Internal Compiler Error: Case branch with a "let...in..." inside returns a different type?

[poeface]

Hey I'm back with another match-statement related bug. I don't think this is the same as my last issue since the pass name (internalisation) and cause appear to be different.

Like before, I trimmed the code down as much as possible. Lines 17-18 contain the offending code. If those lines are replaced with line 20, the error disappears. Something to do with having a let __ in __ in the body of a case ___ -> ___.

This problem happened when compiling for both C and multicore targets.

bug.zip

Finally, here is a printout of compiler's output:

Internal compiler error.  Please report this:
  https://github.com/diku-dk/futhark/issues
After internalisation:
In function makeConvLayer_6036
When checking function body
In expression of statement
  {makeConvLayer_res_6528 : ({}, i64),
   makeConvLayer_res_6529 : ({}, i64),
   makeConvLayer_res_6530 : ({}, i8),
   makeConvLayer_res_6531 : ({}, [makeConvLayer_res_6528][makeConvLayer_res_6529]f64)}
in body of case {1i8}
Type error:
Type annotation is
  {i64, i64, i8, [?0][?1]f64}
But result returns type
  {i64, i64, i8, [NM_6521][NM_6521]f64, [defunc_2_tabulate_2d_arg_6536][defunc_2_tabulate_2d_arg_6535]f64}

Hope that's enough info!

[athas]

This bug was due to two factors we hadn't seen combined before:

1. Using complex initialisation expressions in sum types.
2. Using a sum type that contains arrays.

The first is just a matter of style preference, but the second is often not a good idea. However, it is fine in this case because the arrays along the different constructors have the same type, and it should definitely work. And now it does. (Hopefully. I still suspect there are bugs lurking in defunctionalisation where we don't propagate sizes properly. Fortunately they almost always result in compiler crashes rather than silent miscompilation.)

[poeface]

Thanks for the pointer about array types in sum types--this is definitely something I wouldn't have thought to be an issue but is quite valuable to know about!