replace-triggered-by must reference a valid resource or resource attribute

.changes/v1.12/BUG FIXES-20250331-170607.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: replace-triggered-by must reference a valid resource or resource attribute
+time: 2025-03-31T17:06:07.04296+02:00
+custom:
+    Issue: "36740"

internal/terraform/context_plan2_test.go

@@ -4399,6 +4399,51 @@ resource "test_object" "b" {
 	}
 }
 
+func TestContext2Plan_triggeredByInvalid(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+			resource "test_object" "a" {
+				count = 1
+				test_string = "new"
+			}
+			resource "test_object" "b" {
+				count = 1
+				test_string = test_object.a[count.index].test_string
+				lifecycle {
+					# reference to an invalid attribute "yikes" should cause an error
+					replace_triggered_by = [ test_object.a[count.index].yikes ]
+				}
+			}
+`,
+	})
+
+	p := simpleMockProvider()
+
+	ctx := testContext2(t, &ContextOpts{
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+	})
+
+	state := states.BuildState(func(s *states.SyncState) {
+		s.SetResourceInstanceCurrent(
+			mustResourceInstanceAddr("test_object.a[0]"),
+			&states.ResourceInstanceObjectSrc{
+				AttrsJSON: []byte(`{"test_string":"old"}`),
+				Status:    states.ObjectReady,
+			},
+			mustProviderConfig(`provider["registry.terraform.io/hashicorp/test"]`),
+		)
+	})
+
+	_, diags := ctx.Plan(m, state, &PlanOpts{
+		Mode: plans.NormalMode,
+	})
+	if !diags.HasErrors() {
+		t.Fatalf("expected errors, got none")
+	}
+}
+
 func TestContext2Plan_dataSchemaChange(t *testing.T) {
 	// We can't decode the prior state when a data source upgrades the schema
 	// in an incompatible way. Since prior state for data sources is purely

internal/terraform/eval_context_builtin.go

@@ -407,10 +407,7 @@ func (ctx *BuiltinEvalContext) EvaluateReplaceTriggeredBy(expr hcl.Expression, r
 		return nil, false, diags
 	}
 
-	path, _ := traversalToPath(ref.Remaining)
-	attrBefore, _ := path.Apply(change.Before)
-	attrAfter, _ := path.Apply(change.After)
-
+	attrBefore, attrAfter, diags := evalTriggeredByRefPath(ref, change)
 	if attrBefore == cty.NilVal || attrAfter == cty.NilVal {
 		replace := attrBefore != attrAfter
 		return ref, replace, diags
@@ -421,6 +418,42 @@ func (ctx *BuiltinEvalContext) EvaluateReplaceTriggeredBy(expr hcl.Expression, r
 	return ref, replace, diags
 }
 
+// evalTriggeredByRefPath evaluates the attribute reference path in the context of the
+// resource change objects. It returns the before and after values of the attribute
+// and any diagnostics that occurred during evaluation.
+func evalTriggeredByRefPath(ref *addrs.Reference, change *plans.ResourceInstanceChange) (cty.Value, cty.Value, tfdiags.Diagnostics) {
+	var diags tfdiags.Diagnostics
+	path, key := traversalToPath(ref.Remaining)
+
+	applyPath := func(value cty.Value) (cty.Value, tfdiags.Diagnostics) {
+		attr, err := path.Apply(value)
+		if err != nil {
+			return cty.NilVal, tfdiags.Diagnostics{}.Append(tfdiags.AttributeValue(
+				tfdiags.Error,
+				"Invalid attribute path",
+				fmt.Sprintf(
+					"The specified path %q could not be applied to the object specified in replace_triggered_by:\n"+
+						"Path: %s\n"+
+						"Error: %s\n"+
+						"Please check your configuration and ensure the path is valid.",
+					key, ref.DisplayString(), err.Error(),
+				),
+				path,
+			))
+		}
+		return attr, nil
+	}
+
+	// Apply the path to the "before" and "after" states
+	attrBefore, beforeDiags := applyPath(change.Before)
+	diags = diags.AppendWithoutDuplicates(beforeDiags...)
+
+	attrAfter, afterDiags := applyPath(change.After)
+	diags = diags.AppendWithoutDuplicates(afterDiags...)
+
+	return attrBefore, attrAfter, diags
+}
+
 func (ctx *BuiltinEvalContext) EvaluationScope(self addrs.Referenceable, source addrs.Referenceable, keyData InstanceKeyEvalData) *lang.Scope {
 	switch scope := ctx.scope.(type) {
 	case evalContextModuleInstance: