C++23 #include <format> from libstdc++ fails with "variable template ... cannot appear in its own initializer"

[stbergmann]

Apparently since 91f1830 "[Clang][Sema] Handle invalid variable template specialization whose type depends on itself (#134522)": At least when building agaisnt libstdc++-devel-15.1.1-1.fc42.x86_64,

$ cat test.cc
#include <format>

$ clang++ -std=c++23 -fsyntax-only test.cc
In file included from test.cc:1:
/usr/lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/format:5165:7: error: variable template 'format_kind' declared with deduced type 'const auto' cannot appear in its own initializer
 5165 |       format_kind<_Rg> // you can specialize this for non-const input ranges
      |       ^
1 error generated.

(where therelevant contents of /usr/include/c++/15/format` is

  template<typename _Rg>
    constexpr auto format_kind =
    __primary_template_not_defined(
      format_kind<_Rg> // you can specialize this for non-const input ranges
    );

).

[stbergmann]

duplicate of #139067 "[clang] fails to compile print("{}", 5s) when using libstdc++"

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Stephan Bergmann (stbergmann)

Apparently since <https://github.com/llvm/llvm-project/commit/91f1830cb69cab96201adba6378d64095e251f95> "[Clang][Sema] Handle invalid variable template specialization whose type depends on itself (https://github.com//pull/134522)": At least when building agaisnt `libstdc++-devel-15.1.1-1.fc42.x86_64`, ``` $ cat test.cc #include <format> ``` ``` $ clang++ -std=c++23 -fsyntax-only test.cc In file included from test.cc:1: /usr/lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/format:5165:7: error: variable template 'format_kind' declared with deduced type 'const auto' cannot appear in its own initializer 5165 | format_kind<_Rg> // you can specialize this for non-const input ranges | ^ 1 error generated. ``` (where the` relevant contents of `/usr/include/c++/15/format` is ``` template<typename _Rg> constexpr auto format_kind = __primary_template_not_defined( format_kind<_Rg> // you can specialize this for non-const input ranges ); ``` ).