As described in the PR, in C++ we can have assignments
where both the lhs and rhs are COMPONENT_REFs with TREE_ADDRESSABLE types,
including padding, but the FIELD_DECLs are artificial fields that have
narrower bit sizes.
store_field in this case takes the path of bit-field handling (even when
it has bitpos and bitsize multiples of BITS_PER_UNIT (I think that is
necessarily true for the TREE_ADDRESSABLE types), which is incorrect,
because the rhs is expanded in that case through expand_normal, which
for a result type wider than the FIELD_DECL with forces it into a temporary.
In older GCCs that just generated inefficient code (copy the rhs into a
stack temporary, then copy that to lhs), but GCC trunk ICEs on that.
Fixed by not taking the bit-field path in that case after verifying
we'll be able to expand it properly using the normal store_expr.