4.4.0 add function compilation is wrong

Created by: 719media

The bootstrap 4.4 add function doesn't quite work right with sass compilation.

As an example, paste the following

@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} + #{$value2});
}

$line-height-base:            1.5 !default;
$input-btn-padding-y:         .375rem !default;
$border-width:                1px !default;
$test-height: add($line-height-base * 1em, add($input-btn-padding-y * 2, $border-width, false)) !default;

.test-height {
  height: $test-height;
}

into https://www.sassmeister.com/

You'll notice that the compiled css has some wonkiness:

.test-height {
  height: calc(1.5em + 0.75rem1px);
}

One option is to interpolate the plus sign in the function, like so: @return if($return-calc == true, calc(#{$value1} + #{$value2}), #{$value1 '+' $value2});