[ICE] couldn't promote integral to integral struct

[apache2]

#!!# type a = struct uint<32> { uint<16> field1; uint<16> field2; };
#!!# type b = struct { little uint<32> my_a; }
#!!# var x = open("*data*");
#!!# (b @ x : 0#B) as a;
1:2: internal compiler error: couldn't promote integral to integral struct
Important information has been dumped in /tmp/pokeiQKOL3.
Please attach it to a bug report and send it to poke-devel@gnu.org.

This is a bit surprising, I stumbled into it when trying to write:
#!!# type b = struct { little a my_a; };
which results in:
#!!# type b = struct { little a my_a; }
<stdin>:1:19: warning: useless endianness annotation in field
type b = struct { little a my_a; };
                  ^~~~~~~~~~~~~~

(maybe that's also a bug?)