[Slash Separator] Selector functions throw errors for slash lists

Closes #2706

accepted/slash-separator.changes.md

@@ -2,6 +2,9 @@
 
 * Require at least two arguments for `slash-list()`.
 
+* Require that selector functions throw errors when passed slash-separated
+  lists.
+
 ## Draft 1
 
 * Initial draft.

accepted/slash-separator.md

@@ -22,6 +22,7 @@ operator.
   * [`slash-list()` Function](#slash-list-function)
   * [`rgb()` Function](#rgb-function)
   * [`hsl()` Function](#hsl-function)
+  * [Selector Functions](#selector-functions)
 * [Deprecation Process](#deprecation-process)
   * [Phase 1](#phase-1)
   * [Phase 2](#phase-2)
@@ -348,6 +349,13 @@ overload to be the following:
 
 * Otherwise, proceed with the existing definition of the function.
 
+### Selector Functions
+
+This proposal modifies [the "Parse a Selector From a SassScript Object"
+procedure][] to throw an error whenever it encounters a slash-separated list.
+
+[the "Parse a Selector From a SassScript Object" procedure]: ../spec/functions/selector.md#parse-a-selector-from-a-sassscript-object
+
 ## Deprecation Process
 
 The deprecation process will be divided into three phases:

spec/functions/selector.md

@@ -0,0 +1,52 @@
+# Selector Functions
+
+## Table of Contents
+
+* [Procedures](#procedures)
+  * [Parse a Selector From a SassScript Object](#parse-a-selector-from-a-sassscript-object)
+
+## Procedures
+
+### Parse a Selector From a SassScript Object
+
+This algorithm takes a SassScript object `selector` and returns an abstract
+representation of a CSS selector.
+
+* Set `text` to an empty string.
+
+* If `selector` is a list:
+
+  * If `selector` is bracketed, throw an error.
+
+  * If `selector` is space-separated:
+
+    * If `selector` contains any non-string elements, throw an error.
+
+    * Set `text` to the concatenation of each element of `selector`, separated
+      by spaces.
+
+  * Otherwise, if `selector` is comma-separated:
+
+    * For each element `complex` of `selector`:
+
+      * If `complex` is a list:
+
+        * If `complex` is bracketed or comma-separated, throw an error.
+
+        * Otherwise, if `complex` contains any non-string elements, throw an error.
+
+        * Otherwise, append the concatenation of each element of `selector`, separated
+          by spaces, to `text`.
+
+      * Otherwise, if `complex` is not a string, throw an error.
+
+      * Otherwise, append `complex` to text.
+
+      * Append a comma to `text` unless `complex` is the last element of
+        `selector`.
+      
+* Otherwise, if `selector` is not a string, throw an error.
+
+* Otherwise, set `text` to the contents of `selector`.
+
+* Parse `text` as a selector and return the result.