[Bug c++/91023] Unification Fails for parameter pack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91023

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The trunk we get the following ICE:
<source>: In substitution of 'template<class ... T>
std::optional<std::variant<_Types ...> > parse_variant(const Foo<T,
std::variant<_Types ...> >& ...) [with T = <missing>]':
<source>:34:34:   required from here
<source>:34:34: internal compiler error: tree check: expected class
'expression', have 'type' (record_type) in tree_operand_check, at tree.h:3939
   34 |     const auto x =  parse_variant(a(), b());
      |                     ~~~~~~~~~~~~~^~~~~~~~~~