[MLton] deepFlatten exception

[Matthew Fluet]

>> While compiling a program using the command
>> 
>> mlton -verbose 3 -runtime 'ram-slop 0.4' t.mlb
>> 
>> quite a bit of output is produced (see attached log file) with this at the 
>> end:
>> 
>>           deepFlatten starting
>>           deepFlatten raised in 0.11 + 0.00 (0% GC)
>>        ssa2Simplify raised in 0.35 + 0.00 (0% GC)
>>     pre codegen raised in 28.94 + 5.17 (15% GC)
>>  Compile SML raised in 28.94 + 5.17 (15% GC)
>> MLton raised in 28.94 + 5.17 (15% GC)
>> DeepFlatten.replaceVar x_0
>> 
>> Is this a compiler bug, or something wrong with my program?
>
> It is a compiler bug.  Any errors after the parseAndElaborate (i.e., SML 
> type-checking) phase are indicative of compiler bugs.  (It might be a good 
> idea for us to change the exception printer to explicitly state that the 
> error should be reported to mlton@mlton.org as a compiler bug.)

I should have also mentioned that you can work around the compiler bug by 
instructing the compiler to drop the offending optimization pass:

   mlton -verbose 3 -runtime 'ram-slop 0.4' -drop-pass deepFlatten t.mlb

I'm able to successfully compile the test program with the above command 
line.

When one encounters a compiler bug like this, it is also useful to compile 
with the '-type-check true' option, which instructs the compiler to type 
check the intermediate languages after each transformation and 
optimization pass.  This is useful because some compiler bugs will result 
in a type incorrect program; it can also be useful to pinpoint where the 
compilation went wrong.

Unfortunately, none of this applies to this bug; the SSA2 program into 
deepFlatten type checks.