[Clang] Error on pack indexing in return type

[Sirraide]

Consider (https://godbolt.org/z/3ThcG9r9d):

template <typename... opts>
struct temp  {
    template <unsigned idx>
    static constexpr __SIZE_TYPE__ get_index() { return idx; }

    template <unsigned s>
    static auto x() -> opts...[get_index<s>()] {}
};

Attempting to call x like this

temp<void>::x<0>();

fails with the following (rather weird) error:

<source>:11:5: error: no matching function for call to 'x'
   11 |     temp<void>::x<0>(); // Error
      |     ^~~~~~~~~~~~~~~~
<source>:7:17: note: candidate template ignored: substitution failure [with s = 0]: array size is not a constant expression
    7 |     static auto x() -> opts...[get_index<s>()] {}
      |                 ^              ~~~~~~~~~

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Sirraide)

Consider: ```c++ template <typename... opts> struct temp { template <unsigned idx> static constexpr __SIZE_TYPE__ get_index() { return idx; }

template <unsigned s>
static auto x() -> opts...[get_index<s>()] {}

};

Attempting to call `x` like this
```c++
temp<void>::x<0>();

fails with the following (rather weird) error:

<source>:11:5: error: no matching function for call to 'x'
   11 |     temp<void>::x<0>(); // Error
      |     ^~~~~~~~~~~~~~~~
<source>:7:17: note: candidate template ignored: substitution failure [with s = 0]: array size is not a constant expression
    7 |     static auto x() -> opts...[get_index<s>()] {}
      |                 ^              ~~~~~~~~~

Furthermore, attempting to call x like this

temp<>::x<0>();

outright crashes with the following message:

clang++: /root/llvm-project/clang/lib/Sema/TreeTransform.h:6698: clang::QualType clang::TreeTransform<Derived>::TransformPackIndexingType(clang::TypeLocBuilder&, clang::PackIndexingTypeLoc) [with Derived = {anonymous}::TemplateInstantiator]: Assertion `!Unexpanded.empty() && "Pack expansion without parameter packs?"' 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 -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++26 <source>
1.	<source>:12:18: current parser token ')'
2.	<source>:10:10: parsing function body 'f'
3.	<source>:10:10: in compound statement ('{}')
 #<!-- -->0 0x0000000003b07c88 llvm::sys::PrintStackTrace(llvm::raw_ostream&amp;, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b07c88)
 #<!-- -->1 0x0000000003b0597c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3b0597c)
 #<!-- -->2 0x0000000003a4e8f8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #<!-- -->3 0x000070325c242520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #<!-- -->4 0x000070325c2969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #<!-- -->5 0x000070325c242476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #<!-- -->6 0x000070325c2287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #<!-- -->7 0x000070325c22871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #<!-- -->8 0x000070325c239e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #<!-- -->9 0x0000000007233965 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformPackIndexingType(clang::TypeLocBuilder&amp;, clang::PackIndexingTypeLoc) SemaTemplateInstantiate.cpp:0:0
#<!-- -->10 0x00000000072119d3 clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformType(clang::TypeLocBuilder&amp;, clang::TypeLoc) SemaTemplateInstantiate.cpp:0:0
#<!-- -->11 0x0000000007217cca clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformType(clang::TypeSourceInfo*) SemaTemplateInstantiate.cpp:0:0
#<!-- -->12 0x0000000007217dec clang::TreeTransform&lt;(anonymous namespace)::TemplateInstantiator&gt;::TransformType(clang::QualType) SemaTemplateInstantiate.cpp:0:0
#<!-- -->13 0x0000000007219aab clang::Sema::SubstType(clang::QualType, clang::MultiLevelTemplateArgumentList const&amp;, clang::SourceLocation, clang::DeclarationName) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7219aab)
#<!-- -->14 0x00000000070f281e clang::Sema::SubstituteExplicitTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo&amp;, llvm::SmallVectorImpl&lt;clang::DeducedTemplateArgument&gt;&amp;, llvm::SmallVectorImpl&lt;clang::QualType&gt;&amp;, clang::QualType*, clang::sema::TemplateDeductionInfo&amp;) (.constprop.0) SemaTemplateDeduction.cpp:0:0
#<!-- -->15 0x00000000070f31a0 void llvm::function_ref&lt;void ()&gt;::callback_fn&lt;clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::FunctionDecl*&amp;, clang::sema::TemplateDeductionInfo&amp;, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref&lt;bool (llvm::ArrayRef&lt;clang::QualType&gt;)&gt;)::'lambda'()&gt;(long) SemaTemplateDeduction.cpp:0:0
#<!-- -->16 0x000000000669ff51 clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x669ff51)
#<!-- -->17 0x0000000007154b71 clang::Sema::DeduceTemplateArguments(clang::FunctionTemplateDecl*, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::FunctionDecl*&amp;, clang::sema::TemplateDeductionInfo&amp;, bool, bool, clang::QualType, clang::Expr::Classification, llvm::function_ref&lt;bool (llvm::ArrayRef&lt;clang::QualType&gt;)&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7154b71)
#<!-- -->18 0x0000000006f691d8 clang::Sema::AddTemplateOverloadCandidate(clang::FunctionTemplateDecl*, clang::DeclAccessPair, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::OverloadCandidateSet&amp;, bool, bool, bool, clang::CallExpr::ADLCallKind, clang::OverloadCandidateParamOrder, bool) (.constprop.1) SemaOverload.cpp:0:0
#<!-- -->19 0x0000000006f699db AddOverloadedCallCandidate(clang::Sema&amp;, clang::DeclAccessPair, clang::TemplateArgumentListInfo*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::OverloadCandidateSet&amp;, bool, bool) SemaOverload.cpp:0:0
#<!-- -->20 0x0000000006f69cc9 clang::Sema::AddOverloadedCallCandidates(clang::UnresolvedLookupExpr*, llvm::ArrayRef&lt;clang::Expr*&gt;, clang::OverloadCandidateSet&amp;, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f69cc9)
#<!-- -->21 0x0000000006f69fce clang::Sema::buildOverloadedCallSet(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::OverloadCandidateSet*, clang::ActionResult&lt;clang::Expr*, true&gt;*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f69fce)
#<!-- -->22 0x0000000006f766f5 clang::Sema::BuildOverloadedCallExpr(clang::Scope*, clang::Expr*, clang::UnresolvedLookupExpr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f766f5)
#<!-- -->23 0x0000000006b24028 clang::Sema::BuildCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b24028)
#<!-- -->24 0x0000000006b2658c clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef&lt;clang::Expr*&gt;, clang::SourceLocation, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b2658c)
#<!-- -->25 0x00000000065ae22d clang::Parser::ParsePostfixExpressionSuffix(clang::ActionResult&lt;clang::Expr*, true&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65ae22d)
#<!-- -->26 0x00000000065a6fb1 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&amp;, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65a6fb1)
#<!-- -->27 0x00000000065a910a clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65a910a)
#<!-- -->28 0x00000000065a92a9 clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65a92a9)
#<!-- -->29 0x00000000065ad7e9 clang::Parser::ParseExpression(clang::Parser::TypeCastState) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65ad7e9)
#<!-- -->30 0x000000000662f5b9 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x662f5b9)
#<!-- -->31 0x00000000066260a0 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&lt;clang::Stmt*, 32u&gt;&amp;, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x66260a0)
#<!-- -->32 0x0000000006626fd0 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&lt;clang::Stmt*, 32u&gt;&amp;, clang::Parser::ParsedStmtContext, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6626fd0)
#<!-- -->33 0x0000000006627ea9 clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6627ea9)
#<!-- -->34 0x000000000662962a clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x662962a)
#<!-- -->35 0x000000000653d1f3 clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&amp;, clang::Parser::ParsedTemplateInfo const&amp;, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x653d1f3)
#<!-- -->36 0x00000000065720f8 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&amp;, clang::DeclaratorContext, clang::ParsedAttributes&amp;, clang::Parser::ParsedTemplateInfo&amp;, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x65720f8)
#<!-- -->37 0x0000000006530dbe clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec&amp;, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6530dbe)
#<!-- -->38 0x000000000653156e clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x653156e)
#<!-- -->39 0x0000000006537be7 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&amp;, clang::ParsedAttributes&amp;, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6537be7)
#<!-- -->40 0x0000000006538abd clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&lt;clang::DeclGroupRef&gt;&amp;, clang::Sema::ModuleImportState&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6538abd)
#<!-- -->41 0x000000000652bf5a clang::ParseAST(clang::Sema&amp;, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x652bf5a)
#<!-- -->42 0x0000000004424038 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4424038)
#<!-- -->43 0x00000000046b2fb9 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46b2fb9)
#<!-- -->44 0x000000000463678e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x463678e)
#<!-- -->45 0x000000000479c6be clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x479c6be)
#<!-- -->46 0x0000000000ca698f cc1_main(llvm::ArrayRef&lt;char const*&gt;, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xca698f)
#<!-- -->47 0x0000000000c9fbda ExecuteCC1Tool(llvm::SmallVectorImpl&lt;char const*&gt;&amp;, llvm::ToolContext const&amp;) driver.cpp:0:0
#<!-- -->48 0x00000000044676f9 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
#<!-- -->49 0x0000000003a4eda4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref&lt;void ()&gt;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3a4eda4)
#<!-- -->50 0x0000000004467cef 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
#<!-- -->51 0x000000000442d885 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&amp;, clang::driver::Command const*&amp;, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x442d885)
#<!-- -->52 0x000000000442e2ed 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+++0x442e2ed)
#<!-- -->53 0x0000000004435f05 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+++0x4435f05)
#<!-- -->54 0x0000000000ca3d85 clang_main(int, char**, llvm::ToolContext const&amp;) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xca3d85)

[Sirraide]

Apologies for this rather messy issue. I’m not sure whether these are two different bugs or not, but I haven’t managed to reduce this any further...

Actually, the crash seems unrelated, so I’m moving that to a different issue

[zyn0217]

We need to push a constant evaluation context before recursing into transforming the IndexExpr:

llvm-project/clang/lib/Sema/TreeTransform.h

Lines 6667 to 6672 in 0916ae4

	template <typename Derived>
	QualType
	TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
	PackIndexingTypeLoc TL) {
	// Transform the index
	ExprResult IndexExpr = getDerived().TransformExpr(TL.getIndexExpr());

This way, we could instantiate the function body of get_index() when building up a DeclRefExpr for the transformed CallExpr, which is necessary for the constant evaluation.

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

Lines 17923 to 17926 in 0916ae4

	bool NeedDefinition =
	!IsRecursiveCall &&
	(OdrUse == OdrUseContext::Used ||
	(NeededForConstantEvaluation && !Func->isPureVirtual()));

(In this case, the constant evaluator encounters an error that the constexpr function body is undefined, but unfortunately that error was dropped on the floor during the template parameter deduction)