$a: to-upper-case('abcd');
$b: to-upper-case("abcd");
$c: to-upper-case(abcd);

foo {
    content: #{$a};
    content: #{$b};
    content: #{$c};
    content: '#{$a}';
    content: '#{$b}';
    content: '#{$c}';
    content: "#{$a}";
    content: "#{$b}";
    content: "#{$c}";

    content: #{unquote($a)};
    content: #{unquote($b)};
    content: #{unquote($c)};
    content: '#{unquote($a)}';
    content: '#{unquote($b)}';
    content: '#{unquote($c)}';
    content: "#{unquote($a)}";
    content: "#{unquote($b)}";
    content: "#{unquote($c)}";

    content: #{$a + unquote("efg")};
    content: #{$b + unquote("efg")};
    content: #{$c + unquote("efg")};
    content: '#{$a + unquote("efg")}';
    content: '#{$b + unquote("efg")}';
    content: '#{$c + unquote("efg")}';
    content: "#{$a + unquote("efg")}";
    content: "#{$b + unquote("efg")}";
    content: "#{$c + unquote("efg")}";

    content: #{$a + unquote("")};
    content: #{$b + unquote("")};
    content: #{$c + unquote("")};
    content: '#{$a + unquote("")}';
    content: '#{$b + unquote("")}';
    content: '#{$c + unquote("")}';
    content: "#{$a + unquote("")}";
    content: "#{$b + unquote("")}";
    content: "#{$c + unquote("")}";
}