WHITESPACE_ONLY mode does non-whitespace-only optimizations

Closure compiler v20200614

in.js:

{
	function foo(p) { self["external1"](p) }
	
	console.log(foo());
	console.log(foo());
}

Command: java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level WHITESPACE_ONLY --formatting PRETTY_PRINT

out.js:

 {
  var foo = function(p) {
    self["external1"](p);
  };
  console.log(foo());
  console.log(foo());
}
;

Note that function foo(p) was rewritten as var foo = function(p). This is not a whitespace-only change. It also causes a bug (#3623). Therefore even using the lowest tier of WHITESPACE_ONLY optimizations, we cannot escape this bug. I think it is reasonable to expect WHITESPACE_ONLY to only do what it says and not make other adjustments which alter the way scripts work.

[concavelenz]

Any language_out above ECMASCRIPT_2015 should avoid this issue. “STABLE” as language_out is ECMASCRIPT5 currently (but for no good reason at this point).

I think that I would like to take WHITESPACE_ONLY back to its original intent: simply parse and print. That however, would mean that this would be restricted to a single “module” file input, or any number of scripts (which can be concatenated) but not both.

[ctjlewis]

@AshleyScirra There isn’t supposed to be one - same input/output language should also imply no transpilation. It doesn’t end up being 100% coverage, but it does work usually:

$ google-closure-compiler -O WHITESPACE_ONLY --formatting PRETTY_PRINT --language_in ES_NEXT --language_out ES_NEXT

'use strict';
{
  function foo(p) {
    self["external1"](p);
  }
  console.log(foo());
  console.log(foo());
}
;

Also works for ECMASCRIPT5 and a few others I tested. I think what the issue is here is that both language_in and language_out are set to STABLE by default, but STABLE is actually different for input and output (ends up being STABLE_IN and STABLE_OUT internally).

google-closure-compiler -O WHITESPACE_ONLY --formatting PRETTY_PRINT --language_in STABLE --language_out STABLE

 {
  var foo = function(p) {
    self["external1"](p);
  };
  console.log(foo());
  console.log(foo());
}
;

Using STABLE as input language still, but trying NO_TRANSPILE output target to prevent any unexpected behavior:

google-closure-compiler -O WHITESPACE_ONLY --formatting PRETTY_PRINT --language_in STABLE --language_out NO_TRANSPILE

'use strict';
{
  function foo(p) {
    self["external1"](p);
  }
  console.log(foo());
  console.log(foo());
}
;

It’s definitely a bug, but --NO_TRANSPILE output mode is a good trick to have to work around it.