Incorrect message about unbalanced parentheses in BS7

moved from https://github.com/BuckleScript/bucklescript/issues/4030
This code:

/* 
   Error is adding names to the parameters of the predicate function;
   ReasonML tells me that parentheses are not closed.
*/
let indexOf = (lookFor: 'a, predicate: (toFind: 'a, item: 'a) => bool, arr: array('a)): option(int) => {
  Some(0);
}

produces this error message:

bad_error> bsb -make-world
[2/4] Building src/Demo.reast
FAILED: src/Demo.reast 
/home/david/.nvm/versions/node/v12.13.1/lib/node_modules/bs-platform/lib/bsc.exe  -w -30-40+6+7+27+32..39+44+45+101 -warn-error +101 -color always  -o src/Demo.reast -bs-syntax-only -bs-binary-ast /home/david/reasonlab/bad_error/src/Demo.re
File "/home/david/reasonlab/bad_error/src/Demo.re", line 5, characters 46-47:
Error: Unclosed "(" (opened line 5, column 39)

  We've found a bug for you!
  /home/david/reasonlab/bad_error/src/Demo.re
  
  There's been an error running Reason's parser on a file.
  The error location should be slightly above this message.
  Please file an issue on github.com/facebook/reason. Thanks!
  
FAILED: subcommand failed.

Reproduce the diagnostic using the shown input in src/Demo.re and run bsb -make-world. Trace the Reason parser and diagnostic path responsible for the reported unclosed parenthesis. Done means this valid parameter syntax no longer produces a misleading unbalanced-parentheses message.