[cfe-dev] Isolated TypedefDecl in CXXDeductionGuideDecl parameter?

[Gábor Márton via cfe-dev]

Hi,

Considering the below code:
01
02  struct A {};
03  typedef A AT;
04
05  template <typename T> struct ClassT {
06    ClassT(AT p0, T* p1) {}
07  };
08
09  A a;
10  int x;
11  auto y = ClassT(a, &x);

The deduction guide's first parameter refers to a TypedefDecl that is
isolated
from the first TypedefDecl. First declaration:

|-TypedefDecl *0x5555561eaec0* <line:3:3, col:13> col:13 referenced AT 'A'
| `-RecordType 0x5555561ead80 'A'
|   `-CXXRecord 0x5555561eacf0 'A'

Vs the function prototype of the deduction guide refers to an isolated
(out-of-the-redecl-chain) TypedefDecl:

FunctionProtoType 0x55555621bc80 'auto (AT, T *) -> ClassT<T>' dependent
trailing_return cdecl
|-InjectedClassNameType 0x5555561eb220 'ClassT<T>' dependent
| `-CXXRecord 0x5555561eafe0 'ClassT'
|-TypedefType 0x55555621bb20 'AT' sugar
| |-Typedef *0x55555621bac0* 'AT'
| `-RecordType 0x5555561ead80 'struct A'
|   `-CXXRecord 0x5555561eacf0 'A'
`-PointerType 0x5555561eb410 'T *' dependent
  `-TemplateTypeParmType 0x5555561eafa0 'T' dependent depth 0 index 0
    `-TemplateTypeParm 0x5555561eaf18 'T'

So, in the translation unit, we have two distinct TypedefDecl AST nodes. The
first one's DeclContext is the TranslationUnidDecl, however, the second's is
the CXXDeductionGuideDecl.

This is strange, because when dealing with regular function template
instantiations, the two typedefs are the same and we have only one universal
TypedefDecl node in the AST.
01
02  struct A {};
03  typedef A AT;
04
05  template <typename T>
06  int fun(AT p0, T* p1) {}
07
08  A a;
09  int x;
10  int y = fun(a, &x);

TypedefDecl *0x5555561eaf70* <input.cc:3:3, col:13> col:13 referenced AT 'A'
`-RecordType 0x5555561eae30 'A'
  `-CXXRecord 0x5555561eada0 'A'

FunctionProtoType 0x5555561eb240 'int (AT, T *)' dependent cdecl
|-BuiltinType 0x5555561ab0a0 'int'
|-TypedefType 0x5555561eb090 'AT' sugar
| |-Typedef *0x5555561eaf70* 'AT'
| `-RecordType 0x5555561eae30 'struct A'
|   `-CXXRecord 0x5555561eada0 'A'
`-PointerType 0x5555561eb160 'T *' dependent
  `-TemplateTypeParmType 0x5555561eb050 'T' dependent depth 0 index 0
    `-TemplateTypeParm 0x5555561eafc8 'T'

I was wondering whether this discrepancy is deliberate. IMO, the deduction
guide should refer to the TypedefDecl in the TranslationUnidDecl and it
should
not create a new TypedefDecl. Can someone confirm if this is a bug or not?

Context: I am trying to fix an infinite loop in the ASTImporter that is
related
to importing deduction guidelines.

Any insight would be really appreciated.

Thanks,
Gabor
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