[Bug c++/89301] New: [concepts] requires clause on a template alias is ignored

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89301

            Bug ID: 89301
           Summary: [concepts] requires clause on a template alias is
                    ignored
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

The following code compiles however it should not:

template<bool condition, typename T = void>
    requires(condition)
using enable_if_t = T;

template<typename T, typename = enable_if_t<sizeof(T) < 0>>
void foo();

void test() {
    foo<int>();
}


Slightly changed example also compiles on GCC (but fails to compile on Clang):

template<bool condition, typename T = void>
    requires(condition)
using enable_if_t = T;

template<typename T>
enable_if_t<sizeof(T) < 0> foo();

void test() {
    foo<int>();
}