[Bug c++/89214] [7/8/9 Regression] ICE in digest_init_r, at cp/typeck2.c:1211 with -std=c++17

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89214

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
We're hitting

1198   /* "If T is a class type and the initializer list has a single
1199      element of type cv U, where U is T or a class derived from T,
1200      the object is initialized from that element."  */
1201   if (flag_checking
1202       && cxx_dialect >= cxx11
1203       && BRACE_ENCLOSED_INITIALIZER_P (stripped_init)
1204       && CONSTRUCTOR_NELTS (stripped_init) == 1
1205       && ((CLASS_TYPE_P (type) && !CLASSTYPE_NON_AGGREGATE (type))
1206           || VECTOR_TYPE_P (type)))
1207     {
1208       tree elt = CONSTRUCTOR_ELT (stripped_init, 0)->value;
1209       if (reference_related_p (type, TREE_TYPE (elt)))
1210         /* We should have fixed this in reshape_init.  */
1211         gcc_unreachable ();
1212     }