[MLton] New front-end finds bug in basis library

[Jared M Showalter]

The reader and writer types in BinIO.StreamIO are not the same as in
BinPrimIO.  So, the following program should elaborate if the basis
library is conformant, but it doesn't:

type r = BinIO.StreamIO.reader
val f : r -> r = fn x : BinPrimIO.reader => x

yields:

Error: test.sml 2.5: pattern and expression disagree
   pattern:    [r] -> [r]
   expression: [BinPrimIO.reader] -> [BinPrimIO.reader]
   in: (f): (r -> r) = fn ((x): BinPrimIO.reader) => (x)
compilation aborted: elaborate reported errors


This appears to be a bug in the basis library the is caught by the new
front-end, rather than a bug in the front-end, because of the following
excerpt from the output of 'mlton -show-basis true':

structure BinIO:
   sig
      structure StreamIO:
	 sig
	    type reader = BinIO.StreamIO.reader
	    type writer = BinIO.StreamIO.writer

structure TextIO:
   sig
      structure StreamIO:
	 sig
	    type reader = TextPrimIO.reader
	    type writer = TextPrimIO.writer

TextIO has it right, but BinIO doesn't.

Jared