Can't benchmark constructing an array of lists

[axch]

I can make an array of lists:

ans1 = for i:(Fin 3). "hello"

I can time how long it takes to make an array of lists:

%time
ans2 = for i:(Fin 3). "hello"

Compile time: 23.659 ms
Run time:     6.339 us 

But I can't benchmark making an array of lists:

%bench "foo"
ans3 = for i:(Fin 3). "hello"

foo
Compile time: 26.040 ms
Run time:     0.000 ns 
Compiler bug!
Please report this at github.com/google-research/dex-lang/issues

not implemented
CallStack (from HasCallStack):
  error, called at src/lib/LLVMExec.hs:345:8 in dex-0.1.0.0-8hDfthyGTXmzhkTo2ydOn:LLVMExec