clang assert: "Assertion `getDepthAndIndex(NewParam).first == 0 && "Unexpected template parameter depth"' failed."

[hokein]

https://godbolt.org/z/TPjf1YYY3

A regression from #111143

template <typename = void>
class NewDeleteAllocator;

template <>
struct NewDeleteAllocator<> {
  template <typename>
  NewDeleteAllocator();
};

template <typename>
struct NewDeleteAllocator : NewDeleteAllocator<> {
  using NewDeleteAllocator<>::NewDeleteAllocator;
};

void test() { NewDeleteAllocator abc; }

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

https://godbolt.org/z/TPjf1YYY3

A regression from #111143

template <typename = void>
class NewDeleteAllocator;

template <>
struct NewDeleteAllocator<> {
  template <typename>
  NewDeleteAllocator();
};

template <typename>
struct NewDeleteAllocator : NewDeleteAllocator<> {
  using NewDeleteAllocator<>::NewDeleteAllocator;
};

void test() { NewDeleteAllocator abc; }

[hokein]

cc @zyn0217

[zyn0217]

NewParam->dump()
TemplateTypeParmDecl 0x5555631a8e98 <128691.cpp:8:13> col:21 typename depth 32767 index 1

Which suggests the NewDepth we've calculated is wrong.

[zyn0217]

So... the constructor for which we're trying to synthesize a deduction guide comes from the explicit specialization template <> struct NewDeleteAllocator<>, which has an empty template parameter list. As a result, the function template (the constructor) lvies at depth 0. When building a deduction guide within a class template, we assume there's always at least one level of template depth and therefore use Param->getDepth() - 1, which overflows in this case.

(We should have been doing this wrong prior to 20, but it didn't crash because it called Decl::getTemplateDepth(), which returns 0 for the transformed template parameter declaration)