Implement a rule to remove empty then or else for the linter

src/extension/alterschema/CMakeLists.txt

@@ -54,7 +54,8 @@ sourcemeta_library(NAMESPACE sourcemeta PROJECT core NAME alterschema
     linter/if_without_then_else.h
     linter/max_contains_without_contains.h
     linter/min_contains_without_contains.h
-    linter/then_without_if.h)
+    linter/then_without_if.h
+    linter/then_else_empty.h)
 
 if(SOURCEMETA_CORE_INSTALL)
   sourcemeta_library_install(NAMESPACE sourcemeta PROJECT core NAME alterschema)

src/extension/alterschema/alterschema.cc

@@ -77,6 +77,7 @@ static auto every_item_is_boolean(const T &container) -> bool {
 #include "linter/pattern_properties_default.h"
 #include "linter/properties_default.h"
 #include "linter/single_type_array.h"
+#include "linter/then_else_empty.h"
 #include "linter/then_without_if.h"
 #include "linter/unevaluated_items_default.h"
 #include "linter/unevaluated_properties_default.h"
@@ -118,6 +119,7 @@ auto add(SchemaTransformer &bundle, const AlterSchemaMode mode)
   switch (mode) {
     case AlterSchemaMode::StaticAnalysis:
       bundle.add<BooleanTrue>();
+      bundle.add<ThenElseEmpty>();
       bundle.add<ConstAsEnum>();
       bundle.add<ExclusiveMaximumIntegerToMaximum>();
       bundle.add<ExclusiveMinimumIntegerToMinimum>();
@@ -135,6 +137,7 @@ auto add(SchemaTransformer &bundle, const AlterSchemaMode mode)
       bundle.add<TypeUnionImplicit>();
       break;
     case AlterSchemaMode::Readability:
+      bundle.add<ThenElseEmpty>();
       bundle.add<AdditionalPropertiesDefault>();
       bundle.add<ContentSchemaDefault>();
       bundle.add<DependenciesDefault>();

src/extension/alterschema/linter/if_without_then_else.h

@@ -15,13 +15,18 @@ class IfWithoutThenElse final : public SchemaTransformRule {
             const sourcemeta::core::SchemaWalker &,
             const sourcemeta::core::SchemaResolver &) const
       -> sourcemeta::core::SchemaTransformRule::Result override {
+    const auto trivial_if = [](const sourcemeta::core::JSON &s) {
+      return s.is_boolean() || (s.is_object() && s.empty());
+    };
+
     return contains_any(
                vocabularies,
                {"https://json-schema.org/draft/2020-12/vocab/applicator",
                 "https://json-schema.org/draft/2019-09/vocab/applicator",
                 "http://json-schema.org/draft-07/schema#"}) &&
            schema.is_object() && schema.defines("if") &&
-           !schema.defines("then") && !schema.defines("else");
+           !schema.defines("then") && !schema.defines("else") &&
+           trivial_if(schema.at("if"));
   }
 
   auto transform(JSON &schema) const -> void override { schema.erase("if"); }

src/extension/alterschema/linter/then_else_empty.h

@@ -0,0 +1,44 @@
+class ThenElseEmpty final : public SchemaTransformRule {
+public:
+  ThenElseEmpty()
+      : SchemaTransformRule{"then_else_empty",
+                            "then or else is an empty schema ({} or true) "
+                            "and therefore adds no restriction"} {}
+
+  [[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 {
+
+    const auto empty_object = [](const sourcemeta::core::JSON &s) {
+      return s.is_object() && s.empty();
+    };
+
+    return contains_any(
+               vocabularies,
+               {"https://json-schema.org/draft/2020-12/vocab/applicator",
+                "https://json-schema.org/draft/2019-09/vocab/applicator",
+                "http://json-schema.org/draft-07/schema#"}) &&
+           schema.is_object() && schema.defines("if") &&
+           ((schema.defines("then") && empty_object(schema.at("then"))) ||
+            (schema.defines("else") && empty_object(schema.at("else"))));
+  }
+
+  auto transform(JSON &schema) const -> void override {
+    const auto empty_object = [](const JSON &s) {
+      return s.is_object() && s.empty();
+    };
+
+    if (schema.defines("then") && empty_object(schema.at("then"))) {
+      schema.erase("then");
+    }
+    if (schema.defines("else") && empty_object(schema.at("else"))) {
+      schema.erase("else");
+    }
+  }
+};

test/alterschema/alterschema_lint_2020_12_test.cc

@@ -1842,3 +1842,22 @@ TEST(AlterSchema_lint_2020_12, unnecessary_allof_ref_wrapper_5) {
 
   EXPECT_EQ(document, expected);
 }
+
+TEST(AlterSchema_Lint_2020_12, empty_then_else_1) {
+  sourcemeta::core::JSON document = sourcemeta::core::parse_json(R"JSON(
+    {
+      "$schema": "https://json-schema.org/draft/2020-12/schema",
+      "if": { "properties": { "flag": { "const": true } } },
+      "then": {}
+    })JSON");
+
+  LINT_AND_FIX_FOR_READABILITY(document);
+
+  const sourcemeta::core::JSON expected = sourcemeta::core::parse_json(R"JSON(
+    {
+      "$schema": "https://json-schema.org/draft/2020-12/schema",
+      "if": { "properties": { "flag": { "const": true } } }
+    })JSON");
+
+  EXPECT_EQ(document, expected);
+}