From 6e8cc00030b8165f159d72cc39ccfd67a97c483d Mon Sep 17 00:00:00 2001
From: Mike Farah <mikefarah@gmail.com>
Date: Mon, 14 Jul 2025 16:26:26 +1000
Subject: [PATCH] Added  flag to fix #2110

---
 cmd/root.go                                |  1 +
 pkg/yqlib/operator_anchors_aliases.go      |  5 +-
 pkg/yqlib/operator_anchors_aliases_test.go | 55 ++++++++++++++++++++++
 pkg/yqlib/yaml.go                          |  3 ++
 4 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/cmd/root.go b/cmd/root.go
index b7360d3c..2071e278 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -196,6 +196,7 @@ yq -P -oy sample.json
 		panic(err)
 	}
 	rootCmd.PersistentFlags().BoolVarP(&yqlib.ConfiguredYamlPreferences.LeadingContentPreProcessing, "header-preprocess", "", true, "Slurp any header comments and separators before processing expression.")
+	rootCmd.PersistentFlags().BoolVarP(&yqlib.ConfiguredYamlPreferences.FixMergeAnchorToSpec, "yaml-fix-merge-anchor-to-spec", "", false, "Fix merge anchor to match YAML spec. Will default to true in late 2025")
 
 	rootCmd.PersistentFlags().StringVarP(&splitFileExp, "split-exp", "s", "", "print each result (or doc) into a file named (exp). [exp] argument must return a string. You can use $index in the expression as the result counter. The necessary directories will be created.")
 	if err = rootCmd.RegisterFlagCompletionFunc("split-exp", cobra.NoFileCompletions); err != nil {
diff --git a/pkg/yqlib/operator_anchors_aliases.go b/pkg/yqlib/operator_anchors_aliases.go
index 4a2cdf4e..50744871 100644
--- a/pkg/yqlib/operator_anchors_aliases.go
+++ b/pkg/yqlib/operator_anchors_aliases.go
@@ -272,7 +272,10 @@ func overrideEntry(node *CandidateNode, key *CandidateNode, value *CandidateNode
 		log.Debugf("checking new content %v:%v", keyNode.Value, valueEl.Value.(*CandidateNode).Value)
 		if keyNode.Value == key.Value && keyNode.Alias == nil && key.Alias == nil {
 			log.Debugf("overridign new content")
-			valueEl.Value = value
+			if !ConfiguredYamlPreferences.FixMergeAnchorToSpec {
+				log.Warning("--yaml-fix-merge-anchor-to-spec is false; causing the merge anchor to override the existing value at %v which isn't to the yaml spec. This flag will default to true in late 2025.", keyNode.GetNicePath())
+				valueEl.Value = value
+			}
 			return nil
 		}
 		newEl = valueEl // move forward twice
diff --git a/pkg/yqlib/operator_anchors_aliases_test.go b/pkg/yqlib/operator_anchors_aliases_test.go
index a24f3fdd..c5b2f74b 100644
--- a/pkg/yqlib/operator_anchors_aliases_test.go
+++ b/pkg/yqlib/operator_anchors_aliases_test.go
@@ -53,7 +53,54 @@ foobar:
     thing: foobar_thing
 `
 
+var explodeWhenKeysExistDocument = `objects:
+  - &circle
+    name: circle
+    shape: round
+  - name: ellipse
+    !!merge <<: *circle
+  - !!merge <<: *circle
+    name: egg
+`
+
+var explodeWhenKeysExistLegacy = `D0, P[], (!!map)::objects:
+    - name: circle
+      shape: round
+    - name: circle
+      shape: round
+    - shape: round
+      name: egg
+`
+
+var explodeWhenKeysExistExpected = `D0, P[], (!!map)::objects:
+    - name: circle
+      shape: round
+    - name: ellipse
+      shape: round
+    - shape: round
+      name: egg
+`
+
+var fixedAnchorOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:        true,
+		description:    "merge anchor after existing keys",
+		subdescription: "legacy: overrides existing keys",
+		document:       explodeWhenKeysExistDocument,
+		expression:     "explode(.)",
+		expected:       []string{explodeWhenKeysExistExpected},
+	},
+}
+
 var anchorOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:        true,
+		description:    "merge anchor after existing keys",
+		subdescription: "legacy: overrides existing keys",
+		document:       explodeWhenKeysExistDocument,
+		expression:     "explode(.)",
+		expected:       []string{explodeWhenKeysExistLegacy},
+	},
 	{
 		skipDoc:       true,
 		description:   "merge anchor not map",
@@ -291,3 +338,11 @@ func TestAnchorAliasOperatorScenarios(t *testing.T) {
 	}
 	documentOperatorScenarios(t, "anchor-and-alias-operators", anchorOperatorScenarios)
 }
+
+func TestAnchorAliasOperatorAlignedToSpecScenarios(t *testing.T) {
+	ConfiguredYamlPreferences.FixMergeAnchorToSpec = true
+	for _, tt := range fixedAnchorOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	ConfiguredYamlPreferences.FixMergeAnchorToSpec = false
+}
diff --git a/pkg/yqlib/yaml.go b/pkg/yqlib/yaml.go
index 079bc749..1daba35d 100644
--- a/pkg/yqlib/yaml.go
+++ b/pkg/yqlib/yaml.go
@@ -7,6 +7,7 @@ type YamlPreferences struct {
 	PrintDocSeparators          bool
 	UnwrapScalar                bool
 	EvaluateTogether            bool
+	FixMergeAnchorToSpec        bool
 }
 
 func NewDefaultYamlPreferences() YamlPreferences {
@@ -17,6 +18,7 @@ func NewDefaultYamlPreferences() YamlPreferences {
 		PrintDocSeparators:          true,
 		UnwrapScalar:                true,
 		EvaluateTogether:            false,
+		FixMergeAnchorToSpec:        false,
 	}
 }
 
@@ -28,6 +30,7 @@ func (p *YamlPreferences) Copy() YamlPreferences {
 		PrintDocSeparators:          p.PrintDocSeparators,
 		UnwrapScalar:                p.UnwrapScalar,
 		EvaluateTogether:            p.EvaluateTogether,
+		FixMergeAnchorToSpec:        p.FixMergeAnchorToSpec,
 	}
 }