Clang 19 regression matching constrained template to unconstrained template template parameter

[CaseyCarter]

Compiling this program fragment:

template <template <class, class...> class>
constexpr bool f() { return true; }

template <class> class unconstrained {};
static_assert(f<unconstrained>()); // Fine

template <class> concept True = true;
template <True> class constrained {};
static_assert(f<constrained>()); // error: too many template arguments for class template 'constrained'

with clang 19.1.0-rc2 under -std=c++20 diagnoses:

repro.cpp:9:17: error: too many template arguments for class template 'constrained'
    1 | template <template <class, class...> class>
      |                                    ~
    2 | constexpr bool f() { return true; }
    3 |
    4 | template <class> class unconstrained {};
    5 | static_assert(f<unconstrained>()); // Fine
    6 |
    7 | template <class> concept True = true;
    8 | template <True> class constrained {};
    9 | static_assert(f<constrained>()); // error: too many template arguments for class template 'constrained'
      |                 ^
1 error generated.

The constrained case should be as well-formed as the unconstrained case since [temp.arg.template]/3 says the constraints on the argument should be ignored when the template template parameter is not constrained. Note that the release versions of the "big three" (including Clang 18) compile this fragment without issue (https://godbolt.org/z/qYr8orqz7).

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Casey Carter (CaseyCarter)

Compiling this program fragment: ```c++ template <template <class, class...> class> constexpr bool f() { return true; }

template <class> class unconstrained {};
static_assert(f<unconstrained>()); // Fine

template <class> concept True = true;
template <True> class constrained {};
static_assert(f<constrained>()); // error: too many template arguments for class template 'constrained'

with clang 19.1.0-rc2 under `-std=c++20` diagnoses:

repro.cpp:9:17: error: too many template arguments for class template 'constrained'
1 | template <template <class, class...> class>
| ~
2 | constexpr bool f() { return true; }
3 |
4 | template <class> class unconstrained {};
5 | static_assert(f<unconstrained>()); // Fine
6 |
7 | template <class> concept True = true;
8 | template <True> class constrained {};
9 | static_assert(f<constrained>()); // error: too many template arguments for class template 'constrained'
| ^
1 error generated.

The `constrained` case should be as well-formed as the `unconstrained` case since [temp.arg.template]/3 says the constraints on the argument should be ignored when the template template parameter is not constrained. Note that the release versions of the "big three" (including Clang 18) compile this fragment without issue (https://godbolt.org/z/qYr8orqz7).

</details>

[cor3ntin]

@mizvekov

[mizvekov]

This will be fixed by #96023

That might be too big of a change to backport though.

I am looking for which commit introduced the regression.

[mizvekov]

That was from the enablement of relaxed-template-template args by default: https://godbolt.org/z/ErKj9M3Ga

#96023 is the best fix here.

The other problem is that it has also a lot of merge conflicts with other pending patches, and I put it at the top of other patches that are also taking a while to get reviewed.

[cor3ntin]

@AaronBallman for visibility

[CaseyCarter]

FWIW, this impacts a single MSVCSTL test, and the workaround is to change the template <class, class...> class parameter to template <class...> class. It's not worth heroic effort to backport the fix just for us.

[cor3ntin]

@mizvekov this is not actually fixed fyi

[mizvekov]

Yeah, the patch was reverted a while ago, needs a fixup and reapplying.

[mizvekov]

Fixed by #124137

https://gcc.godbolt.org/z/sdfPzrYoG