Clang: Incomplete representation of type sugar for expanded argument packs in the AST. #56099
Description
When expanding template argument packs, clang does not synthesize TemplateTypeParmTypes for the SubstTemplateTypeParmTypes which correspond to the argument's position in the expanded form.
Consider example 1: https://godbolt.org/z/KvMTbh5T7
template <typename... As> struct Y;
template <typename ...Bs> using Z = Y<Bs...>;
template <typename ...Cs> struct foo {
template <typename ...Ds> using bind = Z<Ds..., Cs...>;
};
using U = foo<int, short>::bind<char, float>;AST:
TemplateSpecializationType 0x559e6e0cafa0 'Y<char, float, int, short>' sugar Y
|-TemplateArgument type 'char':'char'
| `-SubstTemplateTypeParmType 0x559e6e0cada0 'char' sugar
| |-TemplateTypeParmType 0x559e6e0acb60 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x559e6e0acb08 'Bs'
| `-BuiltinType 0x559e6e066e80 'char'
|-TemplateArgument type 'float':'float'
| `-SubstTemplateTypeParmType 0x559e6e0cade0 'float' sugar
| |-TemplateTypeParmType 0x559e6e0acb60 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x559e6e0acb08 'Bs'
| `-BuiltinType 0x559e6e066fe0 'float'
|-TemplateArgument type 'int':'int'
| `-SubstTemplateTypeParmType 0x559e6e0ca810 'int' sugar
| |-TemplateTypeParmType 0x559e6e0acb60 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x559e6e0acb08 'Bs'
| `-BuiltinType 0x559e6e066ee0 'int'
|-TemplateArgument type 'short':'short'
| `-SubstTemplateTypeParmType 0x559e6e0ca850 'short' sugar
| |-TemplateTypeParmType 0x559e6e0acb60 'Bs' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x559e6e0acb08 'Bs'
| `-BuiltinType 0x559e6e066ec0 'short'
`-RecordType 0x559e6e0caf80 'Y<char, float, int, short>'
`-ClassTemplateSpecialization 0x559e6e0cae98 'Y'
Example 2: https://godbolt.org/z/r7v8hYdKx
template<typename ...T> struct D {
template<typename... U> using B = int(int (*...p)(T, U));
};
using foo = D<float, char>::B<int, short>;AST:
FunctionProtoType 0x564b6179e8e0 'int (int (*)(float, int), int (*)(char, short))' cdecl
|-BuiltinType 0x564b61738ee0 'int'
|-PointerType 0x564b6179e620 'int (*)(float, int)'
| `-ParenType 0x564b6179e5c0 'int (float, int)' sugar
| `-FunctionProtoType 0x564b6179e580 'int (float, int)' cdecl
| |-BuiltinType 0x564b61738ee0 'int'
| |-SubstTemplateTypeParmType 0x564b6179cef0 'float' sugar
| | |-TemplateTypeParmType 0x564b6177e6b0 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
| | | `-TemplateTypeParm 0x564b6177e630 'T'
| | `-BuiltinType 0x564b61738fe0 'float'
| `-SubstTemplateTypeParmType 0x564b6179e500 'int' sugar
| |-TemplateTypeParmType 0x564b6179c750 'U' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x564b6179c6f8 'U'
| `-BuiltinType 0x564b61738ee0 'int'
`-PointerType 0x564b6179e830 'int (*)(char, short)'
`-ParenType 0x564b6179e7d0 'int (char, short)' sugar
`-FunctionProtoType 0x564b6179e790 'int (char, short)' cdecl
|-BuiltinType 0x564b61738ee0 'int'
|-SubstTemplateTypeParmType 0x564b6179dd90 'char' sugar
| |-TemplateTypeParmType 0x564b6177e6b0 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
| | `-TemplateTypeParm 0x564b6177e630 'T'
| `-BuiltinType 0x564b61738e80 'char'
`-SubstTemplateTypeParmType 0x564b6179e710 'short' sugar
|-TemplateTypeParmType 0x564b6179c750 'U' dependent contains_unexpanded_pack depth 0 index 0 pack
| `-TemplateTypeParm 0x564b6179c6f8 'U'
`-BuiltinType 0x564b61738ec0 'short'
The TemplateTypeParmType's used are the original ones from the packed template argument, and as such it's pretty hard to map these to the argument list as-written.