Deducing this lambda causes crash when using explicit reference capture

[onelumin]

Compiler Explorer: https://godbolt.org/z/6vMEhdGqx

This little example causes the crash:

auto dothing(int num)
{
    auto fun =  [&num](this auto&& self) -> void {
        auto copy = num;
    };

    fun();
}

Interestingly if the signature of fun is changed to [&](this auto&& self) -> void, it works fine even though in both cases num is captured by reference.

Note that there is another issue at play since changing the explicit capture to the & catch all does not fix the code that I reduced this example from. Instead I still get a crash but this time a "cannot compile this l-value expression yet" from codegen.

clang++: /root/llvm-project/clang/lib/AST/ASTContext.cpp:2299: clang::TypeInfo clang::ASTContext::getTypeInfoImpl(const clang::Type*) const: Assertion `!A->getDeducedType().isNull() && "cannot request the size of an undeduced or dependent auto type"' failed.
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++23 <source>
1.	<eof> parser at end of file
2.	Per-file LLVM IR generation
3.	<source>:3:16: Generating code for declaration 'dothing(int)::(anonymous class)::operator()'
 #0 0x00000000037569a8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x37569a8)
 #1 0x000000000375466c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x375466c)
 #2 0x000000000369d018 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007efe146d7420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #4 0x00007efe1419a00b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b)
 #5 0x00007efe14179859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22859)
 #6 0x00007efe14179729 (/lib/x86_64-linux-gnu/libc.so.6+0x22729)
 #7 0x00007efe1418afd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
 #8 0x0000000006db4355 clang::ASTContext::getTypeInfoImpl(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db4355)
 #9 0x0000000006da2c27 clang::ASTContext::getTypeInfo(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da2c27)
#10 0x0000000006db570b clang::ASTContext::getPreferredTypeAlign(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db570b)
#11 0x0000000006db603b clang::ASTContext::getDeclAlign(clang::Decl const*, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db603b)
#12 0x0000000003e20310 clang::CodeGen::CodeGenFunction::EmitAutoVarAlloca(clang::VarDecl const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e20310)
#13 0x0000000003e2165b clang::CodeGen::CodeGenFunction::EmitVarDecl(clang::VarDecl const&) (.part.0) CGDecl.cpp:0:0
#14 0x0000000003e21900 clang::CodeGen::CodeGenFunction::EmitDecl(clang::Decl const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e21900)
#15 0x0000000003a64e07 clang::CodeGen::CodeGenFunction::EmitDeclStmt(clang::DeclStmt const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a64e07)
#16 0x0000000003a78d87 clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a78d87)
#17 0x0000000003a721d5 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a721d5)
#18 0x0000000003a786bc clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a786bc)
#19 0x0000000003ad7c96 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3ad7c96)
#20 0x0000000003aeacda clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3aeacda)
#21 0x0000000003b4bcc3 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b4bcc3)
#22 0x0000000003b46dc5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b46dc5)
#23 0x0000000003b51cee clang::CodeGen::CodeGenModule::EmitDeferred() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b51cee)
#24 0x0000000003b546f3 clang::CodeGen::CodeGenModule::Release() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b546f3)
#25 0x00000000049c1faa (anonymous namespace)::CodeGeneratorImpl::HandleTranslationUnit(clang::ASTContext&) ModuleBuilder.cpp:0:0
#26 0x00000000049c0975 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49c0975)
#27 0x0000000005ecf3b9 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5ecf3b9)
#28 0x00000000049c0068 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49c0068)
#29 0x0000000004223d29 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4223d29)
#30 0x00000000041a4e0e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41a4e0e)
#31 0x0000000004302cfe clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4302cfe)
#32 0x0000000000bf4a76 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbf4a76)
#33 0x0000000000bec33a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#34 0x0000000004000359 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
#35 0x000000000369d4c4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x369d4c4)
#36 0x000000000400094f 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
#37 0x0000000003fc8955 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fc8955)
#38 0x0000000003fc93bd 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+++0x3fc93bd)
#39 0x0000000003fd12e5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fd12e5)
#40 0x0000000000bf1f1c clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbf1f1c)
#41 0x0000000000aeb0f1 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xaeb0f1)
#42 0x00007efe1417b083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#43 0x0000000000bebe1e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbebe1e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: onelumin (onelumin)

Compiler Explorer: [https://godbolt.org/z/6vMEhdGqx](https://godbolt.org/z/6vMEhdGqx)

This little example causes the crash:

auto dothing(int num)
{
    auto fun =  [&num](this auto&& self) -> void {
        auto copy = num;
    };

    fun();
}

Interestingly if the signature of fun is changed to [&](this auto&& self) -> void, it works fine even though in both cases num is captured by reference.

Note that there is another issue at play since changing the explicit capture to the & catch all does not fix the code that I reduced this example from. Instead I still get a crash but this time a "cannot compile this l-value expression yet" from codegen.

clang++: /root/llvm-project/clang/lib/AST/ASTContext.cpp:2299: clang::TypeInfo clang::ASTContext::getTypeInfoImpl(const clang::Type*) const: Assertion `!A->getDeducedType().isNull() && "cannot request the size of an undeduced or dependent auto type"' failed.
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++23 <source>
1.	<eof> parser at end of file
2.	Per-file LLVM IR generation
3.	<source>:3:16: Generating code for declaration 'dothing(int)::(anonymous class)::operator()'
 #<!-- -->0 0x00000000037569a8 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x37569a8)
 #<!-- -->1 0x000000000375466c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x375466c)
 #<!-- -->2 0x000000000369d018 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x00007efe146d7420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #<!-- -->4 0x00007efe1419a00b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b)
 #<!-- -->5 0x00007efe14179859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22859)
 #<!-- -->6 0x00007efe14179729 (/lib/x86_64-linux-gnu/libc.so.6+0x22729)
 #<!-- -->7 0x00007efe1418afd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
 #<!-- -->8 0x0000000006db4355 clang::ASTContext::getTypeInfoImpl(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db4355)
 #<!-- -->9 0x0000000006da2c27 clang::ASTContext::getTypeInfo(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da2c27)
#<!-- -->10 0x0000000006db570b clang::ASTContext::getPreferredTypeAlign(clang::Type const*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db570b)
#<!-- -->11 0x0000000006db603b clang::ASTContext::getDeclAlign(clang::Decl const*, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db603b)
#<!-- -->12 0x0000000003e20310 clang::CodeGen::CodeGenFunction::EmitAutoVarAlloca(clang::VarDecl const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e20310)
#<!-- -->13 0x0000000003e2165b clang::CodeGen::CodeGenFunction::EmitVarDecl(clang::VarDecl const&amp;) (.part.0) CGDecl.cpp:0:0
#<!-- -->14 0x0000000003e21900 clang::CodeGen::CodeGenFunction::EmitDecl(clang::Decl const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3e21900)
#<!-- -->15 0x0000000003a64e07 clang::CodeGen::CodeGenFunction::EmitDeclStmt(clang::DeclStmt const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a64e07)
#<!-- -->16 0x0000000003a78d87 clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef&lt;clang::Attr const*&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a78d87)
#<!-- -->17 0x0000000003a721d5 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef&lt;clang::Attr const*&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a721d5)
#<!-- -->18 0x0000000003a786bc clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&amp;, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a786bc)
#<!-- -->19 0x0000000003ad7c96 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3ad7c96)
#<!-- -->20 0x0000000003aeacda clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3aeacda)
#<!-- -->21 0x0000000003b4bcc3 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b4bcc3)
#<!-- -->22 0x0000000003b46dc5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b46dc5)
#<!-- -->23 0x0000000003b51cee clang::CodeGen::CodeGenModule::EmitDeferred() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b51cee)
#<!-- -->24 0x0000000003b546f3 clang::CodeGen::CodeGenModule::Release() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b546f3)
#<!-- -->25 0x00000000049c1faa (anonymous namespace)::CodeGeneratorImpl::HandleTranslationUnit(clang::ASTContext&amp;) ModuleBuilder.cpp:0:0
#<!-- -->26 0x00000000049c0975 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49c0975)
#<!-- -->27 0x0000000005ecf3b9 clang::ParseAST(clang::Sema&amp;, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5ecf3b9)
#<!-- -->28 0x00000000049c0068 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49c0068)
#<!-- -->29 0x0000000004223d29 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4223d29)
#<!-- -->30 0x00000000041a4e0e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41a4e0e)
#<!-- -->31 0x0000000004302cfe clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4302cfe)
#<!-- -->32 0x0000000000bf4a76 cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbf4a76)
#<!-- -->33 0x0000000000bec33a ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->34 0x0000000004000359 void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const::'lambda'()&gt;(long) Job.cpp:0:0
#<!-- -->35 0x000000000369d4c4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x369d4c4)
#<!-- -->36 0x000000000400094f clang::driver::CC1Command::Execute(llvm::ArrayRef&lt;std::optional&lt;llvm::StringRef&gt;&gt;, std::__cxx11::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt;&gt;*, bool*) const (.part.0) Job.cpp:0:0
#<!-- -->37 0x0000000003fc8955 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fc8955)
#<!-- -->38 0x0000000003fc93bd clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fc93bd)
#<!-- -->39 0x0000000003fd12e5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&amp;, llvm::SmallVectorImpl&lt;std::pair&lt;int, clang::driver::Command const*&gt;&gt;&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fd12e5)
#<!-- -->40 0x0000000000bf1f1c clang_main(int, char**, llvm::ToolContext const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbf1f1c)
#<!-- -->41 0x0000000000aeb0f1 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xaeb0f1)
#<!-- -->42 0x00007efe1417b083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#<!-- -->43 0x0000000000bebe1e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xbebe1e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[shafik]

CC @cor3ntin

[cor3ntin]

This is probably easy enough to fix. unfortunately, I don't think I'll be able to look at it in the next 3 weeks.

[cygao90]

I looked into this issue. The assertion fails because inside lambda scope num is marked as type dependent, the type of copy cannot be duduced.

Here is where this type-dependent is marked:

llvm-project/clang/lib/Sema/SemaExpr.cpp

Lines 20688 to 20714 in 9c093cb

	// C++ [temp.dep.expr]p3:
	// An id-expression is type-dependent if it contains:
	// - an identifier associated by name lookup with an entity captured by copy
	// in a lambda-expression that has an explicit object parameter whose type
	// is dependent ([dcl.fct]),
	static void FixDependencyOfIdExpressionsInLambdaWithDependentObjectParameter(
	Sema &SemaRef, ValueDecl *D, Expr *E) {
	auto *ID = dyn_cast<DeclRefExpr>(E);
	if (!ID || ID->isTypeDependent())
	return;
	auto IsDependent = [&]() {
	const LambdaScopeInfo *LSI = SemaRef.getCurLambda();
	if (!LSI)
	return false;
	if (!LSI->ExplicitObjectParameter ||
	!LSI->ExplicitObjectParameter->getType()->isDependentType())
	return false;
	if (!LSI->CaptureMap.count(D))
	return false;
	const Capture &Cap = LSI->getCapture(D);
	return !Cap.isCopyCapture();
	}();
	ID->setCapturedByCopyInLambdaWithExplicitObjectParameter(
	IsDependent, SemaRef.getASTContext());
	}

According to the comment above. I think the last return statement in that lambda function should be return Cap.isCopyCapture()
Making this change fixes this assertion but will break one test case:

llvm-project/clang/test/SemaCXX/cxx2b-deducing-this.cpp

Lines 197 to 198 in 79524ba

	[i = 0](this const auto&){ i++; }();
	// expected-error@-1 {{cannot assign to a variable captured by copy in a non-mutable lambda}}

But I don't know how to fix it...

[Sirraide]

This is also just #84473. It’s probably a duplicate of the six-or-so other issues related to this, but I’ve candidly started losing track of which one is for what aspect of this bug, so I’m just going to mark this as fixed by that as well.