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feat(linter): add forbid_empty_enum rule #2287

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feat(linter): add forbid_empty_enum rule #2287

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040c9ed
feat(linter): make forbid_empty_enum auto-fixable by rewriting to false
Vaibhav701161 Mar 13, 2026
cbadb5c
test(linter): update forbid_empty_enum tests for 2019-09 and 2020-12
Vaibhav701161 Mar 13, 2026
b5d92d6
test(linter): update forbid_empty_enum tests for draft4, draft6, and …
Vaibhav701161 Mar 13, 2026
ec6066e
test(linter): add edge cases and draft3 coverage for forbid_empty_enum
Vaibhav701161 Mar 13, 2026
2c90804
feat(linter): preserve schema structure in forbid_empty_enum autofix
Vaibhav701161 Mar 13, 2026
b86dd96
test(linter): extend forbid_empty_enum coverage across dialects and r…
Vaibhav701161 Mar 13, 2026
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1 change: 1 addition & 0 deletions src/extension/alterschema/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -80,6 +80,7 @@ sourcemeta_library(NAMESPACE sourcemeta PROJECT core NAME alterschema
linter/duplicate_examples.h
linter/enum_to_const.h
linter/equal_numeric_bounds_to_const.h
linter/forbid_empty_enum.h
linter/items_array_default.h
linter/items_schema_default.h
linter/multiple_of_default.h
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2 changes: 2 additions & 0 deletions src/extension/alterschema/alterschema.cc
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Expand Up @@ -108,6 +108,7 @@ inline auto APPLIES_TO_POINTERS(std::vector<Pointer> &&keywords)
#include "linter/duplicate_examples.h"
#include "linter/enum_to_const.h"
#include "linter/equal_numeric_bounds_to_const.h"
#include "linter/forbid_empty_enum.h"
#include "linter/items_array_default.h"
#include "linter/items_schema_default.h"
#include "linter/multiple_of_default.h"
Expand Down Expand Up @@ -222,6 +223,7 @@ auto add(SchemaTransformer &bundle, const AlterSchemaMode mode) -> void {
bundle.add<UnsatisfiableMaxContains>();
bundle.add<UnsatisfiableMinProperties>();
bundle.add<EnumToConst>();
bundle.add<ForbidEmptyEnum>();
bundle.add<TopLevelTitle>();
bundle.add<TopLevelDescription>();
bundle.add<TopLevelExamples>();
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35 changes: 35 additions & 0 deletions src/extension/alterschema/linter/forbid_empty_enum.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
class ForbidEmptyEnum final : public SchemaTransformRule {
public:
using mutates = std::true_type;
using reframe_after_transform = std::true_type;
ForbidEmptyEnum()
: SchemaTransformRule{"forbid_empty_enum",
"An empty `enum` validates nothing and is "
"equivalent to `not: {}`"} {};

[[nodiscard]] auto
condition(const sourcemeta::core::JSON &schema,
const sourcemeta::core::JSON &,
const sourcemeta::core::Vocabularies &vocabularies,
const sourcemeta::core::SchemaFrame &,
const sourcemeta::core::SchemaFrame::Location &,
const sourcemeta::core::SchemaWalker &,
const sourcemeta::core::SchemaResolver &) const
-> sourcemeta::core::SchemaTransformRule::Result override {
ONLY_CONTINUE_IF(vocabularies.contains_any(
{Vocabularies::Known::JSON_Schema_2020_12_Validation,
Vocabularies::Known::JSON_Schema_2019_09_Validation,
Vocabularies::Known::JSON_Schema_Draft_7,
Vocabularies::Known::JSON_Schema_Draft_6,
Vocabularies::Known::JSON_Schema_Draft_4}) &&
schema.is_object() && schema.defines("enum") &&
!schema.defines("not") && schema.at("enum").is_array() &&
schema.at("enum").empty());
return APPLIES_TO_KEYWORDS("enum");
}

auto transform(JSON &schema, const Result &) const -> void override {
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@jviotti jviotti Mar 13, 2026 •
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Can you add a LINT_AND_FIX test where you have a $ref pointing directly at the subschema with the empty enum and also one pointing to something INSIDE the subschema with empty enum (maybe by having a $defs sibling to the empty enum)?

I suspect the second will break at the moment.

Also what if you have an empty enum on a subschema that has its own nested $id? By turning the entire thing to false, we would end up getting rid of the identifier, which can break a reference to the subschema by URI.

Can you try to encode as many of these edge cases as you can possibly think of? These are the tricky cases that show up in practice and end up in incorrect linter behaviour or crashes

Given all the above, it might be easier to add not: {} instead of turning the entire thing to false. And you can always update the condition to apply for Draft 4 and later (which will address https://github.com/sourcemeta/core/pull/2287/changes#r2931230208) and only apply of not is not there already (a fine compromise for now?)

schema.erase("enum");
schema.assign("not", JSON::make_object());
}
};
246 changes: 246 additions & 0 deletions test/alterschema/alterschema_lint_2019_09_test.cc
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Expand Up @@ -4494,3 +4494,249 @@ TEST(AlterSchema_lint_2019_09, empty_object_as_true_1) {

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_1) {
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@jviotti jviotti Mar 13, 2026

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Now that the rule is always fixable, let's always exercise it with LINT_AND_FIX. Also closely follow the conventions of the other tests. We have expected after the check for result.first, and we don't usually check the traces anymore.

sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"enum": []
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"not": true
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_2) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"enum": [1, 2]
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"enum": [1, 2]
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_3) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"type": "string"
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [1],
"type": "string"
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_4) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"properties": {
"foo": {
"enum": []
}
}
})JSON");
const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"properties": {
"foo": {
"not": true
}
}
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);
EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_5) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"enum": []
}
},
"$ref": "#/$defs/A"
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"not": true
}
},
"$ref": "#/$defs/A"
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_6) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"enum": [],
"$defs": {
"inner": { "type": "string" }
}
}
},
"$ref": "#/$defs/A/$defs/inner"
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"$defs": {
"inner": { "type": "string" }
},
"not": true
}
},
"$ref": "#/$defs/A/$defs/inner"
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_7) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"$id": "https://example.com/schemas/A",
"enum": []
}
},
"$ref": "https://example.com/schemas/A"
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"$defs": {
"A": {
"$id": "https://example.com/schemas/A",
"not": true
}
},
"$ref": "https://example.com/schemas/A"
})JSON");

EXPECT_EQ(document, expected);
}

TEST(AlterSchema_lint_2019_09, forbid_empty_enum_8) {
sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"properties": {
"foo": {
"x-note": "placeholder",
"enum": []
}
}
})JSON");

LINT_AND_FIX(document, result, traces);

EXPECT_TRUE(result.first);

const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON({
"$schema": "https://json-schema.org/draft/2019-09/schema",
"title": "Example",
"description": "Example schema",
"examples": [{}],
"properties": {
"foo": {
"x-note": "placeholder",
"not": true
}
}
})JSON");

EXPECT_EQ(document, expected);
}
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