Template template parameters and default arguments in C++17 (CWG150 + P0522R0)

[pkeir]

Bugzilla Link	42960
Version	trunk
OS	Linux
Attachments	The MWE code described in the report.
CC	@zygoloid

Extended Description

The code below will not compile with Clang, though it does with GCC (with the -std=c++17 flag and upwards).

template <typename T, typename = void>
struct jam;

template <template typename>
struct mata;

using type = mata;

This relates to CWG 150, which I believe was resolved by P0522R0. That is, since C++17, code such as that above should now be legal.

[zygoloid]

See http://clang.llvm.org/cxx_status.html#p0522

Use -frelaxed-template-template-args to enable support for this feature for now.

[mizvekov]

Duplicate of #42224

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (066609a1-1e53-4cd0-b6cb-efe5048c5913)

| | | | --- | --- | | Bugzilla Link | [42960](https://llvm.org/bz42960) | | Version | trunk | | OS | Linux | | Attachments | [The MWE code described in the report.](https://user-images.githubusercontent.com/92601693/143759618-0a508082-59dd-4dad-b5e7-25ee2c80590a.gz) | | CC | @zygoloid |

Extended Description

The code below will not compile with Clang, though it does with GCC (with the -std=c++17 flag and upwards).

template <typename T, typename = void>
struct jam;

template <template <typename> typename>
struct mata;

using type = mata<jam>;

This relates to CWG 150, which I believe was resolved by P0522R0. That is, since C++17, code such as that above should now be legal.