclsToConsumer()
    @     0x55c78a7f678e clang::ASTReader::FinishedDeserializing()
    @     0x55c78a7f67b9 clang::ASTReader::FinishedDeserializing()
    @     0x55c78a811032 clang::ExternalASTSource::Deserializing::~Deserializing()
    @ 0x55c78a7e6cdc  clang::ASTReader::FindExternalVisibleDeclsByName()
    @ 0x55c78a7e6d41  clang::ASTReader::FindExternalVisibleDeclsByName()
    @ 0x55c78d77c4fc  clang::DeclContext::lookupImpl()
    @     0x55c78d77c0d3 clang::DeclContext::lookup()
    @     0x55c78b9bba58  LookupDirect()
    @ 0x55c78b9b72d5  CppNamespaceLookup()
    @     0x55c78b9b6656 clang::Sema::CppLookupName()
    @     0x55c78b9bb787 clang::Sema::LookupName()
    @     0x55c78b137aae clang::Sema::HandleDeclarator()
    @     0x55c78b136c99 clang::Sema::ActOnDeclarator()
    @     0x55c78a6959da clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes()
    @ 0x55c78a694413  clang::Parser::ParseDeclGroup()
    @     0x55c78a5ab674 clang::Parser::ParseDeclOrFunctionDefInternal()
    @     0x55c78a5aa4b1 clang::Parser::ParseDeclarationOrFunctionDefinition()
    @ 0x55c78a5a9a4c  clang::Parser::ParseExternalDeclaration()
    @ 0x55c78a5a6ec1  clang::Parser::ParseTopLevelDecl()
    @     0x55c78a59f616 clang::ParseAST()
    @     0x55c789f41475 clang::ASTFrontendAction::ExecuteAction()
    @     0x55c789f40a2b clang::FrontendAction::Execute()
    @     0x55c789deca4d clang::CompilerInstance::ExecuteAction()
    @     0x55c78785dea5 clang::ExecuteCompilerInvocation()
    @     0x55c78782b668 cc1_main()
```
</pre>
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