[clang][rejects-valid] unfound constrained partial specialization of class template with variadic NTTP of class type

[ericniebler]

The following valid C++20 code is rejected by clang trunk:

struct Integer
{
  int value{};
};

template <Integer... X>
struct Type;

template <Integer... X>
  requires ((X.value == 0) ||...)
struct Type<X...>
{
};

result:

<source>:11:8: error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]
   11 | struct Type<X...>
      |        ^
<source>:7:8: note: template is declared here
    7 | struct Type;
      |        ^
1 error generated.

This bug requires the class type to be parameterized on a variadic pack of non-type template parameters of class type.

repro:
https://godbolt.org/z/fYKe1MEeK

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eric Niebler (ericniebler)

The following valid C++20 code is rejected by clang trunk:

struct Integer
{
  int value{};
};

template <Integer... X>
struct Type;

template <Integer... X>
  requires ((X.value == 0) ||...)
struct Type<X...>
{
};

result:

<source>:11:8: error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]
   11 | struct Type<X...>
      |        ^
<source>:7:8: note: template is declared here
    7 | struct Type;
      |        ^
1 error generated.

This bug requires the class type to be parameterized on a variadic pack of non-type template parameters of class type.

repro:
https://godbolt.org/z/fYKe1MEeK

[zyn0217]

https://godbolt.org/z/xnGqW84GK

[zyn0217]

@mizvekov It thinks deducing against the primary template using the partial specialization type Type<X...> is a non-deduced mismatch. This is odd.

template <Integer... X>
struct Type;

[mizvekov]

One clue is that we now correctly diagnose this: https://godbolt.org/z/KfcqWMKx5, while the reverse is still accepted.

Are you looking into this problem, or should I take this?

[zyn0217]

No, I'm not. I see there's DeduceTemplateArugments involved so I decided to leave it to you 😄

[mizvekov]

This will be fixed by #134461