<===> input.scss
@function calc($e) { @return custom; }
@function -foo-calc($e) { @return custom; }

.test {
    a: calc(1px * 1%);
    b: -foo-calc(2px * 2%);
    c: call(calc, 3px * 3%);
}

<===> error-dart-sass
Error: Invalid function name.
  ,
1 | @function calc($e) { @return custom; }
  | ^^^^^^^^^^^^^^^^^^
  '
  input.scss 1:1  root stylesheet

<===> output.css
.test {
  a: calc(1px * 1%);
  b: -foo-calc(2px * 2%);
  c: custom;
}

<===> warning
DEPRECATION WARNING on line 1 of /sass/spec/libsass-issues/issue_1706/input.scss:
Naming a function "calc" is disallowed and will be an error in future versions of Sass.
This name conflicts with an existing CSS function with special parse rules.

DEPRECATION WARNING on line 2 of /sass/spec/libsass-issues/issue_1706/input.scss:
Naming a function "-foo-calc" is disallowed and will be an error in future versions of Sass.
This name conflicts with an existing CSS function with special parse rules.