Failure to partially specialize a class when make_index_sequence is involved

[llvmbot]

Bugzilla Link	52586
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid

Extended Description

The following code fails to compile.

#include <utility>

template <typename T, typename Seq>
struct X;

template <typename T>
struct X<T, std::make_index_sequence<sizeof(T)>> {
};

X<char, std::make_index_sequence<1>> x;
// ^^^
// error: implicit instantiation of undefined template
// 'X<char, std::integer_sequence<unsigned long, 0>>

See https://godbolt.org/z/o8ejrTGKE

Replacing make_index_sequence<1> with the equivalent index_sequence<0> in the class template definition works around the issue.

[royjacobson]

Potentially related to #42102 - there's some weirdness about __make_integer_seq and the way it's implemented as a builtin.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[mizvekov]

I think the issue is that if any of the template arguments to __make_integer_seq are dependent, then we create a canonical TemplateSpecializationType of the __make_integer_seq builtin template.

We should instead create a canonical TST based on the first argument of the builtin, if that argument is not dependent.

[mizvekov]

Fixed by: f4ea3bd