This works as expected:

    @if $condition @def $value = "foo";
    @else @def $value = "bar";

but this does not:

    @if $condition {
	@def $value = "foo";
    } @else {
	@def $value = "bar";
    }

because $value is only defined in the blocks, not the surrounding scope,
which makes it useless.

(This is simplified from a practical case, where I'd like the
condition to control several definitions of variables, functions, etc.
Instead I have to push the condition down inside each definition as an
ugly workaround.)