https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122088
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC 15 just says:
122088.cc:13:9: error: expected ‘)’ before ‘...’ token
13 | Bar(Args...) -> Bar<std::common_type_t<Args...>>;
| ~ ^~~
| )
122088.cc: In function ‘int main()’:
122088.cc:16:11: error: class template argument deduction failed:
16 | Bar b{42};
| ^
Clang's error is more helpful:
122088.cc:13:1: error: cannot specify deduction guide for alias template 'Bar'
13 | Bar(Args...) -> Bar<std::common_type_t<Args...>>;
| ^
122088.cc:10:1: note: template is declared here
10 | using Bar = std::variant<Foo<T>>;
| ^
122088.cc:13:1: error: declarator requires an identifier
13 | Bar(Args...) -> Bar<std::common_type_t<Args...>>;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
122088.cc:16:7: error: no viable constructor or deduction guide for deduction
of template arguments of 'Bar'
16 | Bar b{42};
| ^
Even if you _could_ define a deduction guide for an alias template, doing so
for std::variant would be ill-formed because you don't own that type.
