Reject a YAML flow mapping entry that is missing its comma

src/core/yaml/parser.h

@@ -789,7 +789,17 @@ class Parser {
     switch (current_token.type) {
       case TokenType::Scalar: {
         auto next{this->next_token()};
-        if (next.has_value() && next->type == TokenType::BlockMappingValue) {
+        // YAML 1.2.2 Section 7.4.2: the value of an entry of a flow collection
+        // is a single node, and a pair carrying no brackets of its own is a
+        // node only where a flow sequence takes its entries. Leaving the
+        // indicator unread in that position hands the scalar back on its own,
+        // which is what lets the caller report the separator the entry is
+        // really missing
+        const auto pair_without_brackets_allowed{
+            this->lexer_->flow_level() == 0 ||
+            context != JSON::ParseContext::Property};
+        if (next.has_value() && next->type == TokenType::BlockMappingValue &&
+            pair_without_brackets_allowed) {
           if (current_token.multiline) [[unlikely]] {
             throw YAMLParseError{current_token.line, current_token.column,
                                  "Multi-line implicit mapping key"};

test/yaml/yaml_parse_test.cc

@@ -2769,3 +2769,35 @@ TEST(implicit_key_anchor_on_a_plain_scalar_stays_a_string) {
       sourcemeta::core::parse_json(R"JSON({ "foo": 1, "b": "foo" })JSON")};
   EXPECT_EQ(result, expected);
 }
+
+// YAML 1.2.2 Section 7.4.2: the entries of a flow mapping are separated by a
+// comma, and the value of an entry is a single node, so a second pair cannot
+// stand in the place of the first entry's value
+TEST(flow_mapping_entries_without_a_comma) {
+  const std::string input{"{a: 1 b: 2}"};
+  try {
+    sourcemeta::core::parse_yaml(input);
+    FAIL();
+  } catch (const sourcemeta::core::YAMLParseError &error) {
+    EXPECT_STREQ(error.what(), "Missing comma between flow mapping entries");
+  }
+}
+
+TEST(flow_sequence_single_pair_entry_still_parses) {
+  const std::string input{"[a: 1]"};
+  const auto result{sourcemeta::core::parse_yaml(input)};
+  EXPECT_TRUE(result.is_array());
+  EXPECT_EQ(result.size(), 1);
+  EXPECT_TRUE(result.at(0).is_object());
+  EXPECT_EQ(result.at(0).at("a"), sourcemeta::core::JSON{1});
+}
+
+TEST(flow_sequence_single_pair_value_cannot_be_another_pair) {
+  const std::string input{"[a: b: c]"};
+  try {
+    sourcemeta::core::parse_yaml(input);
+    FAIL();
+  } catch (const sourcemeta::core::YAMLParseError &error) {
+    EXPECT_STREQ(error.what(), "Missing comma in flow sequence");
+  }
+}