feat(go): Unions include runtime validation

generators/go/internal/generator/model.go

@@ -474,6 +474,9 @@ func (t *typeVisitor) VisitUnion(union *ir.UnionTypeDeclaration) error {
 
 	// Implement the json.Marshaler interface.
 	t.writer.P("func (", receiver, " ", t.typeName, ") MarshalJSON() ([]byte, error) {")
+	t.writer.P("if err := ", receiver, ".validate(); err != nil {")
+	t.writer.P("return nil, err")
+	t.writer.P("}")
 	if t.unionVersion != UnionVersionV1 {
 		t.writer.P("switch ", receiver, ".", discriminantName, " {")
 	}
@@ -659,6 +662,63 @@ func (t *typeVisitor) VisitUnion(union *ir.UnionTypeDeclaration) error {
 	t.writer.P("}")
 	t.writer.P()
 
+	// Generate the validate method.
+	t.writer.P("func (", receiver, " *", t.typeName, ") validate() error {")
+	t.writer.P("if ", receiver, " == nil {")
+	t.writer.P(`return fmt.Errorf("type %T is nil", `, receiver, ")")
+	t.writer.P("}")
+	t.writer.P("var fields []string")
+	for _, unionType := range union.Types {
+		var (
+			isLiteral  bool
+			isOptional bool
+			date       *date
+		)
+		if unionType.Shape.SingleProperty != nil {
+			isLiteral = isLiteralType(unionType.Shape.SingleProperty.Type, t.writer.types)
+			isOptional = isOptionalType(unionType.Shape.SingleProperty.Type, t.writer.types)
+			date = maybeDate(unionType.Shape.SingleProperty.Type, isOptional)
+		}
+		zeroValue := "nil"
+		if unionType.Shape.PropertiesType == "singleProperty" {
+			zeroValue = zeroValueForTypeReference(unionType.Shape.SingleProperty.Type, t.writer.types)
+		}
+		unionTypeValue := receiver + "." + unionType.DiscriminantValue.Name.PascalCase.UnsafeName
+		if isLiteral {
+			unionTypeValue = receiver + "." + unionType.DiscriminantValue.Name.CamelCase.SafeName
+		}
+		if date != nil && !isOptional {
+			t.writer.P("if !", unionTypeValue, ".IsZero() {")
+		} else {
+			t.writer.P("if ", unionTypeValue, " != ", zeroValue, " {")
+		}
+		t.writer.P(`fields = append(fields, "`, unionType.DiscriminantValue.WireValue, `")`)
+		t.writer.P("}")
+	}
+	t.writer.P("if len(fields) == 0 {")
+	t.writer.P("if ", receiver, ".", discriminantName, ` != "" {`)
+	t.writer.P(`return fmt.Errorf("type %T defines a discriminant set to %q but the field is not set", `, receiver, ", ", receiver, ".", discriminantName, ")")
+	t.writer.P("}")
+	t.writer.P(`return fmt.Errorf("type %T is empty", `, receiver, ")")
+	t.writer.P("}")
+	t.writer.P("if len(fields) > 1 {")
+	t.writer.P(`return fmt.Errorf("type %T defines values for %s, but only one value is allowed", `, receiver, ", fields)")
+	t.writer.P("}")
+	t.writer.P("if ", receiver, ".", discriminantName, ` != "" {`)
+	t.writer.P("field := fields[0]")
+	t.writer.P("if ", receiver, ".", discriminantName, " != field {")
+	t.writer.P("return fmt.Errorf(")
+	t.writer.P(`"type %T defines a discriminant set to %q, but it does not match the %T field; either remove or update the discriminant to match",`)
+	t.writer.P(receiver, ", ")
+	t.writer.P(receiver, ".", discriminantName, ", ")
+	t.writer.P(receiver, ", ")
+	t.writer.P(")")
+	t.writer.P("}")
+	t.writer.P("}")
+	t.writer.P("return nil")
+	t.writer.P("}")
+	t.writer.P()
+
 	return nil
 }
 

generators/go/sdk/versions.yml

@@ -1,3 +1,32 @@
+- version: 0.35.0
+  changelogEntry:
+    - type: feat
+      summary: >-
+        Add runtime validation for discriminated unions to prevent users from accidentally
+        sending the wrong type of value. With this, users will be expected to set exactly
+        one of the union's values like so:
+
+        ```go
+        package example
+
+        type Animal struct {
+          Type string
+          Cat  *Cat
+          Dog  *Dog
+        }
+
+        func do() {
+          union := &Animal{
+            Cat: &Cat{
+              Name: "Fluffy",
+            },
+          }
+        }
+        ```
+
+        If the user sets _both_ `Cat` and `Dog`, the user will receive an error when the
+        type is serialized to JSON (i.e. in the `json.Marshaler` implementation).
+  irVersion: 53
 - version: 0.34.0
   changelogEntry:
     - type: feat