Generated by SassMeister.com.

input.scss

@function add($value1, $value2, $return-calc: true) {
  @if $value1 == null {
    @return $value2;
  }

  @if $value2 == null {
    @return $value1;
  }

  @if type-of($value1) == number and type-of($value2) == number and comparable($value1, $value2) {
    @return $value1 + $value2;
  }

  @return if($return-calc == true, calc(#{$value1} + #{$value2}), $value1 + unquote(" + ") + $value2);
}

@function subtract($value1, $value2, $return-calc: true) {
  @if $value1 == null and $value2 == null {
    @return null;
  }

  @if $value1 == null {
    @return -$value2;
  }

  @if $value2 == null {
    @return $value1;
  }

  @if type-of($value1) == number and type-of($value2) == number and comparable($value1, $value2) {
    @return $value1 - $value2;
  }

  @if type-of($value2) != number {
    $value2: unquote("(") + $value2 + unquote(")");
  }

  @return if($return-calc == true, calc(#{$value1} - #{$value2}), $value1 + unquote(" - ") + $value2);
}

$add: add(1rem, 1em, false);
$subtract: subtract(1rem, 1em, false);

a {
    font-size: add(1vw, $add);
    font-size: add(1vw, $subtract);
    font-size: subtract(1vw, $add);
    font-size: subtract(1vw, $subtract);
}

output.css

a {
  font-size: calc(1vw + 1rem + 1em);
  font-size: calc(1vw + 1rem - 1em);
  font-size: calc(1vw - (1rem + 1em));
  font-size: calc(1vw - (1rem - 1em));
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.32.12",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}