add support for custom tags and filters

CLAUDE.md

@@ -78,6 +78,23 @@ Django parses everything between `{{` and `}}` — and the entire remainder of `
 whitespace first, so a pipe there must be tight. Hence `_filtered_value_spaced` /
 `_filter_expression_spaced`, aliased back to the plain node names so the tree shape stays identical.
 
+### Names a `{% load %}` brought in
+
+A library's tags and filters are registered in Python, so the grammar cannot know their names, their
+arity, or whether a tag is a block tag. Two fallback rules accept them:
+
+- `custom_tag` — `simpleTag($, $.identifier)`, i.e. what `simple_tag`/`inclusion_tag` accept, which is how
+  a tag is registered unless it needs the parser itself.
+- the last alternative of `filter`, whose name is `$.identifier` rather than one of the builtin literals.
+
+**Builtin names must stay keyword-extractable, or the fallbacks swallow them.** `word: $.identifier` turns
+each builtin's name into its own token, which the lexer prefers wherever it is valid, so a builtin commits
+to its own alternative and its argument rules still apply. This is why a filter's name and its `:` are
+separate tokens: while the name was spelled `"add:"` it was not word-shaped, so a bare `add` lexed as an
+identifier and reached the fallback, losing the arity check for all 20 filters that require an argument.
+`FILTERS_WITHOUT_ARGUMENT` / `FILTERS_WITH_ARGUMENT` / `FILTERS_WITH_OPTIONAL_ARGUMENT` carry that arity,
+which Django itself enforces while it parses the template ("add requires 2 arguments, 1 provided").
+
 ### External scanner (`src/scanner.c`)
 
 Nine external tokens, for the constraints a context-free grammar cannot express:
@@ -99,7 +116,8 @@ do not, and belong in a linter instead.
 ### Queries
 
 `queries/highlights.scm` lists tag and filter names explicitly, so adding either to the grammar means
-adding it there too. `queries/locals.scm` relies on the `variable:` field to tell a binding from a
+adding it there too; filter names appear there without their colon. A name the grammar does not know is
+captured through its field instead (`(filter name: (identifier))`, `(custom_tag tag: (identifier))`). `queries/locals.scm` relies on the `variable:` field to tell a binding from a
 reference — another reason to keep `asVariable` uniform.
 
 ## Testing
@@ -137,5 +155,14 @@ documentation or memory. Both checks are worth repeating whenever tags, filters
 - `{% querystring page=2 page=3 %}` (a repeated `**kwargs` key) is a Django error but is not expressible in
   a context-free grammar. Repeats _are_ rejected wherever the signature names its arguments.
 - `{% csp_nonce_attr "a" media="b" %}` is accepted, matching Django, which only rejects it at render time.
+- A misspelled tag or filter cannot be caught: `{{ x|lenght }}` is indistinguishable from a filter some
+  library registered. Cross-referencing names against `{% load %}` needs the project's Python, so it
+  belongs in a linter; the `load` rule already parses the library and `from` names for one to use.
+- `{% mytag %}…{% endmytag %}` parses as two sibling `custom_tag`s. Nesting them would mean guessing that
+  an unknown tag is a block tag.
+- A custom tag's contents are only assumed to be the `simple_tag` shape. `@register.tag` receives the raw
+  token and may parse anything, so `{% mytag <<>> %}` is rejected here and not by Django.
+- A library may override a builtin (`Parser.add_library` is `self.filters.update(...)`), so a project that
+  redefines `length` with a different arity gets a false error here.
 - `partial`/`partialdef` are Django builtins as of Django 6; `elif`/`else`/`empty` are modelled as parts of
   their enclosing tag rather than as separate tags.

grammar.js

@@ -38,7 +38,7 @@ function part(first, ...rest) {
 
 /**
  * A tag, which may hug its delimiters ("{%cycle 1%}") or not.
- * @param {string} tag
+ * @param {RuleOrLiteral} tag
  * @param {...RuleOrLiteral} args
  * @returns {SeqRule}
  */
@@ -121,7 +121,7 @@ function arrangements(items) {
  * @param {boolean|Record<string, RuleOrLiteral>} kwargs true for any
  *   "name=value" pair (**kwargs), false for none, or the accepted names mapped
  *   to their value rules, each of which may be given at most once
- * @param {string} tag
+ * @param {RuleOrLiteral} tag
  * @returns {SeqRule}
  */
 function simpleTag($, tag, args = true, kwargs = true) {
@@ -148,6 +148,79 @@ function simpleTag($, tag, args = true, kwargs = true) {
   return block(tag, ...parts, optional(asVariable($)));
 }
 
+/**
+ * Django checks a filter's argument count against the registered function
+ * while it parses the template, so the arity of a builtin is syntax here:
+ * "length:2" and a bare "add" are both errors.
+ */
+const FILTERS_WITHOUT_ARGUMENT = [
+  "addslashes",
+  "capfirst",
+  "escape",
+  "escapejs",
+  "escapeseq",
+  "filesizeformat",
+  "first",
+  "force_escape",
+  "iriencode",
+  "last",
+  "length",
+  "linebreaks",
+  "linebreaksbr",
+  "linenumbers",
+  "lower",
+  "make_list",
+  "phone2numeric",
+  "pprint",
+  "random",
+  "safe",
+  "safeseq",
+  "slugify",
+  "striptags",
+  "title",
+  "unordered_list",
+  "upper",
+  "urlize",
+  "wordcount",
+];
+
+/** Builtin filters that require an argument. */
+const FILTERS_WITH_ARGUMENT = [
+  "add",
+  "center",
+  "cut",
+  "default",
+  "default_if_none",
+  "dictsort",
+  "dictsortreversed",
+  "divisibleby",
+  "get_digit",
+  "join",
+  "ljust",
+  "rjust",
+  "slice",
+  "stringformat",
+  "truncatechars",
+  "truncatechars_html",
+  "truncatewords",
+  "truncatewords_html",
+  "urlizetrunc",
+  "wordwrap",
+];
+
+/** Builtin filters that take an argument or no argument. */
+const FILTERS_WITH_OPTIONAL_ARGUMENT = [
+  "date",
+  "floatformat",
+  "json_script",
+  "pluralize",
+  "time",
+  "timesince",
+  "timeuntil",
+  "urlencode",
+  "yesno",
+];
+
 /** Django's string constant, which takes any escape but a line break. */
 const STRING = /"(?:[^"\\]|\\[^\n])*"|'(?:[^'\\]|\\[^\n])*'/;
 
@@ -267,6 +340,7 @@ const django = grammar({
         $.comment_group,
         $.csp_nonce_attr,
         $.csrf_token,
+        $.custom_tag,
         $.cycle,
         $.debug,
         $.extends,
@@ -291,65 +365,30 @@ const django = grammar({
         $.width_ratio,
         $.with_group,
       ),
+    /**
+     * A filter is a name and, after a colon, at most one argument. The name is
+     * a token of its own rather than part of a "name:" token so that Django's
+     * builtins are keyword-extracted: a builtin written with the wrong arity
+     * then cannot be read as a filter from a loaded library instead.
+     */
     filter: ($) =>
       choice(
-        seq("add:", $.value),
+        field("name", choice(...FILTERS_WITHOUT_ARGUMENT)),
-        "addslashes",
+        seq(
-        "capfirst",
+          field("name", choice(...FILTERS_WITH_ARGUMENT)),
-        seq("center:", $.value),
+          ":",
-        seq("cut:", $.value),
+          field("argument", $.value),
-        choice(seq("date:", $.value), "date"),
+        ),
-        seq("default:", $.value),
+        seq(
-        seq("default_if_none:", $.value),
+          field("name", choice(...FILTERS_WITH_OPTIONAL_ARGUMENT)),
-        seq("dictsort:", $.value),
+          optional(seq(":", field("argument", $.value))),
-        seq("dictsortreversed:", $.value),
+        ),
-        seq("divisibleby:", $.value),
+        // a filter a "load" brought in: its name is not known here, and so
-        "escape",
+        // neither is whether it takes an argument
-        "escapejs",
+        seq(
-        "escapeseq",
+          field("name", $.identifier),
-        "filesizeformat",
+          optional(seq(":", field("argument", $.value))),
-        "first",
+        ),
-        choice(seq("floatformat:", $.value), "floatformat"),
-        "force_escape",
-        seq("get_digit:", $.value),
-        "iriencode",
-        seq("join:", $.value),
-        choice(seq("json_script:", $.value), "json_script"),
-        "last",
-        "length",
-        "linebreaks",
-        "linebreaksbr",
-        "linenumbers",
-        seq("ljust:", $.value),
-        "lower",
-        "make_list",
-        "phone2numeric",
-        choice(seq("pluralize:", $.value), "pluralize"),
-        "pprint",
-        "random",
-        seq("rjust:", $.value),
-        "safe",
-        "safeseq",
-        seq("slice:", $.value),
-        "slugify",
-        seq("stringformat:", $.value),
-        "striptags",
-        choice(seq("time:", $.value), "time"),
-        choice(seq("timesince:", $.value), "timesince"),
-        choice(seq("timeuntil:", $.value), "timeuntil"),
-        "title",
-        seq("truncatechars:", $.value),
-        seq("truncatechars_html:", $.value),
-        seq("truncatewords:", $.value),
-        seq("truncatewords_html:", $.value),
-        "unordered_list",
-        "upper",
-        choice(seq("urlencode:", $.value), "urlencode"),
-        "urlize",
-        seq("urlizetrunc:", $.value),
-        "wordcount",
-        seq("wordwrap:", $.value),
-        choice(seq("yesno:", $.value), "yesno"),
       ),
     autoescape_group: ($) =>
       seq(
@@ -374,6 +413,12 @@ const django = grammar({
     csp_nonce_attr: ($) =>
       simpleTag($, "csp_nonce_attr", 1, { media: $.filtered_value }),
     csrf_token: ($) => block("csrf_token"),
+    /**
+     * A tag a "load" brought in. Its arguments cannot be known here, so what
+     * is accepted is what Library.simple_tag and Library.inclusion_tag take,
+     * which is how a tag is registered unless it needs the parser itself.
+     */
+    custom_tag: ($) => simpleTag($, $.identifier),
     cycle: ($) =>
       block(
         "cycle",

queries/highlights.scm

@@ -25,6 +25,7 @@ variable: (identifier) @variable.parameter
     ("in")
     ("=")
     ("|")
+    (":")
     (".")] @operator
 
 [
@@ -90,74 +91,72 @@ tag: [
     ("with")
     ("endwith")] @function
 
-(filter .
+(filter
-    [
+    name: [
-        ("add:")
+        ("add")
         ("addslashes")
         ("capfirst")
-        ("center:")
+        ("center")
-        ("cut:")
+        ("cut")
         ("date")
-        ("date:")
+        ("default")
-        ("default:")
+        ("default_if_none")
-        ("default_if_none:")
+        ("dictsort")
-        ("dictsort:")
+        ("dictsortreversed")
-        ("dictsortreversed:")
+        ("divisibleby")
-        ("divisibleby:")
         ("escape")
         ("escapejs")
         ("escapeseq")
         ("filesizeformat")
         ("first")
         ("floatformat")
-        ("floatformat:")
         ("force_escape")
-        ("get_digit:")
+        ("get_digit")
         ("iriencode")
-        ("join:")
+        ("join")
         ("json_script")
-        ("json_script:")
         ("last")
         ("length")
         ("linebreaks")
         ("linebreaksbr")
         ("linenumbers")
-        ("ljust:")
+        ("ljust")
         ("lower")
         ("make_list")
         ("phone2numeric")
         ("pluralize")
-        ("pluralize:")
         ("pprint")
         ("random")
-        ("rjust:")
+        ("rjust")
         ("safe")
         ("safeseq")
-        ("slice:")
+        ("slice")
         ("slugify")
-        ("stringformat:")
+        ("stringformat")
         ("striptags")
         ("time")
-        ("time:")
         ("timesince")
-        ("timesince:")
         ("timeuntil")
-        ("timeuntil:")
         ("title")
-        ("truncatechars:")
+        ("truncatechars")
-        ("truncatechars_html:")
+        ("truncatechars_html")
-        ("truncatewords:")
+        ("truncatewords")
-        ("truncatewords_html:")
+        ("truncatewords_html")
         ("unordered_list")
         ("upper")
         ("urlencode")
-        ("urlencode:")
         ("urlize")
-        ("urlizetrunc:")
+        ("urlizetrunc")
         ("wordcount")
-        ("wordwrap:")
+        ("wordwrap")
-        ("yesno")
+        ("yesno")] @function)
-        ("yesno:")] @function)
+
+; a filter or tag a "load" brought in, whose name the grammar does not know
+(filter
+    name: (identifier) @function.call)
+
+(custom_tag
+    tag: (identifier) @function.call)
 
 (load
     (identifier) @function)

src/node-types.json

@@ -23,6 +23,10 @@
         "type": "csrf_token",
         "named": true
       },
+      {
+        "type": "custom_tag",
+        "named": true
+      },
       {
         "type": "cycle",
         "named": true
@@ -300,6 +304,46 @@
       }
     }
   },
+  {
+    "type": "custom_tag",
+    "named": true,
+    "fields": {
+      "tag": {
+        "multiple": false,
+        "required": true,
+        "types": [
+          {
+            "type": "identifier",
+            "named": true
+          }
+        ]
+      },
+      "variable": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": "identifier",
+            "named": true
+          }
+        ]
+      }
+    },
+    "children": {
+      "multiple": true,
+      "required": false,
+      "types": [
+        {
+          "type": "filtered_value",
+          "named": true
+        },
+        {
+          "type": "identifier",
+          "named": true
+        }
+      ]
+    }
+  },
   {
     "type": "cycle",
     "named": true,
@@ -381,16 +425,255 @@
   {
     "type": "filter",
     "named": true,
-    "fields": {},
+    "fields": {
-    "children": {
+      "argument": {
-      "multiple": false,
+        "multiple": false,
-      "required": false,
+        "required": false,
-      "types": [
+        "types": [
-        {
+          {
-          "type": "value",
+            "type": "value",
-          "named": true
+            "named": true
-        }
+          }
-      ]
+        ]
+      },
+      "name": {
+        "multiple": false,
+        "required": true,
+        "types": [
+          {
+            "type": "add",
+            "named": false
+          },
+          {
+            "type": "addslashes",
+            "named": false
+          },
+          {
+            "type": "capfirst",
+            "named": false
+          },
+          {
+            "type": "center",
+            "named": false
+          },
+          {
+            "type": "cut",
+            "named": false
+          },
+          {
+            "type": "date",
+            "named": false
+          },
+          {
+            "type": "default",
+            "named": false
+          },
+          {
+            "type": "default_if_none",
+            "named": false
+          },
+          {
+            "type": "dictsort",
+            "named": false
+          },
+          {
+            "type": "dictsortreversed",
+            "named": false
+          },
+          {
+            "type": "divisibleby",
+            "named": false
+          },
+          {
+            "type": "escape",
+            "named": false
+          },
+          {
+            "type": "escapejs",
+            "named": false
+          },
+          {
+            "type": "escapeseq",
+            "named": false
+          },
+          {
+            "type": "filesizeformat",
+            "named": false
+          },
+          {
+            "type": "first",
+            "named": false
+          },
+          {
+            "type": "floatformat",
+            "named": false
+          },
+          {
+            "type": "force_escape",
+            "named": false
+          },
+          {
+            "type": "get_digit",
+            "named": false
+          },
+          {
+            "type": "identifier",
+            "named": true
+          },
+          {
+            "type": "iriencode",
+            "named": false
+          },
+          {
+            "type": "join",
+            "named": false
+          },
+          {
+            "type": "json_script",
+            "named": false
+          },
+          {
+            "type": "last",
+            "named": false
+          },
+          {
+            "type": "length",
+            "named": false
+          },
+          {
+            "type": "linebreaks",
+            "named": false
+          },
+          {
+            "type": "linebreaksbr",
+            "named": false
+          },
+          {
+            "type": "linenumbers",
+            "named": false
+          },
+          {
+            "type": "ljust",
+            "named": false
+          },
+          {
+            "type": "lower",
+            "named": false
+          },
+          {
+            "type": "make_list",
+            "named": false
+          },
+          {
+            "type": "phone2numeric",
+            "named": false
+          },
+          {
+            "type": "pluralize",
+            "named": false
+          },
+          {
+            "type": "pprint",
+            "named": false
+          },
+          {
+            "type": "random",
+            "named": false
+          },
+          {
+            "type": "rjust",
+            "named": false
+          },
+          {
+            "type": "safe",
+            "named": false
+          },
+          {
+            "type": "safeseq",
+            "named": false
+          },
+          {
+            "type": "slice",
+            "named": false
+          },
+          {
+            "type": "slugify",
+            "named": false
+          },
+          {
+            "type": "stringformat",
+            "named": false
+          },
+          {
+            "type": "striptags",
+            "named": false
+          },
+          {
+            "type": "time",
+            "named": false
+          },
+          {
+            "type": "timesince",
+            "named": false
+          },
+          {
+            "type": "timeuntil",
+            "named": false
+          },
+          {
+            "type": "title",
+            "named": false
+          },
+          {
+            "type": "truncatechars",
+            "named": false
+          },
+          {
+            "type": "truncatechars_html",
+            "named": false
+          },
+          {
+            "type": "truncatewords",
+            "named": false
+          },
+          {
+            "type": "truncatewords_html",
+            "named": false
+          },
+          {
+            "type": "unordered_list",
+            "named": false
+          },
+          {
+            "type": "upper",
+            "named": false
+          },
+          {
+            "type": "urlencode",
+            "named": false
+          },
+          {
+            "type": "urlize",
+            "named": false
+          },
+          {
+            "type": "urlizetrunc",
+            "named": false
+          },
+          {
+            "type": "wordcount",
+            "named": false
+          },
+          {
+            "type": "wordwrap",
+            "named": false
+          },
+          {
+            "type": "yesno",
+            "named": false
+          }
+        ]
+      }
     }
   },
   {
@@ -1317,6 +1600,10 @@
     "type": ".",
     "named": false
   },
+  {
+    "type": ":",
+    "named": false
+  },
   {
     "type": "<",
     "named": false
@@ -1346,7 +1633,7 @@
     "named": false
   },
   {
-    "type": "add:",
+    "type": "add",
     "named": false
   },
   {
@@ -1386,7 +1673,7 @@
     "named": false
   },
   {
-    "type": "center:",
+    "type": "center",
     "named": false
   },
   {
@@ -1426,7 +1713,7 @@
     "named": false
   },
   {
-    "type": "cut:",
+    "type": "cut",
     "named": false
   },
   {
@@ -1437,32 +1724,28 @@
     "type": "date",
     "named": false
   },
-  {
-    "type": "date:",
-    "named": false
-  },
   {
     "type": "debug",
     "named": false
   },
   {
-    "type": "default:",
+    "type": "default",
     "named": false
   },
   {
-    "type": "default_if_none:",
+    "type": "default_if_none",
     "named": false
   },
   {
-    "type": "dictsort:",
+    "type": "dictsort",
     "named": false
   },
   {
-    "type": "dictsortreversed:",
+    "type": "dictsortreversed",
     "named": false
   },
   {
-    "type": "divisibleby:",
+    "type": "divisibleby",
     "named": false
   },
   {
@@ -1557,10 +1840,6 @@
     "type": "floatformat",
     "named": false
   },
-  {
-    "type": "floatformat:",
-    "named": false
-  },
   {
     "type": "for",
     "named": false
@@ -1574,7 +1853,7 @@
     "named": false
   },
   {
-    "type": "get_digit:",
+    "type": "get_digit",
     "named": false
   },
   {
@@ -1610,17 +1889,13 @@
     "named": false
   },
   {
-    "type": "join:",
+    "type": "join",
     "named": false
   },
   {
     "type": "json_script",
     "named": false
   },
-  {
-    "type": "json_script:",
-    "named": false
-  },
   {
     "type": "last",
     "named": false
@@ -1642,7 +1917,7 @@
     "named": false
   },
   {
-    "type": "ljust:",
+    "type": "ljust",
     "named": false
   },
   {
@@ -1725,10 +2000,6 @@
     "type": "pluralize",
     "named": false
   },
-  {
-    "type": "pluralize:",
-    "named": false
-  },
   {
     "type": "pop_block",
     "named": true
@@ -1774,7 +2045,7 @@
     "named": false
   },
   {
-    "type": "rjust:",
+    "type": "rjust",
     "named": false
   },
   {
@@ -1790,7 +2061,7 @@
     "named": false
   },
   {
-    "type": "slice:",
+    "type": "slice",
     "named": false
   },
   {
@@ -1806,7 +2077,7 @@
     "named": true
   },
   {
-    "type": "stringformat:",
+    "type": "stringformat",
     "named": false
   },
   {
@@ -1821,44 +2092,32 @@
     "type": "time",
     "named": false
   },
-  {
-    "type": "time:",
-    "named": false
-  },
   {
     "type": "timesince",
     "named": false
   },
-  {
-    "type": "timesince:",
-    "named": false
-  },
   {
     "type": "timeuntil",
     "named": false
   },
-  {
-    "type": "timeuntil:",
-    "named": false
-  },
   {
     "type": "title",
     "named": false
   },
   {
-    "type": "truncatechars:",
+    "type": "truncatechars",
     "named": false
   },
   {
-    "type": "truncatechars_html:",
+    "type": "truncatechars_html",
     "named": false
   },
   {
-    "type": "truncatewords:",
+    "type": "truncatewords",
     "named": false
   },
   {
-    "type": "truncatewords_html:",
+    "type": "truncatewords_html",
     "named": false
   },
   {
@@ -1877,16 +2136,12 @@
     "type": "urlencode",
     "named": false
   },
-  {
-    "type": "urlencode:",
-    "named": false
-  },
   {
     "type": "urlize",
     "named": false
   },
   {
-    "type": "urlizetrunc:",
+    "type": "urlizetrunc",
     "named": false
   },
   {
@@ -1914,17 +2169,13 @@
     "named": false
   },
   {
-    "type": "wordwrap:",
+    "type": "wordwrap",
     "named": false
   },
   {
     "type": "yesno",
     "named": false
   },
-  {
-    "type": "yesno:",
-    "named": false
-  },
   {
     "type": "{#",
     "named": false

test/corpus/custom.txt

@@ -0,0 +1,231 @@
+==================
+Custom Tag
+==================
+
+{% mytag %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier))
+  (content))
+
+==================
+Custom Tag With Arguments
+==================
+
+{% mytag a "b" 3 %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier)
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier))))
+    (filtered_value
+      (value
+        (literal
+          (string))))
+    (filtered_value
+      (value
+        (literal
+          (number)))))
+  (content))
+
+==================
+Custom Tag With Keyword Arguments
+==================
+
+{% mytag a b=1 %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier)
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier))))
+    (identifier)
+    (filtered_value
+      (value
+        (literal
+          (number)))))
+  (content))
+
+==================
+Custom Tag With As
+==================
+
+{% mytag a as result %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier)
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier))))
+    variable: (identifier))
+  (content))
+
+==================
+Custom Tag With Filtered Argument
+==================
+
+{% mytag a|upper %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier)
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier)))
+      (filter_expression
+        (filter))))
+  (content))
+
+==================
+Custom Tag Does Not Nest
+==================
+
+{% mytag %}hi{% endmytag %}
+
+---
+
+(template
+  (content)
+  (custom_tag
+    tag: (identifier))
+  (content)
+  (custom_tag
+    tag: (identifier))
+  (content))
+
+==================
+Custom Tag Does Not Cover A Builtin Name
+:error
+==================
+
+{% cycle %}
+
+---
+
+
+==================
+Custom Filter
+==================
+
+{{ x|intcomma }}
+
+---
+
+(template
+  (content)
+  (template_variable
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier)))
+      (filter_expression
+        (filter
+          name: (identifier)))))
+  (content))
+
+==================
+Custom Filter With Argument
+==================
+
+{{ x|naturaltime:"y" }}
+
+---
+
+(template
+  (content)
+  (template_variable
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier)))
+      (filter_expression
+        (filter
+          name: (identifier)
+          argument: (value
+            (literal
+              (string)))))))
+  (content))
+
+==================
+Custom Filter In A Chain With A Builtin
+==================
+
+{{ x|intcomma|upper }}
+
+---
+
+(template
+  (content)
+  (template_variable
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier)))
+      (filter_expression
+        (filter
+          name: (identifier))
+        (filter))))
+  (content))
+
+==================
+Custom Filter In A Tag
+==================
+
+{% firstof a|intcomma %}
+
+---
+
+(template
+  (content)
+  (firstof
+    (filtered_value
+      (value
+        (variable_attribute
+          (identifier)))
+      (filter_expression
+        (filter
+          name: (identifier)))))
+  (content))
+
+==================
+Builtin Filter Without Its Required Argument
+:error
+==================
+
+{{ x|add }}
+
+---
+
+
+==================
+Builtin Filter With An Argument It Does Not Take
+:error
+==================
+
+{{ x|length:2 }}
+
+---