Crash on evaluation consteval user defined conversion function

[shafik]

Given the following in C++20:

struct A {
   consteval A(){}
   consteval // comment out this line and no longer crashes
     operator int() { return 1;}
};

int main() {
   int x = A{};
}

godbolt: https://godbolt.org/z/sfTdPrj8h

This crashes with the following assert:

clang++: /root/llvm-project/clang/lib/Sema/SemaExprCXX.cpp:1453: clang::ExprResult clang::Sema::BuildCXXTypeConstructExpr(clang::TypeSourceInfo*, clang::SourceLocation, clang::MultiExprArg, clang::SourceLocation, bool): Assertion `(!ListInitialization || (Exprs.size() == 1 && isa<InitListExpr>(Exprs[0]))) && "List initialization must have initializer list as expression."' failed.

and backtrace:

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++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++20 <source>
1.	<eof> parser at end of file
2.	<source>:7:12: parsing function body 'main'
 #0 0x00005648cd7968a4 PrintStackTraceSignalHandler(void*) Signals.cpp:0:0
 #1 0x00005648cd79464c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3f9264c)
 #2 0x00005648cd6ca618 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007f61e1a50420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #4 0x00007f61e151d00b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b)
 #5 0x00007f61e14fc859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22859)
 #6 0x00007f61e14fc729 (/lib/x86_64-linux-gnu/libc.so.6+0x22729)
 #7 0x00007f61e150dfd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
 #8 0x00005648d031016f clang::Sema::BuildCXXTypeConstructExpr(clang::TypeSourceInfo*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b0e16f)
 #9 0x00005648d02a7ab9 clang::TreeTransform<RemoveNestedImmediateInvocation(clang::Sema&, clang::Sema::ExpressionEvaluationContextRecord&, std::reverse_iterator<llvm::PointerIntPair<clang::ConstantExpr*, 1u, unsigned int, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>, llvm::PointerIntPairInfo<clang::ConstantExpr*, 1u, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>>>*>)::ComplexRemove>::TransformCXXFunctionalCastExpr(clang::CXXFunctionalCastExpr*) SemaExpr.cpp:0:0
#10 0x00005648d02a9928 clang::TreeTransform<RemoveNestedImmediateInvocation(clang::Sema&, clang::Sema::ExpressionEvaluationContextRecord&, std::reverse_iterator<llvm::PointerIntPair<clang::ConstantExpr*, 1u, unsigned int, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>, llvm::PointerIntPairInfo<clang::ConstantExpr*, 1u, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>>>*>)::ComplexRemove>::TransformMemberExpr(clang::MemberExpr*) SemaExpr.cpp:0:0
#11 0x00005648d02a68d6 clang::TreeTransform<RemoveNestedImmediateInvocation(clang::Sema&, clang::Sema::ExpressionEvaluationContextRecord&, std::reverse_iterator<llvm::PointerIntPair<clang::ConstantExpr*, 1u, unsigned int, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>, llvm::PointerIntPairInfo<clang::ConstantExpr*, 1u, llvm::PointerLikeTypeTraits<clang::ConstantExpr*>>>*>)::ComplexRemove>::TransformCallExpr(clang::CallExpr*) SemaExpr.cpp:0:0
#12 0x00005648d0237dd8 clang::Sema::PopExpressionEvaluationContext() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a35dd8)
#13 0x00005648d0067dc0 clang::Sema::ActOnFinishFunctionBody(clang::Decl*, clang::Stmt*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6865dc0)
#14 0x00005648cfdf7b6d clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65f5b6d)
#15 0x00005648cfd283c1 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65263c1)
#16 0x00005648cfd54f90 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6552f90)
#17 0x00005648cfd1cf92 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x651af92)
#18 0x00005648cfd1d7f0 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (.part.0) Parser.cpp:0:0
#19 0x00005648cfd22feb clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6520feb)
#20 0x00005648cfd23e7a clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6521e7a)
#21 0x00005648cfd1812a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x651612a)
#22 0x00005648ceab8ed8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52b6ed8)
#23 0x00005648ce34e409 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b4c409)
#24 0x00005648ce2d581e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ad381e)
#25 0x00005648ce433af3 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c31af3)
#26 0x00005648cac3bca4 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x1439ca4)
#27 0x00005648cac34dd8 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) driver.cpp:0:0
#28 0x00005648ce1439a9 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#29 0x00005648cd6cadba llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3ec8dba)
#30 0x00005648ce1441ff clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#31 0x00005648ce10dbf9 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x490bbf9)
#32 0x00005648ce10e67d 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+++0x490c67d)
#33 0x00005648ce11819c clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x491619c)
#34 0x00005648cac3a083 clang_main(int, char**) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x1438083)
#35 0x00007f61e14fe083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#36 0x00005648cac349ee _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x14329ee)
clang-16: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[shafik]

This one looks like a bad assumption in Sema::BuildCXXTypeConstructExpr which is assumes that list init must have a InitListExpr but in this case int x = A{} is still list initialization but it is a CXXTemporaryObjectExpr and there is note at the bottom that this is an odd case but represents the functional style cast. So I think the AST is correct and the assert is wrong but I have not managed to convince myself that the other places in the code we do an unconditional cast to InitListExpr are not reachable in this case.

It is interested we only hit this case b/c we have an immediate invocation and we do a RemoveNestedImmediateInvocation so if you remove the consteval from the conversion function we won't hit this case. Although I am wondering if there is a non-consteval path here.

Note that expr.type.conv tells us both () and {} are allowed for a functional conversion and dcl.init.list tells us this is list init.

[shafik]

PR: https://reviews.llvm.org/D138947