Clang frontend crash with deduction guide inside class template

[elbeno]

This seems to be a regression from 17 to 18/trunk.

template <typename, typename...>
struct X {
  template <typename>
  struct Y {
    template <typename T>
    Y(T) {}
  };

  template <typename T, typename... Vs>
  Y(T, Vs...) -> Y<T>;
};

using T = X<int>::Y<int>;

https://godbolt.org/z/K5d3sWGjP

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Ben Deane (elbeno)

This seems to be a regression from 17 to 18/trunk.

template <typename, typename...>
struct X {
  template <typename>
  struct Y {
    template <typename T>
    Y(T) {}
  };

  template <typename T, typename... Vs>
  Y(T, Vs...) -> Y<T>;
};

using T = X<int>::Y<int>;

https://godbolt.org/z/K5d3sWGjP

[lukevalenty]

This breaks an Intel open source repo: https://github.com/intel/compile-time-init-build. At least in message.hpp, maybe more.

[elbeno]

With assertions:

clang++: /root/llvm-project/clang/lib/Sema/SemaTemplateInstantiate.cpp:2533:
clang::QualType {anonymous}::TemplateInstantiator::TransformTemplateTypeParmType(clang::TypeLocBuilder&, clang::TemplateTypeParmTypeLoc, bool):
Assertion `Arg.getKind() == TemplateArgument::Type && "Template argument kind mismatch"' failed.

[shafik]

Maybe related: #63823

It does indeed look like a regression introduced between clang-17 and clang-18.

CC @zyn0217 @hokein