Regression in selector interpolation

[xzyfer]

The follow selector interpolation worked in 3.1.0 but fails with 3.2.0-beta.1 and above

$i: 1;
.foo#{-$i} { a:b }

3.2.0-beta.2 and above produce the following error

Error: invalid selector after .foo-
        on line 1 of test.scss

3.2.0-beta.1 also fails but reports a different line. A git bisect reports this regression was introduced in 7877cfa

The following variations work in all tested releases

$i: 1;
.foo-#{$i} { a:b }
.foo#{-1} { a:b }
.foo-#{1} { a:b }

Spec added sass/sass-spec#309

Can you take a look @mgreter ?

[mgreter]

Error seems to happen in expand, since it seems to be parsed correctly:

String_Schema 0x2c322b0 (0@[0:6]-[1:0]) 4
 String_Quoted : 0x2a41dc0 (0@[0:6]-[1:0]) [.foo] <>
 Unary_Expression 0x2a41e40 (0@[0:6]-[0:7]) [1]
  operand: Variable 0x2c84be0 (0@[0:7]-[0:9]) [$i]

My guess is the unary expression is not correctly or not at all evaluated!
Edit: seems to be a problem in interpolation in eval.cpp (my bad).
Another usefull spec test discovered I would say ahem 😄