Broken order of operations when using less variables in calculation and not using less to evaluate it

[battlesnake]

Demo:

@a: 1em - 1px;
@b: 1em + 1px;

sum {
    actual: calc(@a + 2 * @b);
    expect: calc(1em - 1px + 2 * (1em + 1px));
    fix: calc((@a) + 2 * (@b));
}

diff {
    actual: calc(@a - 2 * @b);
    expect: calc(1em - 1px - 2 * (1em + 1px));
    fix: calc((@a) - 2 * (@b));
}

Test:

$ lessc -su=on -sm=on test.less
sum {
  actual: calc(1em - 1px + 2 * 1em + 1px);
  expect: calc(1em - 1px + 2 * (1em + 1px));
  fix: calc((1em - 1px) + 2 * (1em + 1px));
}
diff {
  actual: calc(1em - 1px - 2 * 1em + 1px);
  expect: calc(1em - 1px - 2 * (1em + 1px));
  fix: calc((1em - 1px) - 2 * (1em + 1px));
}

I understand that this is defined behaviour and not a bug, however it seems a little unintuitive at first so should probably be in the main docs.

[rjgotten]

Numeric expressions that can not be further reduced should probably have their output CSS value changed to add enclosing braces. Solves the problem completely, I think.

[seven-phases-max]

Yes, it would make sense to document but we don't have such "all that thousands common pitfalls" section there.

Speaking of the code itself: with -sm=on it would be almost must-have recommendation to always put parens around a variable value if it's supposed to represent an arithmetic expression, e.g::

// -sm:on
@a:  1 + 2;  // this has nothing to do with arithmetics at all!
@b: (1 + 2); // now this is an arithmetic expression

Also in regards of sm:on: see #1872 (and its #1880 follow up). So to be honest I'd recommend to you revert this ;) - unless you're forced to use sm:on by an external library (e.g. Bootstrap).
(su:on does not probably have any critical problems, but considering that almost nobody uses this mode your just make you code less-compatible, beside since it not quite widely used it's also much less tested en masse for possible inconsistencies).

---

Assuming it's not really a compiler issue but more like a documentation suggestion I wonder if this should be moved to less.js/less-docs tracker.

[seven-phases-max]

@rjgotten

changed to add enclosing braces

The only purpose of sm:on is "to not treat values like font: ... 1em/2px ...; as arithmetic ops". So by automatically adding parens you'll break those.

[seven-phases-max]

OK, before comments start to pile up I'm closing this in favour of #1880 (please do not hesitate to put any ideas there). And for docs suggestions please use less.js/less-docs.

[battlesnake]

sm:on also allows one to use CSS calc. I use su:on as my background is physics and engineering, and I like dimensional consistency.

[seven-phases-max]

Merged to #1880.

[rjgotten]

@seven-phases-max
Oh right. I missed the strictMath option there, in its shorthand form. D'oh! Sorry...