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a5e6590b15
CXXDeductionGuideDecl with a local typedef has its own copy of the
TypedefDecl with the CXXDeductionGuideDecl as the DeclContext of that
TypedefDecl.
```
template <typename T> struct A {
typedef T U;
A(U, T);
};
A a{(int)0, (int)0};
```
Related discussion on cfe-dev:
http://lists.llvm.org/pipermail/cfe-dev/2020-November/067252.html
Without this fix, when we import the CXXDeductionGuideDecl (via
VisitFunctionDecl) then before creating the Decl we must import the
FunctionType. However, the first parameter's type is the afore mentioned
local typedef. So, we then start importing the TypedefDecl whose
DeclContext is the CXXDeductionGuideDecl itself. The infinite loop is
formed.
```
#0 clang::ASTNodeImporter::VisitCXXDeductionGuideDecl(clang::CXXDeductionGuideDecl*) clang/lib/AST/ASTImporter.cpp:3543:0
#1 clang::declvisitor::Base<std::add_pointer, clang::ASTNodeImporter, llvm::Expected<clang::Decl*> >::Visit(clang::Decl*) /home/egbomrt/WORK/llvm5/build/debug/tools/clang/include/clang/AST/DeclNodes.inc:405:0
#2 clang::ASTImporter::ImportImpl(clang::Decl*) clang/lib/AST/ASTImporter.cpp:8038:0
#3 clang::ASTImporter::Import(clang::Decl*) clang/lib/AST/ASTImporter.cpp:8200:0
#4 clang::ASTImporter::ImportContext(clang::DeclContext*) clang/lib/AST/ASTImporter.cpp:8297:0
#5 clang::ASTNodeImporter::ImportDeclContext(clang::Decl*, clang::DeclContext*&, clang::DeclContext*&) clang/lib/AST/ASTImporter.cpp:1852:0
#6 clang::ASTNodeImporter::ImportDeclParts(clang::NamedDecl*, clang::DeclContext*&, clang::DeclContext*&, clang::DeclarationName&, clang::NamedDecl*&, clang::SourceLocation&) clang/lib/AST/ASTImporter.cpp:1628:0
#7 clang::ASTNodeImporter::VisitTypedefNameDecl(clang::TypedefNameDecl*, bool) clang/lib/AST/ASTImporter.cpp:2419:0
#8 clang::ASTNodeImporter::VisitTypedefDecl(clang::TypedefDecl*) clang/lib/AST/ASTImporter.cpp:2500:0
#9 clang::declvisitor::Base<std::add_pointer, clang::ASTNodeImporter, llvm::Expected<clang::Decl*> >::Visit(clang::Decl*) /home/egbomrt/WORK/llvm5/build/debug/tools/clang/include/clang/AST/DeclNodes.inc:315:0
#10 clang::ASTImporter::ImportImpl(clang::Decl*) clang/lib/AST/ASTImporter.cpp:8038:0
#11 clang::ASTImporter::Import(clang::Decl*) clang/lib/AST/ASTImporter.cpp:8200:0
#12 llvm::Expected<clang::TypedefNameDecl*> clang::ASTNodeImporter::import<clang::TypedefNameDecl>(clang::TypedefNameDecl*) clang/lib/AST/ASTImporter.cpp:165:0
#13 clang::ASTNodeImporter::VisitTypedefType(clang::TypedefType const*) clang/lib/AST/ASTImporter.cpp:1304:0
#14 clang::TypeVisitor<clang::ASTNodeImporter, llvm::Expected<clang::QualType> >::Visit(clang::Type const*) /home/egbomrt/WORK/llvm5/build/debug/tools/clang/include/clang/AST/TypeNodes.inc:74:0
#15 clang::ASTImporter::Import(clang::QualType) clang/lib/AST/ASTImporter.cpp:8071:0
#16 llvm::Expected<clang::QualType> clang::ASTNodeImporter::import<clang::QualType>(clang::QualType const&) clang/lib/AST/ASTImporter.cpp:179:0
#17 clang::ASTNodeImporter::VisitFunctionProtoType(clang::FunctionProtoType const*) clang/lib/AST/ASTImporter.cpp:1244:0
#18 clang::TypeVisitor<clang::ASTNodeImporter, llvm::Expected<clang::QualType> >::Visit(clang::Type const*) /home/egbomrt/WORK/llvm5/build/debug/tools/clang/include/clang/AST/TypeNodes.inc:47:0
#19 clang::ASTImporter::Import(clang::QualType) clang/lib/AST/ASTImporter.cpp:8071:0
#20 llvm::Expected<clang::QualType> clang::ASTNodeImporter::import<clang::QualType>(clang::QualType const&) clang/lib/AST/ASTImporter.cpp:179:0
#21 clang::QualType clang::ASTNodeImporter::importChecked<clang::QualType>(llvm::Error&, clang::QualType const&) clang/lib/AST/ASTImporter.cpp:198:0
#22 clang::ASTNodeImporter::VisitFunctionDecl(clang::FunctionDecl*) clang/lib/AST/ASTImporter.cpp:3313:0
#23 clang::ASTNodeImporter::VisitCXXDeductionGuideDecl(clang::CXXDeductionGuideDecl*) clang/lib/AST/ASTImporter.cpp:3543:0
```
The fix is to first create the TypedefDecl and only then start to import
the DeclContext.
Basically, we could do this during the import of all other Decls (not
just for typedefs). But it seems, there is only one another AST
construct that has a similar cycle: a struct defined as a function
parameter:
```
int struct_in_proto(struct data_t{int a;int b;} *d);
```
In that case, however, we had decided to return simply with an error
back then because that seemed to be a very rare construct.
Differential Revision: https://reviews.llvm.org/D92209
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