Resolve collection identity during strict composition

internal/engine/collection_policy.go

@@ -32,6 +32,22 @@ func resolveCollectionPolicy(
 	applicationIdentityFields []string,
 	lower *collectionPolicy,
 ) (collectionPolicyResolution, []Diagnostic) {
+	normalizedDeclarations := cloneSequenceDeclarations(declarations)
+	for index := range normalizedDeclarations {
+		declaration := &normalizedDeclarations[index]
+		if !declaration.identityPresent {
+			continue
+		}
+		fields, err := normalizeSequenceIdentityFields(declaration.identityFields)
+		if err != nil {
+			return collectionPolicyResolution{}, []Diagnostic{{
+				Code: CodeSequenceInvalid, Severity: SeverityError, Path: destination.clone(),
+				Message: "collection identity declaration requires at least one field",
+				Primary: collectionDeclarationLocation(declaration.identityLocation, declaration.location),
+			}}
+		}
+		declaration.identityFields = fields
+	}
 	normalizedApplicationIdentityFields := cloneStrings(applicationIdentityFields)
 	if len(applicationIdentityFields) != 0 {
 		var err error
@@ -44,8 +60,8 @@ func resolveCollectionPolicy(
 		}
 	}
 	result := collectionPolicyResolution{
-		declarations:               cloneSequenceDeclarations(declarations),
-		declarationsPresent:        len(declarations) != 0,
+		declarations:               normalizedDeclarations,
+		declarationsPresent:        len(normalizedDeclarations) != 0,
 		applicationIdentityFields:  normalizedApplicationIdentityFields,
 		applicationIdentityPresent: len(normalizedApplicationIdentityFields) != 0,
 	}
@@ -55,7 +71,7 @@ func resolveCollectionPolicy(
 	var declaredIdentityLocation *Location
 	var declaredMode sequenceMode
 	var declaredModeLocation *Location
-	for _, declaration := range declarations {
+	for _, declaration := range normalizedDeclarations {
 		if declaration.identityPresent {
 			result.identityDeclarationPresent = true
 			if declaredIdentityFields == nil {