[GNATS 21696] Spurious diagnostic for --no_exceptions with default constructors

[jhsedg]

Customer bug ID

(boost) variant_subset gnats(21696 full expanded), 21696a-reduced

Description

​This reduced Boost test case fails with --no_exceptions
​
​template struct b { static constexpr int c = a; };
​template struct conditional;
​template <typename...> struct d;
​template <typename e, typename f> struct d<e, f> : conditional::g {};
​template struct i : b<noexcept(h())> {};
​template struct j : d<h, h> {};
​struct k;
​template struct conditional { typedef i g; };
​struct k {
​ k();
​};
​static_assert(!j::c, "stdis_nothrow_default_constructiblevalue");
​
​[ellen@lin1 status]$ $CPFE --c++17 -w -tused --no_exceptions /tmp/variant_subset.reduced.C
​"/tmp/variant_subset.reduced.C", line 12: error: static assertion failed with
​ "stdis_nothrow_default_constructiblevalue"
​ static_assert(!j::c, "stdis_nothrow_default_constructiblevalue");
​ ^
​
​1 error detected in the compilation of "/tmp/variant_subset.reduced.C".