Fold comma operation when left argument has no side effects.

[leewz]

(Using https://closure-compiler.appspot.com/home with Advanced Optimizations.)

Currently, the compiler shows a warning:

JSC_USELESS_CODE: Suspicious code. This code lacks side-effects. Is there a bug? at line 1 character 13
console.log((1, 0) > 5)
             ^

However, it doesn't (edit:) always constant-fold the comma operator.

This is folded:

console.log((1,0));

This is not:

console.log((1, 0) > 5)

Output:

console.log(5<(1,0));

[MatrixFrog]

If I remember correctly, after reporting that warning, it adds some temporary code to explicitly protect the 1 from being removed. The reason for this is that sometimes an expression that should have no side effects, is actually necessary to workaround a browser bug, e.g. elem.offsetWidth; // force the browser to re-layout

Obviously 1 will never have side effects no matter how buggy your browser is :) so maybe we can make this a little less aggressive.

[leewz]

Since property access is allowed to have side effects, I suppose it should only aggressively fold for literals, identifiers, function expressions, and array/object "literals" containing only foldables. Isn't the exact same judgment needed for identifying dead code? Hm. Implicit string conversion from an operator might cause `#toString` to be called, and maybe you're allowed to override toString on primitive types. But that's a problem for the constant-folder.