Allow declaration of empty Concepts.

Concepts codes field is still required in the parser.

PiperOrigin-RevId: 650334971

interpreter/literal.go

@@ -302,13 +302,13 @@ func (i *interpreter) evalInstance(in *model.Instance) (result.Value, error) {
 			if err != nil {
 				return result.Value{}, err
 			}
-			var codes []result.Code
-			for _, codeObj := range codeObjs {
+			codes := make([]result.Code, len(codeObjs))
+			for i, codeObj := range codeObjs {
 				code, err := result.ToCode(codeObj)
 				if err != nil {
 					return result.Value{}, err
 				}
-				codes = append(codes, code)
+				codes[i] = code
 			}
 			cv.Codes = codes
 		}

parser/expressions_test.go

@@ -133,6 +133,12 @@ func TestParserExpressions_Errors(t *testing.T) {
 			errContains: []string{"unsupported type for maximumString"},
 			errCount:    1,
 		},
+		{
+			name:        "Concept Instance codes not specified",
+			cql:         "Concept{ }",
+			errContains: []string{"no viable alternative at input 'Concept{ }'"},
+			errCount:    1,
+		},
 	}
 
 	for _, test := range tests {

result/value.go

@@ -401,8 +401,8 @@ func New(val any) (Value, error) {
 		}
 		return Value{runtimeType: types.CodeSystem, goValue: v}, nil
 	case Concept:
-		if len(v.Codes) == 0 {
-			return Value{}, fmt.Errorf("%v must have at least one %v", types.Concept, types.Code)
+		if v.Codes == nil {
+			return Value{}, fmt.Errorf("%v must specify the codes field", types.Concept)
 		}
 		return Value{runtimeType: types.Concept, goValue: v}, nil
 	case ValueSet:

result/value_test.go

@@ -782,9 +782,9 @@ func TestNew_Error(t *testing.T) {
 			wantErr: "System.CodeSystem must have an ID",
 		},
 		{
-			name:    "Concept must have one code",
+			name:    "Concept must specify codes",
 			input:   Concept{},
-			wantErr: "System.Concept must have at least one System.Code",
+			wantErr: "System.Concept must specify the codes field",
 		},
 		{
 			name:    "ValueSet missing ID",

tests/enginetests/literal_test.go

@@ -451,6 +451,13 @@ func TestTupleAndInstanceSelector(t *testing.T) {
 			cql:        "CodeSystem{id: 'id', version: '1.0' }",
 			wantResult: newOrFatal(t, result.CodeSystem{ID: "id", Version: "1.0"}),
 		},
+		{
+			name: "Concept Instance no codes",
+			cql:  "Concept{codes: {} }",
+			wantResult: newOrFatal(t, result.Concept{
+				Codes: []result.Code{},
+			}),
+		},
 		{
 			name: "Concept Instance",
 			cql:  "Concept{codes: {Code{code: 'foo', system: 'bar', version: '1.0' }}, display: 'display' }",

tests/enginetests/operator_type_test.go

@@ -733,9 +733,10 @@ func TestToConcept_Error(t *testing.T) {
 		wantError string
 	}{
 		{
+			// TODO: b/301606416 - This shouldn't be an error once we support null values in concepts.
 			name:      "Concept must have a Code",
 			cql:       "ToConcept(List<Code>{null})",
-			wantError: "System.Concept must have at least one System.Code",
+			wantError: "System.Concept must specify the codes field",
 		},
 	}