[GNATS 27816] Boost header spurious expected a declaration

[jhsedg]

Customer bug ID

32 tests with transcode_view.hpp reduced test case is 27816.C

Description

​32 boost tests fail due to an error in transcode_view.hpp
​
​template <typename, typename> bool is_convertible_v;
​template <typename _Derived, typename _Base>
​concept derived_from = is_convertible_v<_Derived, _Base>;
​struct input_iterator_tag;
​namespace __detail {
​template using __iter_concept = _Iter;
​}
​template
​concept input_iterator =
​ derived_from<__detail::__iter_concept<_Iter>, input_iterator_tag>;
​namespace ranges {
​template
​concept input_range = input_iterator<_Tp>;
​}
​template <ranges::input_range, auto> class project_view;
​template <ranges::input_range V> struct char8_view {
​ char8_view() : project_view<V, char8_t{}>{} {}
​};
​
​
​"/tmp/transcode_view.reduced.C", line 17: error: expected a declaration
​ char8_view() : project_view<V, char8_t{}>{} {}
​ ^
​
​1 error detected in the compilation of "/tmp/transcode_view.reduced.C".