Internal compiler error: Type error after pass 'explicit allocations'

[PhilipBorgesen]

I got the following internal compiler error with Futhark v0.21.8:

Internal compiler error.  Please report this:
  https://github.com/diku-dk/futhark/issues
Type error after pass 'explicit allocations':
In function entry_main
When checking function body
In expression of statement
  {ext_mem_4588 : ({A_4584, A_mem_4587}, mem),
   main_res_4585 : ({}, i64),
   main_res_4586 : ({}, [main_res_4585]i64 @ ext_mem_4588 ->
                    {base: [ext_mem_4588]; contiguous: true;
                     LMADs: [{offset: 0i64; strides: [1i64]; rotates: [0i64];
                              shape: [ext_mem_4588]; permutation: [0];
                              monotonicity: [Inc]}]})}
Type error:
Expected apply result type:
  [mem, i64, *[?1]i64 @ ?0->
   {base: [?1]; contiguous: true; LMADs: [{offset: 0i64; strides: [1i64];
                                           rotates: [0i64]; shape: [?1];
                                           permutation: [0];
                                           monotonicity: [Inc]}]}]
But annotation is:
  [mem, i64, *[?0]i64 @ ?0->
   {base: [?0]; contiguous: true; LMADs: [{offset: 0i64; strides: [1i64];
                                           rotates: [0i64]; shape: [?0];
                                           permutation: [0];
                                           monotonicity: [Inc]}]}]

A reduced program that causes the error is:

#[noinline]
def double [n] (A: [n]i64) : *[]i64 =
  A ++ A

def main [n] (A: [n]i64) : *[]i64 =
  double A