feat: Introduce ErrYAMLMergeKey

README.md

@@ -58,6 +58,7 @@ an error annotated with line and column in the YAML file if necessary.
 	the [`encoding.TextUnmarshaler`](https://pkg.go.dev/encoding#TextUnmarshaler) interface.
 	- 🚫 Forbids empty array items ([see rationale](#why-are-empty-array-items-forbidden)).
 	- 🚫 Forbids multi-document files.
+	- 🚫 Forbids [YAML merge keys](https://yaml.org/type/merge.html).
 - Features:
 	- 🪄 If any type within your configuration struct implements the `Validate` interface,
 	then its validation method will be called using reflection

yamagiconf.go

@@ -52,6 +52,7 @@ var (
 		"any other variants of null are not supported")
 	ErrYAMLNonStrOnTextUnmarsh = errors.New("value must be a string because the " +
 		"target type implements encoding.TextUnmarshaler")
+	ErrYAMLMergeKey = errors.New("avoid using YAML merge keys")
 
 	// ErrYAMLEmptyArrayItem applies to both Go arrays and slices even though
 	// an empty item would be parsed correctly as zero-value in case of Go arrays
@@ -730,6 +731,12 @@ func validateYAMLValues(
 				return fmt.Errorf("at %s (as %q): %w",
 					path, yamlTag, ErrYAMLMissingConfig)
 			}
+			for _, n := range contentNode.Content {
+				if n.Tag == "!!merge" {
+					return fmt.Errorf("at %d:%d: %w",
+						n.Line, n.Column, ErrYAMLMergeKey)
+				}
+			}
 			err := validateYAMLValues(anchors, yamlTag, path, f.Type, contentNode)
 			if err != nil {
 				return err

yamagiconf_test.go

@@ -1118,6 +1118,67 @@ func TestValidateTypeErrYAMLTagRedefined(t *testing.T) {
 	require.Equal(t, err, yamagiconf.Validate(TestConfig{}))
 }
 
+func TestErrYAMLMergeKey(t *testing.T) {
+	t.Run("in_struct", func(t *testing.T) {
+		type Server struct {
+			Host string `yaml:"host"`
+			Port uint16 `yaml:"port"`
+		}
+		type TestConfig struct {
+			ServerDefault  Server `yaml:"server-default"`
+			ServerFallback Server `yaml:"server-fallback"`
+		}
+		var c TestConfig
+		err := yamagiconf.Load[TestConfig](`
+server-default: &default
+  host: default.server
+  port: 12345
+server-fallback:
+  port: 54321
+  <<: *default
+`, &c)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMergeKey)
+		require.Equal(t, `at 7:3: `+yamagiconf.ErrYAMLMergeKey.Error(), err.Error())
+	})
+
+	t.Run("in_map", func(t *testing.T) {
+		type TestConfig struct {
+			Map1 map[string]string `yaml:"map1"`
+			Map2 map[string]string `yaml:"map2"`
+		}
+		var c TestConfig
+		err := yamagiconf.Load[TestConfig](`
+map1: &map1
+  foo: bar
+  bazz: fazz
+map2:
+  <<: *map1
+  mar: tar
+`, &c)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMergeKey)
+		require.Equal(t, `at 6:3: `+yamagiconf.ErrYAMLMergeKey.Error(), err.Error())
+	})
+
+	t.Run("map_multi_merge", func(t *testing.T) {
+		type TestConfig struct {
+			Map1 map[string]string `yaml:"map1"`
+			Map2 map[string]string `yaml:"map2"`
+			Map3 map[string]string `yaml:"map3"`
+		}
+		var c TestConfig
+		err := yamagiconf.Load[TestConfig](`
+map1: &map1
+  foo: bar
+map2: &map2
+  bazz: fazz
+map3:
+  <<: [*map1,*map2]
+`, &c)
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLMergeKey)
+		require.Equal(t, `at 7:3: `+yamagiconf.ErrYAMLMergeKey.Error(), err.Error())
+	})
+}
+
 func TestValidateTypeErrTypeEnvTagOnUnexported(t *testing.T) {
 	type TestConfig struct {
 		Ok string `yaml:"okay"`
@@ -1484,10 +1545,20 @@ foo: 1
 		type TestConfig struct {
 			Boolean bool `yaml:"boolean"`
 		}
-		_, err := LoadSrc[TestConfig]("\nboolean: yes")
+		_, err := LoadSrc[TestConfig]("boolean: yes")
+		require.ErrorIs(t, err, yamagiconf.ErrYAMLBadBoolLiteral)
+		require.Equal(t, `at 1:10: "boolean" (TestConfig.Boolean): `+
+			yamagiconf.ErrYAMLBadBoolLiteral.Error(), err.Error())
+	})
+
+	t.Run("unsupported_boolean_literal_in_array", func(t *testing.T) {
+		type TestConfig struct {
+			Booleans []bool `yaml:"booleans"`
+		}
+		_, err := LoadSrc[TestConfig]("booleans:\n  - false\n  - no")
 		require.ErrorIs(t, err, yamagiconf.ErrYAMLBadBoolLiteral)
-		require.True(t, strings.HasPrefix(err.Error(),
-			`at 2:10: "boolean" (TestConfig.Boolean): must be either false or true`))
+		require.Equal(t, `at 3:5: "booleans" (TestConfig.Booleans[1]): `+
+			yamagiconf.ErrYAMLBadBoolLiteral.Error(), err.Error())
 	})
 
 	t.Run("unsupported_null_literal_tilde", func(t *testing.T) {