Refactor error for unsupported feature

lib/pseudo.js

@@ -6,7 +6,7 @@
  * @typedef {import('./types.js').SelectState} SelectState
  */
 
-import {unreachable} from 'devlop'
+import {ok as assert, unreachable} from 'devlop'
 import fauxEsmNthCheck from 'nth-check'
 import {zwitch} from 'zwitch'
 import {parent} from './util.js'
@@ -409,10 +409,12 @@ function getCachedNthCheck(query) {
 
   if (!fn) {
     const value = query.argument
+    assert(value, 'expected `argument`')
 
-    /* c8 ignore next 3 -- never happens with our config */
-    if (!value || value.type !== 'Formula') {
-      unreachable('`:nth` has a formula')
+    if (value.type !== 'Formula') {
+      throw new Error(
+        'Expected `nth` formula, such as `even` or `2n+1` (`of` is not yet supported)'
+      )
     }
 
     fn = nthCheck(value.a + 'n+' + value.b)

test/select-all.js

@@ -438,6 +438,15 @@ test('select.selectAll()', async function (t) {
           [u('b', 'Bravo'), u('b', 'Delta'), u('b', 'Foxtrot')]
         )
       })
+
+      await t.test(
+        'should throw on unsupported `of` syntax',
+        async function () {
+          assert.throws(function () {
+            selectAll(':nth-child(odd of a)', u('a'))
+          }, /Expected `nth` formula, such as `even` or `2n\+1` \(`of` is not yet supported\)/)
+        }
+      )
     })
 
     await t.test(':nth-last-child', async function (t) {