My patch for PR92385 made us use VEC_INIT_EXPR for aggregate initialization
of an array where some elements are not explicitly initialized. Constexpr
handling of that was treating initialization from {} as equivalent to
value-initialization, which is problematic for classes with default member
initializers that make the default constructor non-trivial; in older
standard modes, not initializing all members makes a constructor
non-constexpr, but aggregate initialization is fine.
PR c++/105191
PR c++/92385
gcc/cp/ChangeLog:
* tree.cc (build_vec_init_elt): Do {}-init for aggregates.
* constexpr.cc (cxx_eval_vec_init): Only treat {} as value-init
for non-aggregate types.
(build_vec_init_expr): Also check constancy of explicit
initializer elements.