[Clang] Normalize constraints before checking for satisfaction

[cor3ntin]

In the standard, constraint satisfaction checking is done on the normalized form of a constraint.

Clang instead substitutes on the non-normalized form, which causes us to report substitution failures in template arguments or concept ids, which is non-conforming but unavoidable without a parameter mapping

This patch normalizes before satisfaction checking. However, we preserve concept-id nodes in the normalized form, solely for diagnostics purposes.

This addresses #61811 and related concepts conformance bugs, ideally to make the remaining implementation of concept template parameters easier

Fixes #135190
Fixes #61811

Co-authored-by: Younan Zhang [email protected]

[zmodem]

We're seeing significant (16%) build time increases after this revision. I'll try to come up with some kind of repro, but wanted to give a heads up.

[zmodem]

Here's a random translation unit where compile time increased about 10%: plugin_service_impl.ii.gz

clang++ -cc1 -triple x86_64-unknown-linux-gnu -O0 -emit-obj -mrelocation-model pic -pic-level 2 -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -target-feature +sse3 -tune-cpu generic -debug-info-kind=constructor -dwarf-version=4 -debugger-tuning=gdb -ggnu-pubnames -gsimple-template-names=simple -std=c++20 -fdeprecated-macro -fvisibility=hidden -fvisibility-inlines-hidden -fwrapv -fwrapv-pointer -pthread -stack-protector 1 -ftrivial-auto-var-init=pattern -fno-rtti -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fno-sized-deallocation -Qn -fcomplete-member-pointers -faddrsig -w /tmp/plugin_service_impl.ii

[cor3ntin]

@zmodem Thanks, I'll investigate.

Note that we expected some performance degradation on very concept heavy codebase without having a good way to quantify it - and it's not clear how much of that we can claw back, but we will certainly try to improve that as much as we can - I certainly agree that 10-15% is a lot

There are still places where our concept implementations is non-conforming so in theory we would have to make the performance worse still, but these non-conformances are less observables so we'll try to focus on improving performances before doing anything else to our concepts implementation.

[rnk]

I discussed this with Hans today, and we think one way to evaluate the performance impact would be to bootstrap clang itself with C++20 enabled. I think Chromium, and probably most C++ code in the wild, isn't using C++20 concepts heavily, and is mostly picking them up through libc++.

I really would not want to eat a 16% C++ compile time regression on normal C++ code. As a point of comparison, we're using 32-bit source locations to avoid a 4% peak memory usage increase with 0% measurable regression, which is very conservative.

It's hard, though, since it's a performance vs conformance (arguably correctness) tradeoff, it's just not a correctness regression, which we usually weight very highly when making tradeoff decisions. :)

[rupprecht]

I bisected a breakage to this commit, and while I'm still unsure whether the code is breaking is valid or not, it's behaving unexpectedly: the breakage is affected by other template instantiations. I suspect something is wrong with this patch. Reduced/filed as #164750.

[zyn0217]

I bisected a breakage to this commit, and while I'm still unsure whether the code is breaking is valid or not, it's behaving unexpectedly: the breakage is affected by other template instantiations. I suspect something is wrong with this patch. Reduced/filed as #164750.

@rupprecht #164777 will fix it. Thanks for the heads up!

[zmodem]

Here's another example TU. perf stat -r5 shows a 27% regression here. 72616c5 does not seem to make much difference.

clang -cc1 -triple x86_64-unknown-linux-gnu -O0 -emit-obj -mrelocation-model pic -pic-level 2 -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -target-feature +sse3 -tune-cpu generic -debug-info-kind=constructor -dwarf-version=4 -debugger-tuning=gdb -ggnu-pubnames -gsimple-template-names=simple -mllvm -generate-arange-section -debug-forward-template-params  -std=c++20 -fdeprecated-macro -fvisibility=hidden -fvisibility-inlines-hidden -fwrapv -fwrapv-pointer -pthread -stack-protector 1 -ftrivial-auto-var-init=pattern -fno-rtti -fgnuc-version=4.2.1 -fno-implicit-modules -fskip-odr-check-in-gmf -fno-sized-deallocation -Qn -fuse-ctor-homing -fcomplete-member-pointers -faddrsig -x c++ /tmp/inspector_style_resolver.ii -w

inspector_style_resolver.ii.gz

[zmodem]

one way to evaluate the performance impact would be to bootstrap clang itself with C++20 enabled.

I looked into this, building a stage0 clang and libc++ configured with:

$ cmake -GNinja -Bstage0 -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=OFF -DLLVM_ENABLE_PROJECTS=clang -DLLVM_ENABLE_RUNTIMES='compiler-rt;libcxx;libcxxabi;libunwind' -DLLVM_TARGETS_TO_BUILD=X86 llvm/

and then using that to do a debug shared-library build of clang in c++20 mode:

$ CC=$PWD/stage0/bin/clang CXX=$PWD/stage0/bin/clang++ CXXFLAGS=-stdlib=libc++ cmake -GNinja -Bstage1 -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_STANDARD=20 -DLLVM_USE_LINKER=lld -DBUILD_SHARED_LIBS=ON -DLLVM_ENABLE_PROJECTS=clang -DLLVM_TARGETS_TO_BUILD=X86 llvm/
$ mkdir -p stage1/lib && cp stage0/lib/x86_64-unknown-linux-gnu/*.so* stage1/lib/
$ rm -f stage1/.ninja_log && time ninja -C stage1 clang

The increase from this change is small but measurable: about 5 CPU-minutes, or 1.5 %.

I suppose it makes sense that the difference is smaller, since this codebase has no use of concepts at all besides via the STL, as opposed to Chromium which does seem to have a fair amount of concepts use in its base library, Abseil and so on.

---

I'm not sure how to proceed here. A 16% build time increase (as mentioned up-thread) is very significant for us.

@AaronBallman do you have any thoughts here?

I did see that #61811 mention "We should get performance data for that change and give feedback to WG21 if it looks like this is going to have a significant performance impact." So it sounds like this was somewhat anticipated?

In the standard, constraint satisfaction checking is done on the normalized form of a constraint.

Clang instead substitutes on the non-normalized form, which causes us to report substitution failures in template arguments or concept ids, which is non-conforming but unavoidable without a parameter mapping

I don't know much about concepts, but is there some way we could keep the old, fast, code path for most cases, and only use the new more expensive logic when that's not sufficient?

[AaronBallman]

I'm not sure how to proceed here. A 16% build time increase (as mentioned up-thread) is very significant for us.

@AaronBallman do you have any thoughts here?

16% is a bigger performance regression than I think we were anticipating.

I did see that #61811 mention "We should get performance data for that change and give feedback to WG21 if it looks like this is going to have a significant performance impact." So it sounds like this was somewhat anticipated?

I think we expected a performance hit, but more like 1-3%, not 10+%; that's significant enough that if we can't solve it, it should be feedback given to WG21 because that feels like too much of a regression to swallow. CC @cor3ntin (it may be a while before he responds, the dev conference is next week and WG21 meetings are the week after).

[cor3ntin]

Note that I already pushed something that reduces the performance cost by a few percents. both me and Younan will keep exploring additional mitigations in the coming weeks.

We think there are specific expressions which are unexpectedly costly, we are still trying to isolate exactly the problematic cases.

[zmodem]

Thanks both! See you at the dev meeting :)

[alexfh]

Hi @cor3ntin , we've started seeing Clang crashes after this commit. They only reproduce with our Clang header modules setup on a pretty large input. Reduction has been crawling slowly, so I don't expect to get a nice little test case soon (or ever). The crashes are sporadic, but reproduce more or less consistently when clang is built with msan:

==4090==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x558de04647e5 in clang::Sema::getNormalizedAssociatedConstraints(llvm::PointerUnion<clang::NamedDecl const*, clang::concepts::NestedRequirement const*>, llvm::ArrayRef<clang::AssociatedConstraint>) third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:2413:1
    #1 0x558de045b248 in CheckConstraintSatisfaction(clang::Sema&, clang::NamedDecl const*, llvm::ArrayRef<clang::AssociatedConstraint>, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&, clang::Expr**, clang::ConceptReference const*) third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:1149:9
    #2 0x558de045aa66 in clang::Sema::CheckConstraintSatisfaction(llvm::PointerUnion<clang::NamedDecl const*, clang::concepts::NestedRequirement const*>, llvm::ArrayRef<clang::AssociatedConstraint>, clang::MultiLevelTemplateArgumentList const&, clang::SourceRange, clang::ConstraintSatisfaction&, clang::ConceptReference const*, clang::Expr**) third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:1221:7
    #3 0x558de046112e in CheckFunctionConstraintsWithoutInstantiation third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:1670:18
    #4 0x558de046112e in clang::Sema::CheckFunctionTemplateConstraints(clang::SourceLocation, clang::FunctionDecl*, llvm::ArrayRef<clang::TemplateArgument>, clang::ConstraintSatisfaction&) third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:1682:12
    #5 0x558de151791e in clang::Sema::FinishTemplateArgumentDeduction(clang::FunctionTemplateDecl*, llvm::SmallVectorImpl<clang::DeducedTemplateArgument>&, unsigned int, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, llvm::SmallVectorImpl<clang::Sema::OriginalCallArg> const*, bool, bool, bool, llvm::function_ref<bool (bool)>) third_party/llvm/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:3995:9
    #6 0x558de1600704 in operator() third_party/llvm/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:4736:14
    #7 0x558de1600704 in void llvm::function_ref<void ()>::callback_fn<clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, bool, bool, bool, clang::QualType, clang::Expr::Classification, bool, llvm::function_ref<bool (llvm::ArrayRef<clang::QualType>, bool)>)::$_2>(long) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:12
    #8 0x558de347d9cc in operator() third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:12
    #9 0x558de347d9cc in runWithSufficientStackSpace third_party/llvm/llvm-project/clang/include/clang/Basic/Stack.h:49:7
    #10 0x558de347d9cc in clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) third_party/llvm/llvm-project/clang/lib/Basic/StackExhaustionHandler.cpp:20:3
    #11 0x558de0154faf in clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) third_party/llvm/llvm-project/clang/lib/Sema/Sema.cpp:629:16
    #12 0x558de1520980 in clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, clang::FunctionDecl*&, clang::sema::TemplateDeductionInfo&, bool, bool, bool, clang::QualType, clang::Expr::Classification, bool, llvm::function_ref<bool (llvm::ArrayRef<clang::QualType>, bool)>) third_party/llvm/llvm-project/clang/lib/Sema/SemaTemplateDeduction.cpp:4735:3
    #13 0x558de11f5306 in AddTemplateOverloadCandidateImmediately(clang::Sema&, clang::OverloadCandidateSet&, clang::FunctionTemplateDecl*, clang::DeclAccessPair, clang::TemplateArgumentListInfo*, llvm::ArrayRef<clang::Expr*>, bool, bool, bool, clang::CallExpr::ADLCallKind, clang::OverloadCandidateParamOrder, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaOverload.cpp:8012:42
    #14 0x558de1217b63 in AddTemplateOverloadCandidate third_party/llvm/llvm-project/clang/lib/Sema/SemaOverload.cpp:11242:3
    #15 0x558de1217b63 in clang::OverloadCandidateSet::InjectNonDeducedTemplateCandidates(clang::Sema&) third_party/llvm/llvm-project/clang/lib/Sema/SemaOverload.cpp:11264:7
    #16 0x558de11e67f7 in clang::OverloadCandidateSet::BestViableFunction(clang::Sema&, clang::SourceLocation, clang::OverloadCandidate*&) third_party/llvm/llvm-project/clang/lib/Sema/SemaOverload.cpp:11360:3
    #17 0x558de0df66ec in clang::Sema::LookupSpecialMember(clang::CXXRecordDecl*, clang::CXXSpecialMemberKind, bool, bool, bool, bool, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaLookup.cpp:3578:15
    #18 0x558de089cceb in lookupCallFromSpecialMember(clang::Sema&, clang::CXXRecordDecl*, clang::CXXSpecialMemberKind, unsigned int, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:7412:12
    #19 0x558de081e4ff in findTrivialSpecialMember third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10142:9
    #20 0x558de081e4ff in checkTrivialSubobjectCall(clang::Sema&, clang::SourceLocation, clang::QualType, bool, clang::CXXSpecialMemberKind, TrivialSubobjectKind, clang::TrivialABIHandling, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10210:7
    #21 0x558de08214b1 in checkTrivialClassMembers(clang::Sema&, clang::CXXRecordDecl*, clang::CXXSpecialMemberKind, bool, clang::TrivialABIHandling, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10292:10
    #22 0x558de08207fe in clang::Sema::SpecialMemberIsTrivial(clang::CXXMethodDecl*, clang::CXXSpecialMemberKind, clang::TrivialABIHandling, bool) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10419:8
    #23 0x558de0825dc3 in clang::Sema::DeclareImplicitMoveConstructor(clang::CXXRecordDecl*) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:16034:13
    #24 0x558de0822aef in clang::Sema::AddImplicitlyDeclaredMembersToClass(clang::CXXRecordDecl*) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10817:9
    #25 0x558de06945a2 in clang::Sema::ActOnFields(clang::Scope*, clang::SourceLocation, clang::Decl*, llvm::ArrayRef<clang::Decl*>, clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributesView const&) third_party/llvm/llvm-project/clang/lib/Sema/SemaDecl.cpp:19889:7
    #26 0x558de082259b in clang::Sema::ActOnFinishCXXMemberSpecification(clang::Scope*, clang::SourceLocation, clang::Decl*, clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributesView const&) third_party/llvm/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:10729:3
    #27 0x558ddfa83d11 in clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:3768:13
    #28 0x558ddfa7b9e8 in clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:2174:7
    #29 0x558ddfadf814 in clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) third_party/llvm/llvm-project/clang/lib/Parse/ParseDecl.cpp:4459:7
    #30 0x558ddfa8d77a in ParseDeclarationSpecifiers third_party/llvm/llvm-project/clang/include/clang/Parse/Parser.h:1842:12
    #31 0x558ddfa8d77a in clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:2864:3
    #32 0x558ddfa94bea in clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp
    #33 0x558ddfa839cb in clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:3754:7
    #34 0x558ddfa7b9e8 in clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:2174:7
    #35 0x558ddfadf814 in clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) third_party/llvm/llvm-project/clang/lib/Parse/ParseDecl.cpp:4459:7
    #36 0x558ddf992549 in ParseDeclarationSpecifiers third_party/llvm/llvm-project/clang/include/clang/Parse/Parser.h:1842:12
    #37 0x558ddf992549 in clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1091:3
    #38 0x558ddf991bb9 in clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1209:12
    #39 0x558ddf98f281 in clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1032:14
    #40 0x558ddfa63f04 in clang::Parser::ParseInnerNamespace(llvm::SmallVector<clang::Parser::InnerNamespaceInfo, 4u> const&, unsigned int, clang::SourceLocation&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:240:7
    #41 0x558ddfa614c7 in clang::Parser::ParseNamespace(clang::DeclaratorContext, clang::SourceLocation&, clang::SourceLocation) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:218:3
    #42 0x558ddfad1334 in clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDecl.cpp
    #43 0x558ddf98e4bd in clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp
    #44 0x558ddfa63f04 in clang::Parser::ParseInnerNamespace(llvm::SmallVector<clang::Parser::InnerNamespaceInfo, 4u> const&, unsigned int, clang::SourceLocation&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:240:7
    #45 0x558ddfa614c7 in clang::Parser::ParseNamespace(clang::DeclaratorContext, clang::SourceLocation&, clang::SourceLocation) third_party/llvm/llvm-project/clang/lib/Parse/ParseDeclCXX.cpp:218:3
    #46 0x558ddfad1334 in clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) third_party/llvm/llvm-project/clang/lib/Parse/ParseDecl.cpp
    #47 0x558ddf98e4bd in clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp
    #48 0x558ddf989c0d in clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) third_party/llvm/llvm-project/clang/lib/Parse/Parser.cpp:745:12
    #49 0x558ddf96aeee in clang::ParseAST(clang::Sema&, bool, bool) third_party/llvm/llvm-project/clang/lib/Parse/ParseAST.cpp:170:20
    #50 0x558ddf2278c9 in clang::ASTFrontendAction::ExecuteAction() third_party/llvm/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1431:3
    #51 0x558ddd1246be in clang::CodeGenAction::ExecuteAction() third_party/llvm/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:1109:30
    #52 0x558ddf226532 in clang::FrontendAction::Execute() third_party/llvm/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1311:3
    #53 0x558ddf0acc76 in clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) third_party/llvm/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1008:33
    #54 0x558ddd1108bc in clang::ExecuteCompilerInvocation(clang::CompilerInstance*) third_party/llvm/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:310:25
    #55 0x558ddd0e9923 in cc1_main(llvm::ArrayRef<char const*>, char const*, void*) third_party/llvm/llvm-project/clang/tools/driver/cc1_main.cpp:300:15
    #56 0x558ddd0dee70 in ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>) third_party/llvm/llvm-project/clang/tools/driver/driver.cpp:225:12
    #57 0x558ddd0e60d3 in operator() third_party/llvm/llvm-project/clang/tools/driver/driver.cpp:369:12
    #58 0x558ddd0e60d3 in int llvm::function_ref<int (llvm::SmallVectorImpl<char const*>&)>::callback_fn<clang_main(int, char**, llvm::ToolContext const&)::$_0>(long, llvm::SmallVectorImpl<char const*>&) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:12
    #59 0x558ddf5749e1 in operator() third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:12
    #60 0x558ddf5749e1 in operator() third_party/llvm/llvm-project/clang/lib/Driver/Job.cpp:437:34
    #61 0x558ddf5749e1 in void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::__msan::optional<llvm::StringRef>>, std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char>>*, bool*) const::$_0>(long) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:12
    #62 0x558deccf8397 in llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) () (
    #63 0x558ddf570ff9 in clang::driver::CC1Command::Execute(llvm::ArrayRef<std::__msan::optional<llvm::StringRef>>, std::__msan::basic_string<char, std::__msan::char_traits<char>, std::__msan::allocator<char>>*, bool*) const third_party/llvm/llvm-project/clang/lib/Driver/Job.cpp:437:12
    #64 0x558ddf4b9237 in clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const third_party/llvm/llvm-project/clang/lib/Driver/Compilation.cpp:196:15
    #65 0x558ddf4b980e in clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::__msan::pair<int, clang::driver::Command const*>>&, bool) const third_party/llvm/llvm-project/clang/lib/Driver/Compilation.cpp:246:19
    #66 0x558ddf503068 in clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::__msan::pair<int, clang::driver::Command const*>>&) third_party/llvm/llvm-project/clang/lib/Driver/Driver.cpp:2241:5
    #67 0x558ddd0dd2fa in clang_main(int, char**, llvm::ToolContext const&) third_party/llvm/llvm-project/clang/tools/driver/driver.cpp:407:21
    #68 0x558ddd0d8d6f in main llvm-project/clang/clang-driver.cpp:17:10
    #69 0x7f1225bb1351 in __libc_start_main ()
    #70 0x558ddd049fe9 in _start start.S:120

  Uninitialized value was created by a heap deallocation
    #0 0x558ddd0d8a89 in operator delete(void*, unsigned long, std::align_val_t) third_party/llvm/llvm-project/compiler-rt/lib/msan/msan_new_delete.cpp:108:3
    #1 0x558decd6f8b8 in llvm::deallocate_buffer(void*, unsigned long, unsigned long) () (BuildId: 95156bac43bc56e46756d69535047519)
    #2 0x558ddfeb30ac in deallocateBuckets third_party/llvm/llvm-project/llvm/include/llvm/ADT/DenseMap.h:834:5
    #3 0x558ddfeb30ac in ~DenseMap third_party/llvm/llvm-project/llvm/include/llvm/ADT/DenseMap.h:744:5
    #4 0x558ddfeb30ac in clang::ASTReader::ReadStmtFromStream(clang::serialization::ModuleFile&) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderStmt.cpp:4548:1
    #5 0x558ddfead686 in clang::ASTReader::ReadStmt(clang::serialization::ModuleFile&) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderStmt.cpp:2987:12
    #6 0x558ddfd8a838 in readStmt third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:123:37
    #7 0x558ddfd8a838 in readStmtRef third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:124:32
    #8 0x558ddfd8a838 in readExprRef llvm-project/clang/include/clang/AST/AbstractBasicReader.inc:933:40
    #9 0x558ddfd8a838 in clang::serialization::AbstractTypeReader<clang::ASTRecordReader>::readDecltypeType() llvm-project/clang/include/clang/AST/AbstractTypeReader.inc:325:45
    #10 0x558ddfcc9ce5 in clang::serialization::AbstractTypeReader<clang::ASTRecordReader>::read(clang::Type::TypeClass) llvm-project/clang/include/clang/AST/AbstractTypeReader.inc:50:14
    #11 0x558ddfcc9535 in clang::ASTReader::readTypeRecord(unsigned long) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7197:21
    #12 0x558ddfcb8184 in clang::ASTReader::GetType(unsigned long) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7931:26
    #13 0x558ddfd46ed8 in getLocalType third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7945:10
    #14 0x558ddfd46ed8 in clang::ASTReader::readType(clang::serialization::ModuleFile&, llvm::SmallVector<unsigned long, 64u> const&, unsigned int&) third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTReader.h:2119:12
    #15 0x558ddfd8d76c in readType third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:175:20
    #16 0x558ddfd8d76c in readQualType third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:178:12
    #17 0x558ddfd8d76c in clang::serialization::AbstractTypeReader<clang::ASTRecordReader>::readFunctionProtoType() llvm-project/clang/include/clang/AST/AbstractTypeReader.inc:451:48
    #18 0x558ddfcc9cb9 in clang::serialization::AbstractTypeReader<clang::ASTRecordReader>::read(clang::Type::TypeClass) llvm-project/clang/include/clang/AST/AbstractTypeReader.inc:68:14
    #19 0x558ddfcc9535 in clang::ASTReader::readTypeRecord(unsigned long) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7197:21
    #20 0x558ddfcb8184 in clang::ASTReader::GetType(unsigned long) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7931:26
    #21 0x558ddfd46ed8 in getLocalType third_party/llvm/llvm-project/clang/lib/Serialization/ASTReader.cpp:7945:10
    #22 0x558ddfd46ed8 in clang::ASTReader::readType(clang::serialization::ModuleFile&, llvm::SmallVector<unsigned long, 64u> const&, unsigned int&) third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTReader.h:2119:12
    #23 0x558ddfdd731c in readType third_party/llvm/llvm-project/clang/include/clang/Serialization/ASTRecordReader.h:175:20
    #24 0x558ddfdd731c in clang::ASTDeclReader::VisitDeclaratorDecl(clang::DeclaratorDecl*) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:911:29
    #25 0x558ddfdd90cb in clang::ASTDeclReader::VisitFunctionDecl(clang::FunctionDecl*) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:1026:3
    #26 0x558ddfdcfdb7 in clang::declvisitor::Base<std::__msan::add_pointer, clang::ASTDeclReader, void>::Visit(clang::Decl*) llvm-project/clang/include/clang/AST/DeclNodes.inc:256:1
    #27 0x558ddfdcf435 in clang::ASTDeclReader::Visit(clang::Decl*) third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:527:37
    #28 0x558ddfe5e5ca in operator() third_party/llvm/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:4249:53
    #29 0x558ddfe5e5ca in void llvm::function_ref<void ()>::callback_fn<clang::ASTReader::ReadDeclRecord(clang::GlobalDeclID)::$_1>(long) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:12
    #30 0x558de347d9cc in operator() third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:69:12
    #31 0x558de347d9cc in runWithSufficientStackSpace third_party/llvm/llvm-project/clang/include/clang/Basic/Stack.h:49:7
    #32 0x558de347d9cc in clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) third_party/llvm/llvm-project/clang/lib/Basic/StackExhaustionHandler.cpp:20:3

SUMMARY: MemorySanitizer: use-of-uninitialized-value third_party/llvm/llvm-project/clang/lib/Sema/SemaConcept.cpp:2413:1 in clang::Sema::getNormalizedAssociatedConstraints(llvm::PointerUnion<clang::NamedDecl const*, clang::concepts::NestedRequirement const*>, llvm::ArrayRef<clang::AssociatedConstraint>) (//third_party/llvm/llvm-project/clang:clang)

It looks like serialization or deserialization issue.

[AaronBallman]

Hi @cor3ntin , we've started seeing Clang crashes after this commit.

FYI but @cor3ntin is out at WG21 meetings this week, so it may be a bit before he responds.

[alexfh]

Hi @cor3ntin , we've started seeing Clang crashes after this commit.

FYI but @cor3ntin is out at WG21 meetings this week, so it may be a bit before he responds.

I see. We'll try to dig a bit deeper ourselves, maybe we'll get somewhere before @cor3ntin is back. And the test case reduction is running in the background (it's 30% through now, hopefully, there will be more progress by the end of the week).