Report malformed sequence items at assigned paths

internal/engine/compose.go

@@ -239,7 +239,7 @@ func validateRegisteredSequences(inputs []*document.Node, claims []compositionCl
 		start := lengthsByPath[pathKey]
 		contribution := validatedSequence{items: node.Sequence}
 		for itemIndex, item := range node.Sequence {
-			itemPath := claim.path.Index(itemIndex)
+			itemPath := claim.path.Index(start + itemIndex)
 			if item == nil || item.Kind != document.Mapping {
 				location := (*Location)(nil)
 				if item != nil {

internal/engine/compose_test.go

@@ -339,6 +339,20 @@ func TestComposeDocumentsValidatesRegisteredSequenceSingletons(t *testing.T) {
 	}
 }
 
+func TestComposeDocumentsReportsAssignedPathForMalformedRegisteredSequenceContribution(t *testing.T) {
+	_, diagnostics := composeDocumentsWithPolicy(
+		sequencePolicy{identityFields: map[string]string{"/records": "id"}},
+		parseCompositionDocument(t, "source-1", "records: [{id: first}]\n"),
+		parseCompositionDocument(t, "source-2", "records: [{name: malformed}]\n"),
+	)
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeComposeInvalid || diagnostics[0].Path.String() != "/records/1" {
+		t.Fatalf("diagnostics = %#v, want invalid item at /records/1", diagnostics)
+	}
+	if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-2" {
+		t.Fatalf("malformed item location = %#v, want source-2", diagnostics[0].Primary)
+	}
+}
+
 func TestComposeDocumentsRejectsRegisteredDuplicateIdentities(t *testing.T) {
 	tests := []struct {
 		name   string