https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88977
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
I had missed the example is missing a semicolon after the definition of X (the
error could use improvement). With the semicolon added, the example is
accepted (it's even in the test suite).
The equivalent that isn't accepted but I believe should be is the following:
$ cat pr88977.C && gcc -S -Wall pr88977.C
template <bool B> constexpr bool f() { return B; }
static_assert (f<__builtin_is_constant_evaluated()>());
pr88977.C:2:53: error: no matching function for call to
‘f<__builtin_is_constant_evaluated()>()’
2 | static_assert (f<__builtin_is_constant_evaluated()>());
| ^
pr88977.C:1:34: note: candidate: ‘template<bool B> constexpr bool f()’
1 | template <bool B> constexpr bool f() { return B; }
| ^
pr88977.C:1:34: note: template argument deduction/substitution failed:
