ICE on use of constexpr in lambda parameter type

[ldalessa]

Bugzilla Link	48056
Version	trunk
OS	All
CC	@zygoloid

Extended Description

Forgive me, this one's a bit hard to describe and/or search for, so it's possibly a duplicate.

https://godbolt.org/z/c1P9sr

#include <array>
#include <utility>

struct Foo {
    int n;
};

constexpr bool foo() {
    constexpr std::array<Foo, 2> foos = { 1, 2 };
    [&]<std::size_t... is>(std::index_sequence<is...>) {
        ([&](std::array<int, foos[is].n>) {}, ...);
    }(std::make_index_sequence<size(foos)>());
    return true;
}

static_assert(foo());

[ldalessa]

Was just checking on this and it no-longer ICEs on trunk, but it does fail to compile where gcc reports no errors. I'm not sure that I understand the clang diagnostic, so I'm not sure who is correct here.

[shafik]

This looks like a clang bug to: https://godbolt.org/z/6vc4no76e

Interesting to note this ICEs MSVC.

CC @cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[Endilll]

Trunk doesn't crash indeed, but trunk with assertions does: https://godbolt.org/z/naYodvxxf
Assertion:

clang++: /root/llvm-project/clang/lib/Sema/SemaExprCXX.cpp:8268:
void CheckIfAnyEnclosingLambdasMustCaptureAnyPotentialCaptures(clang::Expr*, clang::sema::LambdaScopeInfo*, clang::Sema&):
Assertion `CurrentLSI->CallOperator == DC && "The current call operator must be synchronized with Sema's CurContext"' failed.

Stack:

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.	<source>:11:19: at annotation token
2.	<source>:8:22: parsing function body 'foo'
3.	<source>:8:22: in compound statement ('{}')
4.	<source>:10:5: lambda expression parsing
5.	<source>:10:56: in compound statement ('{}')
6.	<source>:11:10: lambda expression parsing
 #0 0x00005604f847ddbf llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3c12dbf)
 #1 0x00005604f847bb2c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3c10b2c)
 #2 0x00005604f83c51a8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007f646ff23420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #4 0x00007f646f9f000b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b)
 #5 0x00007f646f9cf859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22859)
 #6 0x00007f646f9cf729 (/lib/x86_64-linux-gnu/libc.so.6+0x22729)
 #7 0x00007f646f9e0fd6 (/lib/x86_64-linux-gnu/libc.so.6+0x33fd6)
 #8 0x00005604fb267d02 clang::Sema::ActOnFinishFullExpr(clang::Expr*, clang::SourceLocation, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69fcd02)
 #9 0x00005604fb4d3b02 CheckConvertedConstantExpression(clang::Sema&, clang::Expr*, clang::QualType, clang::APValue&, clang::Sema::CCEKind, bool, clang::NamedDecl*) SemaOverload.cpp:0:0
#10 0x00005604fb4d42b2 clang::Sema::CheckConvertedConstantExpression(clang::Expr*, clang::QualType, clang::APValue&, clang::Sema::CCEKind, clang::NamedDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c692b2)
#11 0x00005604fb580236 clang::Sema::CheckTemplateArgument(clang::NonTypeTemplateParmDecl*, clang::QualType, clang::Expr*, clang::TemplateArgument&, clang::TemplateArgument&, clang::Sema::CheckTemplateArgumentKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d15236)
#12 0x00005604fb588faa clang::Sema::CheckTemplateArgument(clang::NamedDecl*, clang::TemplateArgumentLoc&, clang::NamedDecl*, clang::SourceLocation, clang::SourceLocation, unsigned int, llvm::SmallVectorImpl<clang::TemplateArgument>&, llvm::SmallVectorImpl<clang::TemplateArgument>&, clang::Sema::CheckTemplateArgumentKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d1dfaa)
#13 0x00005604fb58b511 clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo&, bool, llvm::SmallVectorImpl<clang::TemplateArgument>&, llvm::SmallVectorImpl<clang::TemplateArgument>&, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d20511)
#14 0x00005604fb58ea11 clang::Sema::CheckTemplateIdType(clang::TemplateName, clang::SourceLocation, clang::TemplateArgumentListInfo&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d23a11)
#15 0x00005604fb59154a clang::Sema::ActOnTemplateIdType(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::OpaquePtr<clang::TemplateName>, clang::IdentifierInfo*, clang::SourceLocation, clang::SourceLocation, llvm::MutableArrayRef<clang::ParsedTemplateArgument>, clang::SourceLocation, bool, bool, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d2654a)
#16 0x00005604facbdc8b clang::Parser::AnnotateTemplateIdTokenAsType(clang::CXXScopeSpec&, clang::ImplicitTypenameContext, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6452c8b)
#17 0x00005604fac02f66 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo const&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6397f66)
#18 0x00005604fac07ab4 clang::Parser::ParseParameterDeclarationClause(clang::DeclaratorContext, clang::ParsedAttributes&, llvm::SmallVectorImpl<clang::DeclaratorChunk::ParamInfo>&, clang::SourceLocation&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x639cab4)
#19 0x00005604fac51e46 clang::Parser::ParseLambdaExpressionAfterIntroducer(clang::LambdaIntroducer&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63e6e46)
#20 0x00005604fac523ab clang::Parser::ParseLambdaExpression() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63e73ab)
#21 0x00005604fac2f914 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c4914)
#22 0x00005604fac3138b clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c638b)
#23 0x00005604fac3143d clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c643d)
#24 0x00005604fac355fd clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ca5fd)
#25 0x00005604fac3983d clang::Parser::ParseParenExpression(clang::Parser::ParenParseOption&, bool, bool, clang::OpaquePtr<clang::QualType>&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ce83d)
#26 0x00005604fac2f4a6 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c44a6)
#27 0x00005604fac3138b clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c638b)
#28 0x00005604fac3143d clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c643d)
#29 0x00005604fac355fd clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ca5fd)
#30 0x00005604facb3a40 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6448a40)
#31 0x00005604faca9e13 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643ee13)
#32 0x00005604facaad11 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643fd11)
#33 0x00005604facabd09 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6440d09)
#34 0x00005604fac518e2 clang::Parser::ParseLambdaExpressionAfterIntroducer(clang::LambdaIntroducer&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63e68e2)
#35 0x00005604fac523ab clang::Parser::ParseLambdaExpression() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63e73ab)
#36 0x00005604fac2f914 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c4914)
#37 0x00005604fac3138b clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c638b)
#38 0x00005604fac3143d clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63c643d)
#39 0x00005604fac355fd clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63ca5fd)
#40 0x00005604facb3a40 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6448a40)
#41 0x00005604faca9e13 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643ee13)
#42 0x00005604facaad11 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x643fd11)
#43 0x00005604facabd09 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6440d09)
#44 0x00005604facad6ba clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x64426ba)
#45 0x00005604fabd8421 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x636d421)
#46 0x00005604fabff2f0 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x63942f0)
#47 0x00005604fabcbcc1 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6360cc1)
#48 0x00005604fabcc57f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (.part.0) Parser.cpp:0:0
#49 0x00005604fabd2f21 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6367f21)
#50 0x00005604fabd3896 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6368896)
#51 0x00005604fabc765a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x635c65a)
#52 0x00005604f96cb7c8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e607c8)
#53 0x00005604f8f22f09 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46b7f09)
#54 0x00005604f8ea73d6 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x463c3d6)
#55 0x00005604f90059e6 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x479a9e6)
#56 0x00005604f593946d cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x10ce46d)
#57 0x00005604f593516a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#58 0x00005604f8d0f39d 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
#59 0x00005604f83c56b0 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b5a6b0)
#60 0x00005604f8d0f9bf 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
#61 0x00005604f8cd70ec clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x446c0ec)
#62 0x00005604f8cd7b7d 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+++0x446cb7d)
#63 0x00005604f8cdfc5d clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4474c5d)
#64 0x00005604f59376ca clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x10cc6ca)
#65 0x00005604f583e235 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xfd3235)
#66 0x00007f646f9d1083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#67 0x00005604f592ff4e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x10c4f4e)

[shafik]

We have a dup here with a slightly smaller reproducer: #61596

[alinas]

Gentle ping on this issue?

[shafik]

CC @Fznamznon might be interesting

[Fznamznon]

Reduced to

 struct Foo {
 };

 template <int n, Foo f>
 struct Arr {
 };

 template <int n> struct s {
 };

 constexpr bool foo() {
     constexpr Foo fff;
     [&]<int is>(s<is>) {
         [&](Arr<is, fff>) {}; // error: reference to local variable 'fff' declared in enclosing function 'foo'
     }(s<1>{});

     return true;
 }

 static_assert(foo());

The problem is that we have a generic lambda and consider fff usage from inner lambda parameter list as its capture. At the point we meet fff usage we have inner lambda on the top of the stack of lambdas, but the Sema::CurContext points to outer lambda due to PopDeclContext() call at the end of Sema::ActOnLambdaExpressionAfterIntroducer(). Due to this mismatch we have the mentioned assertion fail when we later process "potential" capture. This mismatch also causes this "potential" capture never transformed to "real" capture and hence the error.
I'm kind of confused about what can be done here, probably it makes sense to consider fff usage as a capture of outer and not inner lambda but I'm not sure.

cc @cor3ntin , @AaronBallman , @shafik , @zygoloid in case you have some insights on how this can be properly fixed.

[zygoloid]

The relevant rule here is https://eel.is/c++draft/basic.def.odr#10.2.2 -- note in particular that we're only supposed to look through the function parameter scope of a lambda if the block scope of the lambda also intervenes between the point of declaration of fff and the point of use. Which is to say, fff is not odr-usable within the lambda's function prototype scope, so by https://eel.is/c++draft/expr.prim.lambda.capture#7 that mention of fff does not result in any lambda capture. (If that mention of fff results in odr-use, then the reference to fff is ill-formed, and if it does not result in odr-use, then it's fine. Either way no capture is required.)